In a broader sense, co-creation means the joint effort of bringing something new to fruition through acts of collective creativity (Sanders & Stappers, 2008) which can be manifested in both tangible (making something together) or intangible (learning something together) outcomes (Puerari et al., 2018). Recently, the concepts of co-creation or co- production have been applied to describe the processes of participation in urban planning and design. Both terms place particular emphasis on the partnerships formed between citizens and the public sector, in which a high level of citizen involvement is pivotal. Participation has been defined through its different levels of citizen involvement, ranging from non-participation to greater degrees of citizen control (Arnstein, 1969) indicating the different levels of influence a participant can have on a participatory process. From the perspective of urban planning, citizen participation is beginning to be described as co-creation when citizens’ roles become more prominent, presenting aspects of self-organisation, increased commitment and a sense of ownership of the process (Puerari et al., 2018). Recent research is exploring new methods of urban planning in which citizens, the municipality and private organisations co-create new planning rules (Bisschops & Beunen, 2019). However, co-creation along with co-production and participation, often used interchangeably, have become popular catchphrases and are considered as processes which are of virtue in themselves. Furthermore, while there is substantial research on these processes, the research conducted on the outcomes of enhanced participation remains rather limited (Voorberg et al., 2015). This highlights the ambiguity in terms of interpretation; is co-creation a methodology, a set of tools to enhance and drive a process, or a goal in itself? (Puerari et al., 2018). There have often been cases where participation, co-creation and co-production have been used decoratively, as a form of justification and validation of decisions already made (Armeni, 2016). In the provision of public spaces, co-creation/co-production may specifically involve housing (Brandsen & Helderman, 2012; Chatterton, 2016) and placemaking: “placemaking in public space implies engaging in the practice of urban planning and design beyond an expert culture. Such collaboration can be described as co-creation.” (Eggertsen Teder, 2019, p.290). As in participation, co-creation requires the sharing of decision-making powers, the creation of  joint knowledge and the assignation of abilities between communities, while urban professionals and local authorities should draw attention to the active involvement of community members. Furthermore, co-creation does not take place in a vacuum, but always occurs within socio- spatial contexts. This points to the objective of co-creation as a tool to influence locally relevant policy through innovation that is “place-based”. To conclude, co-creation can be perceived as a process that is both transdisciplinary in its application, and as a tool for achieving transdisciplinarity on a broader scale through a systematic integration in existing standard practices in urban planning, housing design and architecture. Despite the persisting ambiguity in its definition, co-creation processes can provide more inclusive platforms for revisiting and informing formal and informal knowledge on sustainable and affordable housing.

According to the Oxford English Dictionary, participation is “the act of taking part in an activity or event”. Likewise, it can also mean “the fact of sharing or the act of receiving or having a part of something.” It derives from old French participacion which in turn comes from late Latin participationem, which means “partaking” (Harper, 2000).  References to participation can be found in many fields, including social sciences, economics, politics, and culture. It is often related to the idea of citizenship and its different representations in society. Hence, it could be explained as an umbrella concept, in which several others can be encompassed, including methodologies, philosophical discourses, and tools. Despite the complexity in providing a holistic definition, the intrinsic relation between participation and power is widely recognised. Its ultimate objective is to empower those involved in the process (Nikkhah & Redzuan, 2009). An early application of participatory approaches was the Participatory Rural Appraisal (PRA) which exerted a significant influence in developing new discourses and practices of urban settings (Chambers, 1994; Friedmann, 1994). In the late 1970s increasing attention was paid to the concept by scholars, and several associated principles and terminologies evolved, such as the participation in design and planning with the Scandinavian approach of cooperative design (Bφdker et al., 1995; Gregory, 2003). Participation in design or participatory design is a process and strategy that entails all stakeholders (e.g. partners, citizens, and end-users) partaking in the design process. It is a democratic process for design based on the assumption that users should be involved in the designs they will go on to use (Bannon & Ehn, 2012; Cipan, 2019; Sanoff, 2000, 2006, 2007). Likewise, participatory planning is an alternative paradigm that emerged in response to the rationalistic and centralized – top-down – approaches. Participatory planning aims to integrate the technical expertise with the preferences and knowledge of community members (e.g., citizens, non-governmental organizations, and social movements) directly and centrally in the planning and development processes, producing outcomes that respond to the community's needs (Lane, 2005). Understanding participation through the roles of participants is a vital concept. The work of Sherry Arnstein’s (1969) Ladder of Citizen Participation has long been the cornerstone to understand participation from the perspective of the redistribution of power between the haves and the have-nots. Her most influential typological categorisation work yet distinguishes eight degrees of participation as seen in Figure 1: manipulation, therapy, placation, consultation, informing, citizen control, delegated power and partnership. Applied to a participatory planning context, this classification refers to the range of influence that participants can have in the decision-making process. In this case, no-participation is defined as designers deciding based upon assumptions of the users’ needs and full-participation refers to users defining the quality criteria themselves (Geddes et al., 2019). A more recent classification framework that also grounds the conceptual approach to the design practice and its complex reality has been developed by Archon Fung (2006) upon three key dimensions: who participates; how participants communicate with one another and make decisions together, and how discussions are linked with policy or public action. This three-dimensional approach which Fung describes as a democracy cube (Figure 2), constitutes a more analytic space where any mechanism of participation can be located. Such frameworks of thinking allow for more creative interpretations of the interrelations between participants, participation tools (including immersive digital tools) and contemporary approaches to policymaking. Aligned with Arnstein’s views when describing the lower rungs of the ladder (i.e., nonparticipation and tokenism), other authors have highlighted the perils of incorporating participatory processes as part of pre-defined agendas, as box-ticking exercises, or for political manipulation. By turning to eye-catching epithets to describe it (Participation: The New Tyranny? by Cooke & Kothari, 2001; or The Nightmare of Participation by Miessen, 2010), these authors attempt to raise awareness on the overuse of the term participation and the possible disempowering effects that can bring upon the participating communities, such as frustration and lack of trust. Examples that must exhort practitioners to reassess their role and focus on eliminating rather than reinforcing inequalities (Cooke & Kothari, 2001).

Affordability is defined as the state of being cheap enough for people to be able to buy (Combley, 2011). Applied to housing, affordability, housing unaffordability and the mounting housing affordability crisis, are concepts that have come to the fore, especially in the contexts of free-market economies and housing systems led by private initiatives, due to the spiralling house prices that residents of major urban agglomerations across the world have experienced in recent years (Galster & Ok Lee, 2021). Notwithstanding, the seeming simplicity of the concept, the definition of housing affordability can vary depending on the context and approach to the issue, rendering its applicability in practice difficult. Likewise, its measurement implies a multidimensional and multi-disciplinary lens (Haffner & Hulse, 2021). One definition widely referred to of housing affordability is the one provided by Maclennan and Williams (1990, p.9): “‘Affordability’ is concerned with securing some given standard of housing (or different standards) at a price or a rent which does not impose, in the eyes of some third party (usually government) an unreasonable burden on household incomes”. Hence, the maximum expenditure a household should pay for housing is no more than 30% of its income (Paris, 2006). Otherwise, housing is deemed unaffordable. This measure of affordability reduces a complex issue to a simple calculation of the rent-to-income ratio or house-price-to-income ratio. In reality, a plethora of variables can affect affordability and should be considered when assessing it holistically, especially when judging what is acceptable or not in the context of specific individual and societal norms (see Haffner & Hulse, 2021; Hancock, 1993). Other approaches to measure housing affordability consider how much ‘non-housing’ expenditures are unattended after paying for housing. Whether this residual income is not sufficient to adequately cover other household’s needs, then there is an affordability problem (Stone, 2006). These approaches also distinguish between “purchase affordability” (the ability to borrow funds to purchase a house) and “repayment affordability” (the ability to afford housing finance repayments) (Bieri, 2014). Furthermore, housing production and, ultimately affordability, rely upon demand and supply factors that affect both the developers and home buyers. On the supply side, aspects such as the cost of land, high construction costs, stiff land-use regulations, and zoning codes have a crucial role in determining the ultimate price of housing (Paris, 2006). Likewise, on the policy side, insufficient government subsidies and lengthy approval processes may deter smaller developers from embarking on new projects. On the other hand, the demand for affordable housing keeps increasing alongside the prices, which remain high, as a consequence of the, sometimes deliberate incapacity of the construction sector to meet the consumers' needs (Halligan, 2021). Similarly, the difficulty of decreasing household expenditures while increasing incomes exacerbates the unaffordability of housing (Anacker, 2019). In the end, as more recent scholarship has pointed out (see Haffner & Hulse, 2021; Mulliner & Maliene, 2014), the issue of housing affordability has complex implications that go beyond the purely economic or financial ones. The authors argue that it has a direct impact on the quality of life and well-being of the affected and their relationship with the city, and thus, it requires a multidimensional assessment. Urban and spatial inequalities in the access to city services and resources, gentrification, segregation, fuel and commuting poverty, and suburbanisation are amongst its most notorious consequences. Brysch and Czischke, for example, found through a comparative analysis of 16 collaborative housing projects in Europe that affordability was increased by “strategic design decisions and self-organised activities aiming to reduce building costs” (2021, p.18). This demonstrates that there is a great potential for design and urban planning tools and mechanisms to contribute to the generation of innovative solutions to enable housing affordability considering all the dimensions involved, i.e., spatial, urban, social and economic. Examples range from public-private partnerships, new materials and building techniques, alternative housing schemes and tenure models (e.g., cohousing, housing cooperatives, Community Land Trusts, ‘Baugruppen’), to efficient interior design, (e.g., flexible design, design by layers[1]). Considering affordability from a design point of view can activate different levers to catalyse and bring forward housing solutions for cities; and stakeholders such as socially engaged real estate developers, policymakers, and municipal authorities have a decisive stake in creating an adequate environment for fostering, producing and delivering sustainable and affordable housing.   [1] (see Brand, 1995; Schneider & Till, 2007)

Transdisciplinarity is a research methodology crossing several disciplinary boundaries, creating a holistic approach to solve complex problems. A transdisciplinary approach fosters bottom-up collaboration, provides an environment for mutual learning, and enhances the knowledge of all participants (Klein et al., 2001, Summary and Synthesis). Transdisciplinarity is a relatively young term, first used just over fifty years ago at the Organisation for Economic Co-operation and Development (OECD) congress by Jean Piaget, who described it in a broader sense as “a higher stage succeeding interdisciplinary relationships…without any firm boundaries between disciplines” (Piaget, 1972, p.135). Transdisciplinarity goes beyond interdisciplinarity through a fusion of academic and non- academic knowledge, theory and practice, discipline and profession (Doucet & Janssens, 2011). Stokols (2006) asserts transdisciplinarity is inextricability linked to action research; a term coined by Lewin (1946) as comparative research leading to social action. Lewin sought to empower and enhance the self-esteem of participants, which included residents of minority communities, through horizontal and democratic exchange between the researcher and participants. Familiar devices rooted in action research, such as surveys, questionnaires, and interviews are common in transdisciplinary research (Klein et al., 2001). A transdisciplinarity approach has been used to address complex global concerns in recent decades, beginning with climate change and extending into many areas including socio-political problems (Bernstein, 2015). Lawrence et al. (2010) stress that in addressing community related issues such as housing, it is crucial a transdisciplinary approach is adopted not only to integrate various expert opinions but to ensure the inclusion of affected communities such as the residents themselves. Housing is a complex social issue, therefore requiring such an approach to foster participation of non-academics to provide socially relevant solutions. Salama (2011) advocates for the use of transdisciplinarity in the creation of affordable and sustainable housing, which is often restricted by stakeholders working in silos, the oversimplification of housing-related issues, and a disconnect from local communities.

Building Information Modelling (BIM) is the process of creating a set of digital representations which consists of both graphical and non-graphical data for the entire building cycle  (Eastman et al., 2011). This process involves documenting, gathering, organising, and updating this information throughout the whole life cycle of a building from conception to demolition (Eschenbruch & Bodden, 2018). Beyond the demolition stage BIM can also support circular principles; managing the re-use, recovery, and recycling-potential of a building (Akbarieh et al., 2020; Xue et al., 2021). Whilst the concept of BIM as a process is supported by the International Organisation for Standardisation in ISO 19650-1:2018 (ISO, 2018), the National BIM Standard describes BIM as a digital technology (NBIMS-US, 2015). Despite the origins of BIM dating back to the 1970s, it did not become widely adopted by the Architecture, Engineering and Construction (AEC) industry as a computer design tool until the 2000s (Costa, 2017). The digital building information model uses intelligent objects to store information in the form of three-dimensional geometric components along with its functional characteristics such as type, materials, technical properties, or costs (Eschenbruch & Bodden, 2018). This model forms the basis of a shared knowledge resource to support the various digital workflows of multidisciplinary stakeholders (Chong, Lee and Wang, 2017; Barile et al., 2018). Moreover, it serves the purpose of visualisation, clash detection between different building components, code criteria checking, environmental analysis, and cost estimation to name a few (Kamel & Memari, 2019; Krygiel & Nies, 2008). Therefore, utilising BIM can improve construction accuracy and enhance the built asset’s performance (Kubba, 2017; Love et al., 2013). The building information model facilitates the knowledge transfer between experts and project participants to satisfy end-user needs and support early-stage decision-making (Chong et al., 2017; Lu et al., 2017). Therefore, BIM can be considered a transdisciplinary practice as it communicates AEC, computation, and science (Correia et al., 2017). In the AEC industry implementing BIM involves several stages, which are known as BIM maturity models. The maturity here means the extent of the user’s ability to produce and exchange information. These stages are the milestones, or levels, of collaboration and sharing of information that teams, and organisations aspire to. Defining these milestones is the main purpose of the different BIM maturity models that exist nowadays (Succar et al., 2012). The European Commission (EC) encourages step-by-step maturity models starting from BIM level 0 up to 4, to move the industry from a traditional modelling approach towards an open BIM approach. According to the EC, to reach BIM level 4 “all project, operational documentation and history are linked to objects in the model” (European Commission, 2017). Due to growing concerns over the environmental, economic, and social impacts of the built environment, BIM is increasingly used to facilitate various sustainability analyses. In this regard, the concept of Green BIM initiated as the systematic digitalisation of building life cycles to accomplish established sustainability goals (Barile et al., 2018; Wong & Zhou, 2015). As such BIM has been integrated with Life Cycle Analysis (LCA), Life Cycle Costing Analysis (LCCA), and recently with Social Life Cycle Analysis (S-LCA) (Llatas et al., 2020). Today several BIM applications perform sustainability analysis in conjunction with Green Building Rating Systems (Sartori et al., 2021). In relation to housing BIM plays a crucial role in addressing affordability and sustainability issues from creation to maintenance, as well as the beyond end-of-life phases. However, many challenges remain for it to be fully and inclusively integrated within the AEC practice and for the full potential of BIM to be realised.

Life Cycle Costing (LCC) is a method used to estimate the overall cost of a building during its different life cycle stages, whether from cradle to grave or within a predetermined timeframe (Nucci et al., 2016; Wouterszoon Jansen et al., 2020). The Standardised Method of Life Cycle Costing (SMLCC) identifies LCC in line with the International Standard ISO 15686-5:2008 as "Methodology for the systematic economic evaluation of life cycle costs over a period of analysis, as defined in the agreed scope." (RICS, 2016). This evaluation can provide a useful breakdown of all costs associated with designing, constructing, operating, maintaining and disposing of this building (Dwaikat & Ali, 2018). Life cycle costs of an asset can be divided into two categories: (1) Initial costs, which are all the costs incurred before the occupation of the house, such as capital investment costs, purchase costs, and construction and installation costs (Goh & Sun, 2016; Kubba, 2010); (2) Future costs, which are those that occur after the occupancy phase of the dwelling. The future costs may involve operational costs, maintenance, occupancy and capital replacement (RICS, 2016). They may also include financing, resale, salvage, and end-of-life costs (Karatas & El-Rayes, 2014; Kubba, 2010; Rad et al., 2021). The costs to be included in a LCC analysis vary depending on its objective, scope and time period. Both the LCC objective and scope also determine whether the assessment will be conducted for the whole building, or for a certain building component or equipment (Liu & Qian, 2019; RICS, 2016). When LCC combines initial and future costs, it needs to consider the time value of money (Islam et al., 2015; Korpi & Ala-Risku, 2008). To do so, future costs need to be discounted to present value using what is known as "Discount Rate" (Islam et al., 2015; Korpi & Ala-Risku, 2008). LCC responds to the needs of the Architectural Engineering Construction (AEC) industry to recognise that value on the long term, as opposed to initial price, should be the focus of project financial assessments (Higham et al., 2015). LCC can be seen as a suitable management method to assess costs and available resources for housing projects, regardless of whether they are new or already exist. LCC looks beyond initial capital investment as it takes future operating and maintenance costs into account (Goh & Sun, 2016). Operating an asset over a 30-year lifespan could cost up to four times as much as the initial design and construction costs (Zanni et al., 2019). The costs associated with energy consumption often represent a large proportion of a building’s life cycle costs. For instance, the cumulative value of utility bills is almost half of the cost of a total building life cycle over a 50-year period in some countries (Ahmad & Thaheem, 2018; Inchauste et al., 2018). Prioritising initial cost reduction when selecting a design alternative, regardless of future costs, may not lead to an economically efficient building in the long run (Rad et al., 2021). LCC is a valuable appraising technique for an existing building to predict and assess "whether a project meets the client's performance requirements" (ISO, 2008). Similarly, during the design stages, LCC analysis can be applied to predict the long-term cost performance of a new building or a refurbishing project (Islam et al., 2015; RICS, 2016). Conducting LCC supports the decision-making in the design development stages has a number of benefits (Kubba, 2010). Decisions on building programme requirements, specifications, and systems can affect up to 80% of its environmental performance and operating costs (Bogenstätter, 2000; Goh & Sun, 2016). The absence of comprehensive information about the building's operational performance may result in uninformed decision-making that impacts its life cycle costs and future performance (Alsaadani & Bleil De Souza, 2018; Zanni et al., 2019). LCC can improve the selection of materials in order to reduce negative environmental impact and positively contribute to resourcing efficiency (Rad et al., 2021; Wouterszoon Jansen et al., 2020), in particular when combined with Life Cycle Assessment (LCA). LCA is concerned with the environmental aspects and impacts and the use of resources throughout a product's life cycle (ISO, 2006). Together, LCC and LCA contribute to adopt more comprehensive decisions to promote the sustainability of buildings (Kim, 2014). Therefore, both are part of the requirements of some green building certificates, such as LEED (Hajare & Elwakil, 2020).     LCC can be used to compare design, material, and/or equipment alternatives to find economically compelling solutions that respond to building performance goals, such as maximising human comfort and enhancing energy efficiency (Karatas & El-Rayes, 2014; Rad et al., 2021). Such solutions may have high initial costs but would decrease recurring future cost obligations by selecting the alternative that maximises net savings (Atmaca, 2016; Kubba, 2010; Zanni et al., 2019). LCC is particularly relevant for decisions on energy efficiency measures investments for both new buildings and building retrofitting. Such investments have been argued to be a dominant factor in lowering a building's life cycle cost (Fantozzi et al., 2019; Kazem et al., 2021). The financial effectiveness of such measures on decreasing energy-related operating costs, can be investigated using LCC analysis to compare air-condition systems, glazing options, etc. (Aktacir et al., 2006; Rad et al., 2021). Thus, LCC can be seen as a risk mitigation strategy for owners and occupants to overcome challenges associated with increasing energy prices (Kubba, 2010). The price of investing in energy-efficient measures increase over time. Therefore, LCC has the potential to significantly contribute to tackling housing affordability issues by not only making design decisions based on the building's initial costs but also its impact on future costs – for example energy bills - that will be paid by occupants (Cambier et al., 2021). The input data for a LCC analysis are useful for stakeholders involved in procurement and tendering processes as well as the long-term management of built assets (Korpi & Ala-Risku, 2008). Depending on the LCC scope, these data are extracted from information on installation, operating and maintenance costs and schedules as well as the life cycle performance and the quantity of materials, components and systems, (Goh & Sun, 2016) These information is then translated into cost data along with each element life expectancy in a typical life cycle cost plan (ISO, 2008). Such a process assists the procurement decisions whether for buildings, materials, or systems and/or hiring contractors and labour, in addition to supporting future decisions when needed (RICS, 2016). All this information can be organised using Building Information Modelling (BIM) technology (Kim, 2014; RICS, 2016). BIM is used to organise and structure building information in a digital model. In some countries, it has become mandatory that any procured project by a public sector be delivered in a BIM model to make informed decisions about that project (Government, 2012). Thus, conducting LCC aligns with the adoption purposes of BIM to facilitate the communication and  transfer of building information and data among various stakeholders (Juan & Hsing, 2017; Marzouk et al., 2018). However, conducting LCC is still challenging and not widely adopted in practice. The reliability and various formats of building related-data are some of the main barriers hindering the adoption of LCCs (Goh & Sun, 2016; Islam et al., 2015; Kehily & Underwood, 2017; Zanni et al., 2019).

Industrialised Construction, also referred to as Modern Methods of Construction in the UK (Ministry of Housing, 2019) and Conceptueel Bouwen (Conceptual Building) in the Netherlands (NCB, n.d.), is a broad and dynamic term encompassing innovative techniques and processes that are transforming the construction industry (Lessing, 2006; Smith & Quale, 2017). It is a product-based approach that reinforces continuous improvement, rather than a project-based one, and emphasises the use of standardised components and systems to improve build quality and achieve sustainability goals (Kieran & Timberlake, 2004).  Industrialised Construction can be based on using a kit-of parts and is often likened to a LEGO set, as well as the automotive industry's assembly line and lean production. Industrialisation in the construction sector presents a paradigm shift, driven by advancements in technology (Bock & Linner, 2015). It involves both off-site and on-site processes, with a significant portion occurring in factory-controlled conditions (Andersson & Lessing, 2017). Off-site construction entails the prefabrication of building components manufactured using a combination of two-dimensional (2D), three-dimensional (3D), and hybrid methods, where traditional construction techniques meet cutting-edge technologies such as robotic automation. Industrialised construction is not limited to off-site production, it also encompasses on-site production, including the emerging use of 3D printing or the deployment of temporary or mobile factories. Industrialised Construction increasingly leverages digital and industry 4.0 technologies, such as Building Information Modelling (BIM), Internet of Things, big data, and predictive analysis (Qi et al., 2021). These processes and digital tools enable accurate planning, simulation, and optimisation of construction processes, resulting in enhanced productivity, quality, and resource management. It is important to stress that Industrialised Construction is not only about the physical construction methods, but also the intangible processes involved in the design and delivery of buildings. Industrialised construction offers several benefits across economic, social, and environmental dimensions. From an economic perspective, it reduces construction time and costs in comparison to traditional methods, while providing safer working conditions and eliminates delays due to adverse weather. By employing standardisation and efficient manufacturing processes, it enables affordable and social housing projects to be delivered in a shorter timeframe through economies of scale (Frandsen, 2017). On the social front, Industrialised Construction can enable mass customisation and customer-centric approaches, to provide more flexible solutions while maintaining economic feasibility (Piller, 2004). From an environmental standpoint, industrialised construction minimises waste generation during production by optimising material usage and facilitates the incorporation of Design for Disassembly (Crowther, 2005) and the potential reusability of building elements, promoting both flexibility and a Circular Economy (EC, 2020). This capability aligns with the principles of cradle-to-cradle design, wherein materials and components are continuously repurposed to reduce resource depletion and waste accumulation. Challenges remain in terms of overcoming misconceptions and gaining social acceptance, the slow digital transformation of the construction industry, high factory set-up costs, the lack of interdisciplinary integration of stakeholders from the initial stages, and adapting to unconventional workflows. However, Industrialised Construction will undoubtedly shape the future of the built environment, providing solutions for the increasing demand for sustainable and affordable housing (Bertram et al., 2019).

Open Building is a term that was coined in the mid-1980s but is rooted in ideas from some twenty years earlier, when John Habraken first introduced the Support/Infill concept as a response to the rigidity and uniformity of the post-war mass-housing produced in the Netherlands (Habraken, 1961). Its fundamental principle involves separating the supporting structure of a building, considered a collective resource designed for durability, from the infill components, such as the walls and partitions that can be easily adapted to individual preferences and changing needs. This design approach places a strong emphasis on flexibility and adaptability, allowing buildings to evolve over time and be effortlessly modified or renovated to meet changing requirements. Furthermore, it encourages the participation of building occupants in the design and management of their homes, and it emphasizes the importance of creating buildings that are well-suited to their local context (Kendall, 2021). The Open Building concept introduces a holistic approach to enhancing the adaptability of the built environment, considering social, technical, and organizational aspects (Cuperus, 2001). From a social perspective, Open Building advocates for an open architecture that empowers users to customize their living spaces according to their needs and preferences, accommodating unforeseen changes in the future. On an organisational level, it proposes a redistribution of the design control, enabling top-down decisions to establish a framework within which bottom-up processes can thrive. Lastly, from a technical perspective, it pursues a systematisation of building that allows for the installation, upgrading, or removal of industrialized sub-systems with minimal implications for the overall stability of the building. This approach addresses some of the pressing challenges of the construction industry, offering the potential to enhance housing affordability and sustainability. By allowing greater flexibility in interior design and layouts, spaces can be easily reconfigured to meet changing needs, encouraging a shift towards long-term planning and fostering adaptable, future-ready living environments. Moreover, this strategy reduces the need for costly renovations and discourages demolitions, thus improving construction resilience and facilitating the seamless integration of new technologies. It successfully aligns the diverse objectives of multiple stakeholders, providing builders with a consistent support system, offering developers the freedom to experiment with layouts and ensure long-term functional performance, and granting users the possibility to make personalized choices. For decades, this inherent adaptability has been successfully applied in diverse building types, including shopping centres, office buildings, and hospitals. These buildings necessitate facilities that are 'change-ready', capable of accommodating changes over time, with a focus on long-term adaptability rather than short-term design adequacy (Kendall, 2017; Leupen, 2004). Open Building promotes environmental sustainability through its ‘levels concept’, acknowledging that building components have varying lifespans. The disentanglement and clarity of these hierarchical levels and their interfaces promotes the longevity of infrastructures while enabling incremental renewal and innovation, an increasingly common need in the construction sector. Higher levels provide a framework for the lower levels, setting the overall parameters and constraints in which the lower ones can operate (Habraken, 1998). Additionally, Open Building encourages the separation of building elements into the ‘Shearing layers of change’ articulated by Steward Brand in 1994 (Brand, 1994). These layers provide flexibility and adaptability to the buildings as they can be designed, built, and maintained independently from each other, facilitating design for disassembly practices. Additionally, through a modular coordination of standardised components, not only it is possible to increase the collaboration in the design and construction process of housing, but also to encourage a proliferation of technical subsystems that can be continuously upgraded and scaled-up within an open framework (Kendall & Dale, 2023b). In the housing realm, a key difference between traditional design and the Open Building approach is their underlying methods. Traditional design examines diverse household types and lifestyles from an anthropologic perspective, suggesting various typologies. In contrast, Open Building focuses on creating an open system with no predefined designs. Instead, it operates with a framework of rules, zones and categories to enable the customisation of each dwelling by the user (Habraken, 1976). The adoption of Open Building was a response to the rigidity and waste caused by continued adherence to functionalism where buildings were designed according to the “form-follows-function” principle and became obsolete or impractical for the coming generations and costly to maintain. On the other hand, open architecture can cater to local and cultural demands, embracing the complexity of the built environment by acknowledging that it cannot be fully controlled or shaped by a single agent (Kendall, 2013; Kendall & Dale, 2023a; Paulichen et al., 2019). This encourages community involvement in the design and construction process, creating a sense of ownership and fostering inclusivity. There are many examples across Europe of residential Open Building such as Gleis 21 in Austria, R50 Cohousing in Germany, or Stories in Netherlands. Other cases have been developed as open systems rather than individual projects, replicated and adapted to diverse locations but following the same strategy, as for example the Superlofts by Mark Koehler Architects, which since 2016 has built seven projects in the Netherlands out of this system. Determining whether a project is an Open Building and the degree of flexibility it offers can be measured through a classification chart developed by the Open Building Collective, which is based in the principles showcased in their Manifesto. The dissemination of these exemplary projects through publications (Schneider & Till, 2007), awards, conferences and the Open Building Collective, has stimulated the exchange of knowledge between researchers, practitioners and other stakeholders, spreading the interest in this concept and its practical implementation. Despite its potential benefits, the implementation of Open Building in multi-family housing faces challenges due to entrenched traditional practices, regulatory barriers favouring fixed layouts, and the short-term perspectives among developers, investors, and clients (De Paris & Lopes, 2018; Montaner et al., 2015). However, successful Open Building projects around the globe demonstrate that its capacity to address holistically the social, technical, and organizational aspects of a changing society. It encourages the space appropriation at the infill level while ensuring resilience and robustness in the support level, fostering enduring and inclusive buildings that allow diverse households to coexist and evolve over time (Kendall, 2022).

A precise and definitive definition of environmentally sustainable social housing remains elusive. Instead, it encompasses a bundle of interrelated terms such as low-impact buildings, sustainable buildings and environmentally responsible buildings, all of which are interwoven with the characteristics of social housing and its policy and development. This review examines the theoretical underpinnings of social housing and environmental sustainability at the EU level, outlines the challenges of integrating sustainability into housing and proposes an overarching definition of environmentally sustainable social housing. Social housing narratives Elsinga (2012) explains that social housing in the European Union is broadly described as a set of initiatives to provide high-quality and affordable housing for disadvantaged and middle-income groups, usually managed by public authorities (Elsinga, 2012). In the UK and the Netherlands, however, the management of social housing has largely been entrusted to non-profit organisations. This approach contrasts with that of Germany and Spain, where public subsidies are provided to commercial landlords in exchange for a fixed social rent and thus constitute a form of social housing. Granath Hansson and Lundgren (2019) further note that the historical development of social housing in the EU has involved a significant transfer of responsibility from local authorities to non-municipal providers, albeit under highly regulated practices such as the UK's managerialist approach (Granath Hansson & Lundgren, 2019). Priemus (2013) offers a definition that emphasises the regulatory framework and the role of the public sector in regulating social housing (Priemus, 2013). This definition identifies the target group as households unable to compete in the private housing market due to financial, physical or mental health problems or belonging to an ethnic minority or immigrant group. Bengtsson (2017), adopting a target group perspective, characterises social housing as a "system" designed to provide housing to resource-constrained households, with the requirement for their needs to be confirmed (Bengtsson, 2017). Although there is no universally accepted definition of social housing, it can be assumed that social housing functions as a system that supports households with limited financial resources by providing long-term accommodation. This system requires a mechanism to assess the needs of the target groups, ensuring that the housing is provided as a subsidy and not as a self-sustaining unit. Consequently, rents or prices within this system must be affordable and below market prices. Environmental sustainability narratives While there is no definitive definition of environmental sustainability specific to the EU in the literature, several scholars have contributed to understanding this concept from a global perspective and thus influenced its interpretation at the EU level. Notable contributions include those by Hey (2005), Portney (2015), Purvis et al. (2019) and Morelli (2011). Purvis et al. (2019) emphasise that environmental sustainability results from describing environmental protection goals and their interrelationships with broader concepts of the built environment. Environmental sustainability has evolved into a dynamic and multidisciplinary concept that is closely linked to concepts such as resilience, durability and renewability. Morelli (2011) states that environmental sustainability can be applied at different levels and encompasses tangible and intangible aspects (Morelli, 2011). Portney (2015) argues that environmental sustainability goals include conserving natural resources, improving people’s well-being, and promoting industrial efficiency without compromising societal development. The contemporary approach to implementing sustainability focuses on reducing the resource consumption of buildings (such as water and energy) and minimising waste production while improving the quality of the built environment. This approach goes beyond individual buildings and extends to the urban fabric of cities (Berardi, 2012; McLennan, 2004). The EU's approach to environmental sustainability is reflected in its directives, policies, initiatives and guidelines. An example of these initiatives is the European Green Deal (EC, 2019), which aims for a carbon-neutrality across Europe by 2050 while promoting sustainable economic growth (Fetting, 2020; Siddi, 2020). In addition, the EU emphasises the importance of integrating environmental concerns into various policy areas, including energy, transport, agriculture and industry. The EU Circular Economy Action Plan, for example, promotes an economy that minimises waste and supports sustainable consumption and production patterns (EC, 2020). Overall, the EU's approach to environmental sustainability emphasises the need for a comprehensive, integrated, and long-term perspective (Hermoso et al., 2022; Johansson, 2021). This approach considers the economic, social, and environmental dimensions of sustainability and emphasises the importance of international cooperation in addressing global environmental challenges (Fetting, 2020; Hermoso et al., 2022; Siddi, 2020). Integration imperatives and its challenges The realisation of environmentally sustainable social housing presents numerous challenges. The initial investment in sustainable building technologies and materials is often considerable, especially given the limited funds available for social housing projects. Compliance with ever-evolving environmental regulations further complicates the delivery of sustainable social housing. Consequently, there is an urgent need to adapt sustainable practices to different scales of social housing projects, which requires careful planning and adaptation to the specific needs and context of different developments (Oyebanji, 2014). Despite these challenges, the field of sustainable social housing offers significant opportunities for innovation and improvement. Technological progress continuously offers more efficient, cost-effective and sustainable solutions (IEA, 2022). In addition, robust policy frameworks and incentives are crucial for the adoption of sustainable practices (Fetting, 2020). Another crucial element is the active participation of different stakeholders in the design and maintenance of housing, which can significantly improve both sustainability and social cohesion (Shirazi & Keivani, 2019). The way forward Environmentally Sustainable social housing is becoming increasingly important as it represents both a possible future and an ambitious goal. It envisions an environmentally responsible housing sector without compromising its development capacity (Morgan & Talbot, 2001; Oyebanji, 2014; Winston, 2021). It aims to create housing that minimises its environmental footprint, promotes the well-being of its residents and provides affordable housing opportunities. It also aims to meet the housing needs of vulnerable and low-income groups while promoting sustainable development and addressing climate and environmental issues (Udomiaye et al., 2018).

  Product platforms are a set of standardised components, processes, and knowledge used to create a variety of products and services. Borrowed from the software and manufacturing industries, this concept supports rapid innovation and growth by leveraging shared elements to achieve economies of scale and production flexibility (Lessing, 2006).  A product platform is closely related to the concept of solution space, highlighted by many authors as being one of the fundamental capabilities to implement mass customisation strategies (Salvador et al., 2009). A solution space refers to the range of potential designs or configurations that can be generated within the constraints of a given product platform. It encompasses all the possible variations and customisations that can be achieved using the standardised components and processes defined by the platform (Piller, 2004). Therefore, the product platform provides the kit-of-parts, production processes and knowledge, while the solution space defines the extent to which those elements can be varied to meet specific needs and preferences. Product platforms are central to the development of customised and industrialised housing solutions. By sharing standardised components across various housing products, companies can achieve significant cost reductions while allowing for customisation to meet specific market demands. This balance enhances the ability to provide affordable and tailored housing without sacrificing quality or functionality. Other industries, such as automotive and electronics, have demonstrated the efficiency benefits of product platforms by streamlining production processes, reducing costs, and quickly adapting to market changes. Adopting a similar approach in housing can accelerate innovation and reduce overall costs in the construction sector. Product platforms provide a structure for predefined technical solutions, requiring thorough documentation and continuous improvements, and serving as a backbone for technical information and related processes in a company and its supply chain (Jansson et al., 2014). Robertson and Ulrich (1998) identified four elements that constitute a product platform: components, processes, knowledge, people and relationships. These platforms must integrate common elements and technologies across a range of products, considering manufacturing capabilities and constraints early in the process. This integration ensures that the product platform is not only flexible in the early definition of a housing solution but is also practical and efficient to produce. Flexibility is both key to the success of a product platform in housing and a challenge for scaling manufacturing. It is crucial to find the right balance between standardisation and customisation to meet customer demands efficiently. Therefore, it is vital to integrate customer focus in product-oriented house-building processes (Barlow et al., 2003) and to define the Customer Order Decoupling Point (CODP) in the production process – the point in which the product will be customised to meet specific needs. The CODP determines the production strategy of a product platform, which will consequently affect its inventory management, lead times, and overall supply chain strategy. The production strategy defines the boundaries and degrees of customisation within a product platform, classified into four levels: Made-to-Stock (MTS), Assembled-to-Stock (ATS), Made-to-Order (MTO) or Engineered-to-Order (ETO) (Barlow, 1998; Smith, 2019). Product platforms allow us to understand a building in a systematic way, as a group of components or smaller subsystems that can be designed independently yet function together as a whole. This approach enables continuous improvement of the platform, as insights from one project can drive more efficient use of components in subsequent projects, creating learning loops that enhance overall productivity and innovation. Additionally, a product platform developed with Design for Manufacture and Assembly (DfMA) and Design for Disassembly (DfD) principles can significantly contribute to a circular economy. Standardised components can be easily repurposed or reconfigured, reducing waste and promoting environmental sustainability. This flexibility ensures that buildings can adapt to changing needs over time, extending their lifespan and minimising the environmental impact of demolition and new construction. Finally, there are four principles that should be considered when developing product platforms for the delivery of housing: (1) Modularity:  Product modularity enables a manufacturer to absorb changes in customer needs by reconfiguring and adapting modules based on a set of parameters within a defined solution space.  (2) Automation: Integrating digital workflows to automate repetitive tasks such as manufacturing instructions, building reports or a bill of quantities would ease the development of a variety of housing solutions in an efficient way. (3) Platform rules: The rules and relationships between platform components would have to be properly defined to ensure that consistency in quality and performance are maintained even when designs are customised or scaled. (4) Parametric software tools: The success of a product platform relies on how data generated in the manufacturing and assembly phases is encapsulated within the components and incorporated into the early stages and project planning. Parametric software can facilitate the iteration of options without leaving the product platform’s solution space, optimising the design based on performance data, environmental parameters, or user feedback.