Housing is usually deemed unaffordable when it consumes more than a set percentage of a household's monthly income. The Eurostat (2022) and the OECD (Chung et al., 2018) follow this threshold approach and define households overburdened with housing costs as those that spend more than 40% of their disposable income on housing. However, this indicator fails to capture financial hardship, particularly among lower-income households. In fact, lower-income households may be spending less than 40% of their income on housing and yet failing to meet adequate consumption levels for other goods. As a response, the residual income approach ascertains housing (un)affordability by defining a minimum level of consumption for a set of goods according to particular household types. The residual income approach builds on consumption data to define the minimum level of income necessary for a household to survive after housing costs. The main shortcoming of this approach is that relies on subjective measures of what constitutes the necessary minimal expenses for a household. These two definitions of affordability navigate two tensions 1) between housing and other types of consumption and 2) between the individual conceptions of what is affordable and what the government considers to be affordable (Haffner & Hulse, 2021). More recently, scholars have emphasized the multi-faceted nature of affordability to include commuting and transport costs together with energy costs (Haffner & Boumeester, 2010). Other approaches focus on supply-side measures, for instance on the share of the housing stock that a household can afford (Chung et al., 2018). Evolutions in the measurement of affordability bear witness to the complexity of housing systems. Affordability is not only dependent on housing consumption but also on housing supply, particularly in inelastic markets where providers have considerable power, see for example Kunovac & Zilic (2021). At the same time, displacement pressures and rising energy costs in an older and inefficient stock add pressure on households to access affordable housing.

A universal definition of social housing is difficult, as it is a country-specific and locally contextualised topic (Braga & Palvarini, 2013). This review of the concept focuses on social housing in the context of the UK from the late 1980s, which Malpass (2005) refers to as the phase of ‘restructuring the housing and welfare state’, to the early 2000s, known as the phase of the ‘new organisation of social housing’. In response to previous demands for housing, such as those arising during the Industrial Revolution, and recognising the persistent need to address the substandard quality of housing provided by private landlords in the UK (Scanlon et al., 2015), the primary objective of social housing has historically been to enhance the overall health conditions of workers and low-income populations (Malpass, 2014; Scanlon et al., 2015). However, this philanthropic approach to social housing changed after the Second World War when it became a key instrument to address the housing demand crisis. Private initiatives, housing associations, cooperatives and local governments then became responsible for providing social housing (Carswell, 2012; Scanlon et al., 2015). Social housing in the UK can be viewed from two perspectives: the legal and the academic (Granath Hansson & Lundgren, 2019). Along these two perspectives, social housing is often analysed based on four main criteria: the legal status of the landlord or provider, the tenancy system or tenure, the funding mechanism or subsidies, and the target group or beneficiaries (Braga & Palvarini, 2013; Carswell, 2012; Granath Hansson & Lundgren, 2019). From a legal perspective, social housing maintained its original goals of affordability and accessibility during the restructuring period in the late 1980s. However, citing the economic crisis, the responsibility for developing social housing shifted from local authorities to non-municipal providers with highly regulated practices aligned with the managerialist approach of the welfare state (Granath Hansson & Lundgren, 2019; Malpass, 2005; Malpass & Victory, 2010). Despite the several housing policy reviews and government changes, current definitions of social housing have maintained the same approach as during the restructuring period. Section 68 of the Housing and Regeneration Act 2008, updated in 2017, defines social housing as low-cost accommodation provided to people whose rental or ownership needs are not met by the commercial market (HoC, 2008; 2017, pp. 50-51). The Regulator of Social Housing, formerly the Homes and Communities Agency, has adopted the earlier definition of social housing and clarified which organisations provide it across the UK. These organisations include local authorities, not-for-profit housing associations, cooperatives, and for-profit organisations (RSH, 2021). In contrast, the National Housing Federation emphasises the affordability of social housing regardless of the type of tenure or provider (NHF, 2021). From an academic perspective, Malpass (2005) explains that during the restructuring phase, social housing was defined as a welfare-supported service – although it did have limitations, which meant that funding principles shifted from general subsidy to means-tested support for housing costs only, which later formed the basis for the Right to Buy Act introduced by the Thatcher government in the early 1980s (Malpass, 2005, 2008). The restructuring phase, however, came as a response to the housing 'bifurcation' process that began in the mid-1970s and accelerated sharply from the 1980s to 1990s (Kleinman et al., 1998; Malpass, 2005). During this phase, the role of social housing in the housing system was predominantly residual, with greater emphasis placed on market-based solutions, and social housing ownership concerned both local authorities and housing associations (Malpass & Victory, 2010). This mix has influenced the perception of social housing in the 'new organisation' phase as a framework that regulates public housing intervention for specific groups and focuses on enabling non-municipal providers (Malpass, 2005, 2008; Malpass & Victory, 2010). Currently, as Carswell (2012) explains, social housing plays an important role in nurturing a variety of initiatives aimed at providing ‘good-quality’ and ‘affordable’ housing for vulnerable and low-income groups (Carswell, 2012). Oyebanji (2014) sees social housing as any form of government-regulated housing provided by public institutions, including non-profit organisations (Oyebanji, 2014). Additionally, Bengtsson (2017) describes social housing as a system that aims to provide households with limited means, but only after their need has been confirmed through testing (Bengtsson, B, 2017 as cited in Granath Hansson & Lundgren, 2019). To a great extent, social housing in the UK can be seen as a service system that is intricately linked to the welfare state and influenced by political, economic, and social components. Despite being somehow determined by common factors and actors,  the relationship between social housing and the welfare state can sometimes be complex and subject to fluctuations (Malpass, 2008). In this context, the government plays a vital role in shaping and implementing the mechanisms and practices of social housing. While the pre-restructuring phase focused on meeting the needs of the people by increasing subsidies and introducing the right to buy (Stamsø, 2010), the aim of the restructuring phase was to meet the needs of the market by promoting economic growth (privatisation, market-oriented policies and reducing the role of local authorities) (Stamsø, 2010; Malpass, 2005) . The new organisational phase, on the other hand, works to meet and balance the needs of all, with people, politics and the economy becoming more intertwined. Welfare reform legislation passed in 2010 aims to enable people to meet their needs, but through 'responsible' subsidies, leading to a new policy stance that has been described as 'neoliberal' thinking (Hickman et al., 2018). However, there are still no strict legal requirements for the organisation and development of social housing as an independent service system, and most of the barriers to development are closely related to the political orientation of the government, rapid changes in housing policy and challenges arising from providers' perceptions of existing housing policy structures (Stasiak et al., 2021).

Measuring housing affordability refers to assessing the extent to which households can secure suitable housing in relation to their financial resources and other relevant factors. To date there is no global agreement on measuring housing affordability, nor is there a single metric which comprehensively encompasses all the considerations regarding households' ability to access suitable housing in a convenient location at an affordable cost (Ezennia & Hoskara, 2019; OECD, 2021b). Several approaches exist to measure housing affordability, with two popular approaches, namely the Income Ratio Method (IRM), and the Residual Income Method (RIM) (Ezennia & Hoskara, 2019; Stone et al., 2011). Both are recommended to be accompanied by housing quality standards to evaluate what a household is paying for and a measure of housing satisfaction (Haffner & Heylen, 2011; OECD, 2021b). However, the perception of what constitutes satisfactory, quality, or affordable housing is subjective. This perception can be influenced by economic and social circumstances that policymakers may not perceive as directly relevant to housing policy (OECD, 2021b). The Income Ratio Method (IRM) is the most commonly used in policy and housing market-relevant statistics, as it is easy to measure and compare among different countries. It is based on the housing costs to income ratio defined by national authorities not to exceed a certain proportion (Haidar & Bahammam, 2021; Smith, 2007; Stone, 2006). The official EU indicator for IRM is the "Housing Cost Overburden" index. It considers households suffering from affordability issues if more than 40% of their net income is spent on housing costs (AHC, 2019; Hick et al., 2022; OECD, 2020). However, IRM has been widely criticised as it does not reflect if the household could afford non-housing costs and for how long. The focus on housing costs neglects non-housing costs of utility bills, schools, health, transportation, and so on (AHC, 2019). In this sense, Ezennia & Hoskara, (2019) investigation of the weaknesses of measuring housing affordability emphasised the need to reflect a household's capability to balance current and future costs to attain a house – "access to a house at a certain period" while maintaining other basic expenses without experiencing any financial hardship. The Residual Income Method (RIM) is the second dominant approach. It recognises that after paying the housing costs, a household might be unable to satisfy its non-housing requirements. Thus, the RIM is the remaining income after subtracting housing costs, based on the idea that Housing Affordability is the households' ability to cover their housing costs while still being able to pay their non-housing expenditures (Stone et al., 2011; Stone, 2006). The residual income method took a step closer to resonating with non-housing costs. However, both Haffner & Heylen (2011) and Bramley (2012) advised that the IRM and RIM approaches "are not interchangeable" and need to be combined to provide a comprehensive perception of housing affordability. This combination becomes apparent when comparing both for different household compositions, health, or work conditions. For instance, a house might be affordable when measured using the IRM from the housing costs standpoint, but it might not be affordable utilising the RIM, which is connected with non-housing costs. This combination is referred to as the Composite Method from which several advanced economic modelling approaches to measure housing affordability were developed (Ezennia & Hoskara, 2019). However, relying solely on economic criteria to assess affordability and thus overlooking quality and sustainability may not prove sufficient. A poor-quality house can impose hardships on its residents, and an unsustainable dwelling can strain the environment. Mitigating this issue may involve complementing affordability measurements with indicators reflecting housing quality and sustainability to expand the purely economic view (Ezennia & Hoskara, 2019; Haffner & Heylen, 2011; Mulliner et al., 2013; Salama, 2011). Various indicators can be used to assess housing quality beyond just its cost. These indicators could be seen as serving three primary purposes: (1) to measure the quality of a housing scheme and compare it to others within a country (Homes and Communities Agency, 2011), (2) to measure the quality of housing in one country and compare it to other countries (OECD, 2021b), and (3) to measure housing satisfaction across groups (OECD, 2021a; Riazi & Emami, 2018). An example of the first purpose is England's Housing Quality Indicators (HQIs) system (Homes and Communities Agency, 2011), which is currently withdrawn. HQIs served  as “ measurement and assessment tool to evaluate housing schemes on the basis of quality rather than just cost” design standards mandated for affordable housing providers funded through the National Affordable Housing Programme from 2008 to 2011 and the Affordable Homes Programme from 2011 to 2015. The system comprised ten indicators, which can be categorized into four groups. The first category focused on the location and proximity to amenities and services. The second dealt with site-related aspects such as landscaping, open spaces, and pathways. The third pertained to the housing unit itself, encompassing factors like noise, lighting, accessibility, and sustainability. Lastly, the fourth category addressed the external environment (Homes and Communities Agency, 2011). To enable meaningful cross-country comparisons, it is crucial that the data used for measuring and assessing these indicators are both available and up-to-date. However, it is important to acknowledge that this may not be the case in all countries, as pointed out by the OECD in 2021 (OECD, 2021b). Consequently, to accurately determine what residents are paying for in terms of quality and to facilitate meaningful comparisons, the OECD 2021 Policy Brief on Affordable Housing has emphasized the necessity of two additional housing quality indicators to complement affordability measurements. The first proposed indicator is the "Overcrowding Rate," which evaluates whether a dwelling provides sufficient space for household members based on their composition. This metric assesses whether residents have adequate living space according to the size and structure of their household. The second indicator is the "Housing Deprivation Rates," which gauge inadequate housing conditions. This encompasses issues related to maintenance, such as roofs, walls, floors, foundations, and deteriorating window frames. Moreover, these rates consider the absence of essential amenities, including sanitary facilities. By taking all these factors into account, this indicator offers a comprehensive perspective on the overall quality and habitability of housing in a specific area. Considering subjective measures of housing affordability can be advantageous when assessing housing affordability and quality based on household perceptions. These measures aim to capture housing satisfaction, reflecting the quality of the dwelling as accommodation (OECD, 2021a). In a broader context, housing satisfaction might be termed residential satisfaction, encompassing not just the dwelling but also its surroundings, including places and people. Residential satisfaction assesses how well the current residence and surrounding environment align with the household's desired living conditions (Riazi & Emami, 2018). Therefore, incorporating subjective measures is valuable in assessing housing affordability, helping to identify the determinants of housing satisfaction. Indicators such as satisfaction with the availability of good and affordable housing are crucial aspects to consider in this context (OECD, 2021a). When it comes to sustainability indicators, incorporating them into the measurement of housing affordability remains a wicked  problem. Finding a single comprehensive measure that encompasses the multifaceted aspects of sustainability related to housing affordability is challenging. The technical complexity stems from the necessity to integrate assessments of household characteristics, environmental impacts, financing, and financial aspects, along with housing stress factors. This challenge is exacerbated by the persistent fluctuations in housing prices and recurring expenses like water and energy bills (AHC, 2019). Hence, easily calculable methods such as the Income-to-Rent Ratio (IRM) and Residual Income Model (RIM) continue to be widely used for assessing housing affordability from a top-down perspective at a macro level. Although imperfect, these methods still provide valuable support for policy decision-making to a certain extent (AHC, 2019; Haffner & Heylen, 2011; OECD, 2021a).