Lit Review · Gemini 3.5 Flash

INTRODUCTION In the current global urbanization trend, satisfying the basic need for housing has transformed into one of the major problems in the fields of sociology and economics for the 21st century. The shortage of affordable urban housing have consequences on low-income households, but the situation is rapidly getting worse for the middle class as well. Favorable conditions for economic growth tend to make real estate prices skyrocket in cities. If wages don’t increase at the same rate, the gap between the cost of housing and the income of a typical city resident widens. This situation jeopardizes urban sustainability and deepens the divisions in society and increases the general distress in urban residents. Examining the situation of housing affordability is important for policymakers, urban planners, and sociologists. The study of urban housing affordability has expanded with new definitions, methods, and schools of thought. In the past, affordability was determined with ratio indicators like the “30 percent of income” rule. Recently, scholars have critique the use of static metrics. The literature has moved towards more complex approaches. For example, the residual income framework considers other spending, and the literature has incorporated the cost of transport to assess whether housing is affordable given the cost to travel to other parts of the city. Scholars have come to understand that housing affordability is a combination of many elements including the economy and infrastructure of an area, and the housing needs of the population it serves. Researchers are heavily divided on the structural dimensions of this crisis and the dispute persists on the most appropriate interventions. One major strand of the literature explains the affordability gap on the supply side, and focuses on how various restrictions on zoning and land use, as well as natural impediments, limit the supply of housing and drive up prices. Another major strand of the literature focuses on the demand side and explains the affordable housing crisis by the financialization of housing, in which the global capital markets treat housing as an investment rather than serving its requisite function as a good for the community. Finding a balance between market forces and state intervention is at the heart of the current housing policy debate. This literature review brings together the different strands of research on urban housing affordability. It starts by looking at how the concept and assessment of affordability has evolved, and then considers the debate on the supply side and the financialization of housing. It then looks at the effects of different policies, including zoning policies and rent control, to alleviate housing affordability. It identifies the main shortcomings of the existing research, and suggests areas where research is most needed, given the unstable and tumultuous nature of the global housing market.

Introduction In the age of rapid global urbanisation, housing has gone from being a basic human need to one of our greatest socio-economic challenges in the 21st century. The affordability of urban housing has become a pressing crisis not just for poor households but also the metropolitan middle class in cities worldwide. Cities spur economic growth, and with it fierce competition for space that send real estate values surging much faster than the average wage increase. This growing gap between the costs of shelter and household incomes has far-reaching consequences, challenging urban sustainability, worsening socio-spatial segregation or gentrification processes and lowering the overall quality of life for city inhabitants. Therefore, grasping the dynamics behind how housing is affordable is essential for developers, city planners and sociologists—policymakers alike. Urban housing affordability is a topic that has attracted considerable academic discourse since the mid-20th century, including defining what constitutes "affordability," approaching it from alternative methodological and epistemological perspectives (including quantitative vs qualitative) and ideological frameworks. In the past, affordability was examined through very rudimentary ratio indicators — think of the common 30 percent of income rule. But the static indicators are no longer suited to a modern urban society, contemporary scholars argue. Recent literature increasingly endorses more comprehensive frameworks such as the residual income framework, which includes non-housing need expenditures and transport costs to better assess true spatial affordability. Such evolving paradigms reflect an increasing acknowledgment that housing affordability is not alone a financial equation but also a multidimensional issue inextricably linked to place, the adequacy of infrastructure and broader socio-economic vulnerability. In addition, researchers are also sharply divided into those who identify the roots of this crisis as structural and those whose evidence-base leads them to more clinical solutions. The most popular body of literature says the supply side is to blame — that excessive zoning laws, land-use restrictions, and geographic obstacles are limiting housing from being built and raising prices. On the other hand, an opposite but no less important school rejects demand-side forces and instead highlights financialization of housing where (global) capital markets perceive owners of residential real estate as a formal investment asset rather than responsible citizens. The crucible of a modern housing policy debate lies in this tension between market-led solutions and state-interventionist policies. This literature review uses a systematic approach to bring together the diverse research available on urban housing affordability. The review starts with the emerging definitions of affordability and approaches for its measurement, before moving onto to give an account of the contentious divide between supply-side constraints versus financialization debates. In conclusion, it reviews the effects of any policy measures—such as inclusionary zoning or even rent control—to help guide future research toward areas in need to fill gaps identified by this study within an increasingly turbulent international housing market.

**Introduction** In today's rapidly urbanizing globe, housing is no longer just a basic human need, but one of the most serious socio-economic problems of the 21st century. Urban housing affordability is becoming a critical problem, which creates difficult conditions not only for people from low-income households, but for the middle class as well. Urban areas become engines of economic growth, but they also bring about fierce competition for land, which leads to increasing real estate prices, which develop faster than average growth in wages. Such a growing gap between housing costs and household incomes have serious consequences for urban sustainability, socio-spatial segregation, and poor quality of life of urban dwellers. Thus, it becomes crucial to understand the factors of housing affordability for policymakers, urban planners, and sociologists. The academic literature on urban housing affordability is very rich and diverse, with different definitions and research approaches. In the past, housing affordability was estimated through simple ratio indicators, which gave birth to the so-called “30 percent rule,” used widely by many researchers. However, currently, academics hold the opinion that these old measures of finance fail to explain the real complexities of modern urban life. Today's scholars increasingly see the reasons for the affordability problem in the absence of a comprehensive approach such as "residual income," which considers not just housing expenses but also other nonhousing essentials. Moreover, a modern approach should include transportation expenses as well. In addition to the above-mentioned contradictions about the ways to measure affordability, the scholars are quite divided with regard to the structural causes of the problem. The first approach emphasizes the supply-side problems and considers opposition such as zoning laws and land use regulations as the main factors for the non-affordability problem. The second position criticizes the demand-side problems such as making “real estate” an investment issue. The current research aims at providing a comprehensive reading on existing literature and identifying the opportunities for future studies in housing affordability. The first two parts of research will analyze the definition of housing affordability to prove that it is becoming an increasingly pressing issue today, and will be followed by an analysis of existing policy interventions, such as inclusionary zoning, to provide an overview of ongoing debates.

**Introduction** Urban housing has become a major socio-economic challenge of the 21st century as the need for housing has become elevated by the rapid process of global urbanization. As the economic powerhouse of the 21st century, cities are becoming increasingly unaffordable for both low-income households and the growing number of middle-income households in metropolitan areas around the world. Housing has become a commodity that is greatly sought after by global financial investors who see huge returns on investment as they drive up the cost of space in cities, far outstripping increases in average wages. The resulting discrepancy between the cost of housing and the incomes of householders has become a major challenge to urban sustainability and increases inequality and causes further segregation in cities around the world. Therefore, the issue of affordability of housing in cities has become a major challenge for those involved in the planning and management of cities around the world and for academics who study cities. There is a vast amount of literature on urban housing affordability, with scholars defining the topic, analyzing the issues and the ways to measure it using different methodologies and adopting a variety of perspectives. Traditionally, the issue of affordability has been dealt with by using simple ratios to measure the extent to which unaffordability affects different households. The 30 percent of income rule, which is perhaps the most popular ratio used, has been widely criticized recently. Affordability, as currently analyzed by most of the recent studies, can be dealt with using more holistic frameworks to measure housing affordability as ‘residual income’ to cover other non-housing essential expenditure, and even to include the cost of transport to make a proper assessment of affordability in relation to location. The way in which different researchers perceive housing affordability shows that the issue is far from being merely economic. This conflict exists, for example, on whether the main cause for the negative affordability deficit is to be found on the supply-side, for example within strict zoning rules, complex land use regulations, or geographical constraints, as is argued by for example Urban Land Institute (2018), or whether housing affordability has more to do with demand-side factors, i.e. how global capital markets and other financial actors treat residential real estate as an attractive investment object (ʽhousing as assetsʼ), an issue discussed by the financial analyst Journal The Finanser (2018). The way housing policies respond to this urban challenge is debated intensively. This literature review will explore existing studies on urban housing affordability in order to address several key points: first, an overview of affordability concepts and methodologies will be presented; subsequently, supply constraints on the housing market and the phenomenon of financialization of housing will be compared and contrasted. Finally, in order to assess their effectiveness and identify any research gaps, several current and widely used housing market interventions—such as inclusionary zoning and rent control—will be examined and their implications for policy and future research outlined within the ever-changing context of global housing markets.

**Introduction** In today's era of urbanization across the globe, housing has ceased to be one of the basic necessities of life and evolved into one of the main socio-economic issues of the 21st century. Urban housing affordability became a crisis not just for low-income groups but has become a challenge facing middle-class people in metropolitan cities all over the world. The increasing costs of housing as a result of rapid urban growth are pushing the prices for real estate much higher than the rate of wages growth. This growing gap has significant consequences and affects not only the sustainable development of cities but also contributes to social segregation and the deterioration of the quality of life in urban environment. Therefore, it is necessary to comprehend the mechanisms of housing affordability. There is a great amount of literature concerning urban housing affordability that is characterized by constantly changing definitions and methodologies used in researching, as well as ideological approaches. Traditionally, affordability was estimated through simple ratios – such as the famous "30 percent of income rule". Nowadays, the contemporary academic literature calls into question these methods arguing that these metrics are unable to represent the modern realities of urban housing market. Contemporary literature promotes more holistic approach such as the use of the "residual income approach" taking into account other expenses for basics and inclusion of transport costs in order to estimate spatial housing affordability. Moreover, academics are sharply divided about the reasons of housing affordability crisis and about the effectiveness of possible interventions. One group of studies claims that affordability problem is due to supply-side problems, namely zoning laws, geographic limitations and restrictions, which make housing more expensive. At the same time, another important school of thought suggests that affordability crisis should be analyzed from the perspective of the financialization of housing and global capital markets treating housing as investment. This discussion makes a background of modern housing policy debates. This literature review is a brief synthesis of the broad range of the contemporary scientific publications analyzing the topic of urban housing affordability. First of all, the review will examine the evolution of concepts and measures of housing affordability. Next, it will analyze the debate about the causes of housing affordability crisis in terms of supply-side problems and financialization. Lastly, it will discuss the results of policy interventions and identify gaps in the current literature.

Housing was once thought of as a simple need, but now it is recognized as one of the greatest challenges of the twenty-first century. Along with the rapid urbanization happening globally, cities around the world are facing an affordability crisis in which even middle-class populations are being priced out of the housing market. Urban areas are sources of economic prosperity, but the competition for space within these regions causes an increase in property values and disproportionately high housing prices compared to wages. Rapidly increasing real estate prices in relation to wage growth has created serious concerns for cities because it endangers the long-term viability of our cities, promotes socio-spatial segregation, and decreases quality of life for city-dwellers. It is extremely important for us to understand the current issues regarding the affordability of housing in order to help lower-income families and vulnerable populations. Housing affordability literature is extensive and ever-changing. The ways in which affordability is defined, calculated, and debated are hot topics among researchers. There has been recent push back against the use of ratios as indicator variables for affordability. For many years the 30 percent of income rule has been used to gauge affordability. Although this logic still permeates a lot of popular conversations on housing, researchers have been moving away from this ratio as a sufficient measure of affordability. There has been a shift to use “residual income” as a way to measure affordability while taking into consideration other necessary expenses a person may have. Researchers also began to look at housing and transport expenses together to better understand spatial affordability. In addition to changing ideas about how to measure affordability, there is also extensive research concerning the factors that cause housing to be unaffordable for so many. Some research has found that there is a lack of housing supply due to strict zoning laws, land use regulations, and natural geographic barriers. Other research points to the ways in which housing has been “financialized” as a commodity to be invested in rather than a public good. This research reviews the current literature on housing affordability. The review starts by discussing how affordability is defined and calculated, then goes into the arguments over housing supply and financialization, and lastly reviews the effectiveness of policies that have been tried to address the issue of affordability.

Housing is a basic human necessity, however in recent years there has been a dramatic rise in housing costs and affordability challenges are becoming widespread in metropolitan areas worldwide. As cities expand, so too does competition for land and space within cities driving up property values at a rate that often outpaces income gains. The growing gap between household incomes and the high costs of living has wide-ranging implications. These challenges not only reduce quality of life, they also exacerbate socio-spatial segregation and threaten the sustainability of cities, and affordability concerns that initially applied to the poor are increasingly affecting the middle classes. Measuring the affordability of housing is not necessarily a straightforward process. This has been an evolving concern, and is an area of debate among academic scholars. The standard approach, of defining affordability as "30 percent of income" has been critiqued as outdated and overly simplistic. Contemporary scholars advocate a multidimensional approach to defining and measuring affordability and the effects on quality of life. This literature review examines the contemporary scholarship on housing affordability. This includes definitions and methods of measuring affordability, factors that contribute to housing affordability challenges, and strategies to address these challenges. The paper is organized in three parts. First, different approaches to measuring and conceptualizing housing affordability are reviewed. In this part we will see how this literature has moved away from a simplified cost burden to a more "residual income" approach that takes into account other basic expenditure needs. It will also review research that incorporates commuting costs to measure housing affordability. The next part will review literature that discusses factors that contribute to the housing affordability challenge. Here I will show that there is a divide in the literature between supply-side and demand-side arguments. Supply side approaches often advocate for the deregulation of land use and zoning regulations and point to geographic constraints as the main issues driving prices and affordability challenges, as they decrease the supply of housing. Contrarily, demand-side approaches suggest that housing is being increasingly treated as an investment good to be bought and sold on the global market, obscuring it's value as a social good. The final part will look at how scholars are attempting to reconcile these approaches to understand housing affordability challenges. In particular, this review will examine literature that looks at the effectiveness of market-led and state intervention policies to combat housing affordability challenges, such as rent control and inclusionary zoning.

Introduction At present, under the wave of global urbanization, housing has gradually changed from a human basic requirement to the "most painful socio-economic problem" of the 21st century, which becomes another aspect of major crisis. The Urban Housing Affordability is gradually becoming serious for the low-income people and it is engulfing the middle class in most of the world cities. On the one hand, the city, the growth point, has brought intense competition for land, making housing values much higher than wages; on the other hand, the disparity between housing costs and income has a devastating impact on sustainable urban development, aggravating socio-spatial segregation and increasing the living stress of urban residents. Therefore, understanding urban housing affordability is vital for urban planners, policymakers and sociologists. The academic discussion on urban housing affordability is enormous, with debates ranging from definition, index to ideology. Early studies were represented by the use of simple statistical ratios to measure affordable cost, such as the frequently used housing expenditure accounting for 30 percent of disposable income. However, for those scholars living in modern cities, this is insufficient. At present, a lot of modern studies advocate the use of more comprehensive indicators of urban housing, such as residual income measurement which involves accounting for a portion of essential expenses, or comprehensive spatial affordability measurement by taking the transport costs into consideration. These new definitions also Reflect that urban housing affordability has changed not merely from financial problem to spatial and social problem. In addition,scholars are divided over the structural factors and their solutions for housing affordability problems, with one stream stressing the impact of supply-side constraints such as strict zonning regulations, land use control, or the nature of cities. While others emphasize the influence of demand side impacts on the financialization of housing from globalthat property markets act more like an investment that a social provision. These market vs states conflict shapes today's housing policy debates. This literature reviews on urban housing affordability will introduce various perspectives of it ranging from the definitional evolution and metric to supply-side constraints and the financialization of housing, then it will review outcomes of housing affordability policies. And identify what are missing in current study as well as propose potential research directions in a highly volatile global housing market.

This study examines a complex phenomenon through the lenses of academic discourse; and, aims to critically analyze the current literature on the issues and mechanisms through which housing becomes a crisis, to analyze the most current approaches to conceptualizing, measuring and operationalizing the concept, to debate the primary causes of the housing affordability crisis, supply-side versus financialization of housing, to review the effects of a range of policy responses to affordability, and to pinpoint key issues, gaps and future research directions in an evolving global context.

**Introduction** Today, with the fast-paced pace of global urbanization, housing is no longer a basic human right, but one of the most urgent socio-economic issues of the 21st century. Housing affordability is now a critical issue, not just for low-income families, but for middle-class Americans in urban areas, and for the middle class in most cities across the nation. Cities have to contend with intense competition for space, as well as rapidly rising real estate values, factors that outpace the growth of average wages. This gap between the cost of housing and household incomes has far reaching consequences, which affect the sustainability of urban life, increase socio-spatial segregation and reduce the quality of life of residents. Thus, it is essential that policymakers, urban planners, and sociologists have a good understanding of the dynamics of housing affordability. There is a great deal of academic literature on the issue of housing affordability in the city, with changing meanings, approaches, and ideologies. Affordability has traditionally been determined by ratios – for example, a popular measure was the "30 percent of income rule". But modern scholars give argument that these measures are too simple to take into account the complexities of today's urban life. The current literature increasingly calls for more comprehensive approaches, including the use of the "residual income" model, which also considers other essential costs of living that are not related to housing, and the inclusion of transport costs to better assess the real spatial affordability. These evolving ideas have been the result of a larger understanding that housing affordability is not just a financial equation, but a multifaceted issue that involves geography, infrastructure, and socio-economic vulnerability. Moreover, there is significant disagreement among researchers about what factors are driving the crisis structurally, and on the effectiveness of different interventions. The other major strand of the literature highlights the role of supply-side constraints in explaining the affordability gap, as effectively capping housing supply and driving up costs through the imposition of tight zoning laws, land use regulations and geographic barriers. The other major camp is critical of the demand side dynamics, which sees the financialization of housing; as seen in global capital markets, housing is treated as an investment good and not a social good. This conflict between market-oriented approaches to housing and state-interventionist policies is the cauldron of contemporary housing policy debate. The body of literature on housing affordability in cities is complex, with this literature review being an attempt to draw together the various strands. It starts by reviewing the changing understandings and measurement of affordability and then moves on to consider the debate over supply-side constraints and the financialization of housing. Lastly, it reviews the results of different policy measures, from inclusionary zoning to rent control, to suggest key areas of underresearched literature and avenues for future research in the context of today's ever-changing global housing market.

**Intro** The contemporary world is witnessing a fast-paced global urbanization, housing is shifting from a basic human need to one of the most urgent socio-economic challenges of the twenty-first century. The crisis surrounding urban housing affordability is affecting not just low-income households but progressively engulfing the middle class in major cities around the world. As cities generate economic growth, they also create vigorous competition for space. As a result, real estate prices escalate well in advance of average wage growth. The vast gap between housing prices and household incomes has far reaching consequences on urban sustainability, socio-spatial segregation, and overall quality of life in cities. Therefore, it is vital for housing affordability that policymakers urban planners and sociologists to understand it. The academic discussion on urban housing affordability is very large, which has continuously changing definitions, approaches and philosophies. Traditionally, affordability was evaluated through simple ratio indicators, such as the classic 30 percent of income indicator. Nevertheless, modern scholars say that these static parameters do not do justice to the contemporary urban realities. According to the emerging literature the current approach of using ‘residual income’ approach which includes expenditure on non-housing essentials is further complemented by including transport costs to measure spatial affordability. These changing frameworks allow for a wider acknowledgement of this issue to go beyond a mere issue of finances, economic vulnerability, geography, and infrastructure. Moreover, researchers are deeply divided over structural drivers of this crisis and which measures work or do not. One large literature attribute the affordability deficit to constraints on the supply side, especially how tough zoning laws and land-use regulations and barriers that geography impose restrict housing construction and lead to higher prices. In contrast, another significant school of thought critiques the demand side and emphasises on financialization of housing. The school argues that residential real estate has been a portfolio investment for global capital markets. The conflict between market-led solutions and state-interventionist policies fuels the modern debate on housing policy. The literature reviewing article shows a wide angle of many literatures on housing. The assessment starts by looking at the changing definitions and measures of affordability, then it discusses the problem of supply-side constraints versus the financialization of housing. The paper then examines the effects of a range of policy interventions, such as inclusionary zoning and rent control, to ascertain important gaps in the literature and identify opportunities for future research in an increasingly uncertain global housing market.

(Introduction)** Urban housing affordability has become one of the most challenging social/economic issues of our time. As urbanization intensifies globally and housing is no longer simply a basic human necessity, but rather a major challenge faced by both lower- and middle-income families in many metropolitan cities around the world. Cities continue to be the engines of national economies; however, this also results in increased pressures for housing, which leads to rising real estate prices far beyond the rate of average wages. The resulting gap between the cost of housing and the ability of households to afford it has serious impacts on urban sustainability, exacerbates socio-spatial inequalities and diminishes the quality-of-life of urban residents. Therefore, understanding the factors contributing to affordable housing needs to be addressed by all stakeholders including urban planners, government officials and sociologists. There is considerable variability in the academic study of urban housing affordability. In particular, there have been changes in how housing affordability is defined and calculated over the years. The earlier studies typically utilized very simplistic measures of affordability; examples include ratios that compare housing costs as a percentage of household income. For example, the commonly referenced 'housing affordability' metric that defines housing as affordable when less than 30% of household income goes toward housing costs. More recent scholarship suggests these traditional methods do not provide a complete picture of what makes for affordable housing today. There is growing support among current scholars for using more comprehensive approaches like the 'residual income' method. This residual income method considers other important household expenses in addition to housing, while also considering transportation costs as part of evaluating whether or not housing is truly spatially affordable. The shift towards these alternative methodologies reflects a greater realization that measuring the affordability of housing involves more than just assessing a family's ability to pay their mortgage or rent, but is influenced by geographic location, availability of public transit and socio-economic vulnerability. Scholars are also sharply divided regarding what contributes to the decline in housing affordability and/or effectiveness of potential remedies. On one hand, some researchers believe that the primary cause of declining affordability is due to supply-side problems. Specifically, they contend that restrictive zoning regulations and/or land use restrictions reduce the number of new homes available to purchase/rent and subsequently increase home prices. On the other hand, other researchers point to demand-side problems as being responsible for declining housing affordability. They argue that with the increasing trend of treating residential property as an investment vehicle for foreign investors, domestic hedge funds and private equity firms, combined with aggressive marketing campaigns touting "real estate investments," the housing market has experienced an unprecedented level of financial speculation. Consequently, there exists ongoing debate as to whether addressing declining affordability requires either market-based solutions or state based regulatory policies. This literature review will synthesize the many bodies of research on urban housing affordability. First, we will examine the evolution of conceptualizing and calculating affordability. Second, we will analyze the competing perspectives on why housing affordability declined - whether due to supply side problems such as restrictive zoning/land use regulations and/or demand side problems related to financialization of housing. Third, we will assess a wide variety of policy responses intended to address declining affordability such as inclusionary zoning, rent controls, etc. and ultimately determine where there may exist knowledge gaps in the existing literature that could potentially lead to fruitful avenues of research for the future in a rapidly changing global marketplace.

**Introduction** In today's world, where global urbanization is speeding up, housing has changed from being a basic human necessity to becoming one of the most serious socio-economic problems of the twenty-first century. The issue of affordable housing has become critical, impacting not just low-income families but also increasingly affecting the middle class in cities all over the world. Although cities promote economic growth, they at the same time create fierce competition for space, causing property values to rise much faster than average wages do. The growing gap between the cost of housing and household incomes has serious consequences, for it endangers the sustainability of cities, worsens socio-spatial segregation, and reduces the general standard of life for people living in urban areas. As a result, it is essential for policymakers, urban planners and sociologists to understand the factors involved in housing affordability. The amount of academic discussion concerning the affordability of urban housing is huge and is marked by changing definitions, methods, and ideological disagreements. In the past, affordability was assessed using simple ratio figures—for example, the commonly applied "30 percent of income" rule. Yet today's scholars claim that these fixed kinds of measures do not take into account the complicated situation that exists in modern urban areas. The literature available now tends to support more comprehensive approaches, such as the "residual income" method, which takes into consideration expenses other than those needed for housing and also includes transport costs when judging true spatial affordability. This shift in thinking shows a wider understanding that housing affordability is not just a matter of financial calculation but a multidimensional problem affected by geography, infrastructure, and socio-economic vulnerability. There is also a deep division among researchers regarding the structural causes of the crisis and the effectiveness of different interventions. One major area of literature blames the affordability problem on supply-side limitations, stressing that strict zoning laws, land-use regulations, and geographical obstacles hinder the building of housing and thus drive prices up. On the other hand, a different important group of thinkers criticise the demand-side factors, citing the financialisation of housing, in which global capital markets regard residential property as an investment asset rather than as a social good. The conflict between solutions based on the market and those that call for state intervention is what shapes the current debate on housing policy. The literature review brings together the various aspects of the research concerning urban housing affordability. It first looks at the way the concepts and measures of affordability have developed, then examines the controversy between supply-side limitations and the financialization of housing. Lastly, it looks at the results of different policy interventions—such as inclusionary zoning and rent control—with the aim of spotting the major shortcomings in the current body of literature and suggesting possible directions for future research in the light of the ever-more volatile global housing market.

Housing used to be a basic need, now it's one of the biggest socio‑economic headaches of the 21st century. Fast‑paced urbanisation around the world has pushed prices sky‑high while wages crawl along, and the squeeze is hitting not just low‑income families but the middle class too. Cities churn out jobs, but the rush for space drives real‑estate values way past what most households earn, choking sustainability, deepening segregation and dimming quality of life for city dwellers. Scholars have debated what "affordable" really means for decades. Early on the rule‑of‑thumb was simple: spend no more than 30 percent of income on rent or mortgage. Today most researchers say that snapshot misses a lot, it ignores what people spend on everything else, from groceries to transit. Newer approaches count residual income after essential non‑housing costs, or fold in commuting expenses to get a truer picture of spatial affordability. In other words, it's not just numbers on a spreadsheet; geography, infrastructure and vulnerability all play a part. The literature splits in two camps. One side blames the shortage of supply, pointing to tight zoning, land‑use rules and natural barriers that curb new construction and jack up prices. The other side says the problem lives on the demand side, where global investors treat homes like financial assets instead of places to live. This clash between market‑driven fixes and government‑led interventions fuels today's policy wars. What follows is a sweep of the research. First, we'll trace how the concept and the metrics have evolved. Then we'll unpack the supply‑versus‑demand debate, and finally we'll look at what policies, from inclusionary zoning to rent caps, have actually done. The goal is to spot where the scholarship falls short and point toward the questions that still need answering in a wildly volatile global housing market.