Housing Affordability Index
Also known as: Median Multiple, Housing Cost Burden Ratio, Residual Income Affordability, NAR Housing Affordability Index
A housing affordability index summarises how the cost of housing in a city or market relates to what households can pay, condensing prices, rents and incomes into a single interpretable number. The simplest forms are ratios — the median house price divided by median income, or housing outlays as a share of income — while the residual-income approach championed by Michael Stone instead asks what is left for everything else after housing is paid. Together these measures let analysts compare affordability across places and over time, flag cost-burdened populations, and track housing stress as markets shift.
Key highlights
- Reduces prices, rents and incomes to a single number that is easy to compare across places and time.
- Ratio thresholds (the median multiple, the thirty-percent rule) are transparent and embedded in policy.
- The residual-income variant corrects the ratio's unfairness to low-income households.
- The NAR-style index integrates interest rates, prices and incomes so it tracks owner affordability dynamically.
Intuition
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How it works
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When to use it
Use a housing affordability index when you need to compare housing stress across cities or neighbourhoods, monitor a market over time, or estimate how many households are priced out. Ratio measures such as the median multiple and cost-burden share are ideal for quick, transparent cross-market comparison and for policy thresholds; the NAR-style index is suited to tracking owner affordability as interest rates and prices move together. Prefer the residual-income approach when distributional fairness matters — when low-income households are the focus and a flat percentage would misclassify them. Affordability indices are weaker when income or price data are coarse, when imputed costs (utilities, taxes, maintenance) are omitted, or when housing quality and location differences are ignored, so a cheap but inaccessible or substandard dwelling looks artificially affordable.
Strengths & limitations
- Reduces prices, rents and incomes to a single number that is easy to compare across places and time.
- Ratio thresholds (the median multiple, the thirty-percent rule) are transparent and embedded in policy.
- The residual-income variant corrects the ratio's unfairness to low-income households.
- The NAR-style index integrates interest rates, prices and incomes so it tracks owner affordability dynamically.
- Fixed-percentage rules treat all households alike regardless of size or income level.
- Median-on-median ratios hide within-market inequality and the experience of low-income groups.
- Quality, location and access to jobs and services are usually ignored, distorting comparisons.
- Results are sensitive to which costs (utilities, taxes, insurance, maintenance) are counted as housing.
Common pitfalls
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Frequently asked
What is the difference between the median multiple and the cost-burden ratio?
The median multiple is a stock measure: it compares the price of buying a typical home with a typical annual income, ignoring financing. The cost-burden ratio is a flow measure: it compares ongoing housing outlays (rent or mortgage payments) with income over the same period. The multiple speaks to purchase affordability and is unaffected by interest rates; the cost-burden ratio speaks to ongoing housing stress and rises or falls with rents and mortgage terms.
Why does the residual-income approach criticise the thirty-percent rule?
Because a flat percentage ignores how much income is left over. A household earning very little may spend only twenty-five percent on rent yet still lack enough for food, transport and clothing, so it is genuinely unable to afford that housing; a high-earning household can spend forty-five percent and remain comfortable. Stone's residual-income method subtracts a non-housing budget standard, so its affordability threshold scales with household size and income instead of applying one ratio to everyone.
Does a higher affordability index always mean housing is more affordable?
Only if you know how the index is built. For ratio measures like the median multiple, a higher value means worse affordability. For the NAR-style index, a higher value means better affordability, because it is income relative to the income needed to qualify. Always check the direction of the scale and what it includes before comparing two numbers.
Sources
- 1.Stone, M. E. (2006). What is housing affordability? The case for the residual income approach. Housing Policy Debate, 17(1), 151–184.
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Cite this page
ScholarGate. (2026, June 22). Housing Affordability Index. ScholarGate. https://scholargate.app/urban-studies/housing-affordability-index