Housing: A Look at "Affordability" Indexes

The measures I typically use to compare house prices over time are 1) real house prices (nominal house prices adjusted for inflation), 2) a Price-to-rent ratio, and 3) House prices to median income.

Another measure that is commonly used is an “affordability index”. These indexes calculate affordability based on income and mortgage rates. I’ll present two commonly referenced affordability indexes, and then present my own calculations and some comments on the usefulness of these indexes.

NAR Affordability Index

The National Association of REALTORS®  (NAR) releases several affordability indexes. Here is the most recent blog post by NAR Research Data Specialist Michael Hyman on the monthly index: Housing Affordability Weakens in June as Home Prices Ascend

At the national level, housing affordability declined in June compared to a year ago according to NAR's Housing Affordability Index. Affordability declined in June compared to one year ago as the median family incomes rose by 2.3%, while the monthly mortgage payment increased 21.5%. The effective 30-year fixed mortgage rate1 was 3.03% this June compared to 3.22% one year ago, but the median existing-home sales price rose 24.4% from one year ago.

The higher the number in this index, the more affordable a house. Here is the NAR explanation:

To interpret the indices, a value of 100 means that a family with the median income has exactly enough income to qualify for a mortgage on a median-priced home. An index above 100 signifies that family earning the median income has more than enough income to qualify for a mortgage loan on a median-priced home, assuming a 20 percent down payment. For example, a composite HAI of 120.0 means a family earning the median family income has 120% of the income necessary to qualify for a conventional loan covering 80 percent of a median-priced existing single-family home. An increase in the HAI, then, shows that this family is more able to afford the median priced home.

The calculation assumes a down payment of 20 percent of the home price and it assumes a qualifying ratio of 25 percent. That means the monthly P&I payment cannot exceed 25 percent of the median family monthly income.

First American Real House Price Index

IMPORTANT: Although “Real” is in the title of the index, this is NOT inflation adjusted.

Another type of index measures house prices based on affordability. Here is a tweet from FirstAm Deputy Chief Economist Odeta Kushi.

She is referencing a blog post by FirstAm Chief Economist Mark Fleming: Today's House Prices Are Over 40 Percent More Affordable Than The Housing Boom Peak

The Real House Price Index (RHPI) measures affordability in the context of changes in consumer house-buying power, incorporating fluctuations in household income, mortgage rates and nominal house prices. In June, housing affordability declined on a year-over-year basis for the fourth month in a row, following two years of increasing affordability. The decline in June occurred even as two of the three key drivers of the RHPI, household income and mortgage rates, swung in favor of greater affordability relative to one year ago.

Here is a description from FirstAm:

The First American Real House Price Index (RHPI) measures the price changes of single-family properties throughout the U.S. adjusted for the impact of income and interest rate changes on consumer house-buying power over time and across the United States at the national, state and metropolitan area level. Because the RHPI adjusts for house-buying power, it is also a measure of housing affordability.

Calculated Risk Affordability Index

I’ve put together my own affordability index - since 1976 - that is similar to the FirstAm approach (more of a house price index adjusted by mortgage rates and the median household income).


I used median income from the Census Bureau (estimated 2020 and 2021), assumed a 15% down payment, and used a 2% estimate for property taxes, insurance and maintenance. This is probably low for high property tax states like New Jersey and Texas, and too high for lower property tax states. If we were including condos, we’d also include HOA fees too (this is excluded).

Update: For house prices, I used the Case-Shiller National Index, Seasonally Adjusted (SA). Also, for the down payment - there wasn’t a significant difference between 15% and 20%.

For mortgage rates, I used the Freddie Mac PMMS (30 year fixed rates). Here is a graph of mortgage rates since 1976:

Mortgage rates have been declining for 40 years, and are currently close to 3%. In a falling mortgage rate environment, buyers could refinance at lower rates - and that would mean already purchased houses would become more affordable over time (not included in this calculation).

So here is what the index looks like (lower is more affordable like the FirstAm index):

Note that by this index, during the early ‘80s, homes were very unaffordable due to the very high mortgage rates. During the housing bubble, houses were also less affordable using 30 year mortgage rates, however, during the bubble, there were many “affordability products” that allowed borrowers to be qualified at the teaser rate (usually around 1%) that made houses seem more affordable.

In general, this would suggest houses are fairly affordable right now.

But this says nothing about if now is a good time to buy! Here is a graph showing affordability, and the 5 year real return (red) on the house purchase (annualized).

Update: The 5 year real return is the future return on the Case-Shiller index, adjusted for inflation.

When affordability was poor in the early ‘80s, it was actually a good time to buy (and that was before refinancing!). Another good time to buy was in the mid-to-late ‘90s. And another good time was around 2011 or 2012 (When I wrote The Housing Bottom is Here) and also the next several years. The worst time to buy (using a 5 year real return) was in the runup to and during the housing bubble.

Since the real return is based on 5 years, we don’t know the return after mid-2016.

The bottom line is affordability probably hints at the level of demand (although there was plenty of demand during the housing bubble due to loose lending practices). But affordability tells you nothing about the future return.