What has happened to the bottom of the US housing market?

Housing quality has improved dramatically for most low-income households, but they are paying much larger shares of their income for it. Many discussions of the bottom of the market focus on either the rising costs (the 'bad' news) or the rising quality (the 'good' news). Both points of view have some merit. This paper takes a positivist approach to the question: what's happening to the bottom of the housing market. We use a variety of indicators to look at whether the supply of low-cost units has fallen. We then focus on whether this dwindling supply reflects a market failure. If the market for low-income housing is indeed failing, we would observe: relatively high prices per unit of housing services provided by low-quality housing; obstructions to the supply of low-cost housing, which arises largely from filtering; and high vacancy rates. We find evidence of market failure in some markets but not in others. Finally, we focus on one possible source of market failure - excessive regulation - to see whether it is a common thread across those markets that have difficulty producing low-cost housing.
Two central facts about the bottom of the US housing market are easy to characterise: housing quality has improved dramatically for most low-income households, but they are paying much larger [...]