Harvard University's Joint Center for Housing Studies is out with a big new survey on the state of housing in America in 2015.

It's full of interesting facts, but this one chart is the best illustration I've seen of an issue that doesn't quite get as much attention as it should — the fact that in some key metropolitan areas, affordable housing has become a problem for the middle class:

Here's why that matters. Suffering from housing "cost burden" is defined in this report (and the affordable housing community writ large) as spending over 30 percent of your income on housing.

By that standard, it's obviously true that many low-income households struggle to afford housing. And it's also sad. But in an important sense this isn't really a problem that has anything to do with housing specifically.

If you only earn $15,000 a year, then 30 percent of your annual income is just a really low number. It's hard to afford anything at all when you're poor, and the solution needs to involve either giving poor people subsidies or else enhancing their overall incomes.

But if you're making $55,000 a year, you're not poor. In fact, your average income is slightly above average for the United States. If non-poor people can't afford something as basic as housing, then that's really a specific issue about housing.

And what this shows is that while middle-class housing affordability isn't a huge problem in most of the country, it's a very big challenge in the 10 highest-cost metropolitan areas. These are places — places like the Bay Area and Greater New York City — where people who aren't poor still struggle to afford a decent place to live.

Those are the places where anti-development regulations have created a systematic undersupply of market-rate housing, costing billions in economic damage to the economy as a whole.