One of legendary urban theorist Jane Jacobs’s most famous insights is the pithy observation, “New ideas need old buildings.” As Jacobs wrote in The Death and Life of Great American Cities:

Cities need old buildings so badly it is probably impossible for vigorous streets and districts to grow without them…. for really new ideas of any kind—no matter how ultimately profitable or otherwise successful some of them might prove to be—there is no leeway for such chancy trial, error and experimentation in the high-overhead economy of new construction. Old ideas can sometimes use new buildings. New ideas must use old buildings.

This is true not just in the sense of affordable commercial or studio space for artists, entrepreneurs and other creatives. It is also true that innovative people—like anyone just looking to get a start and build their own wealth from near nothing—need affordable places to live. New construction is an expensive undertaking, so the primary way that cities acquire a stock of low-cost homes for those who need them is through a process called filtering: as homes age, they tend to become less desirable and comparatively less expensive, and occupied by lower-income households. This process is essential to a thriving, dynamic city that affords opportunity to all its residents.

In many American cities, we’ve got a big old-buildings problem: namely, these buildings, and the old urban neighborhoods they comprise, are not affordable enough to serve the function Jacobs so valued anymore.

New research out of Freddie Mac examines the filtering process and how it differs from city to city, as well as ZIP code to ZIP code within cities. It turns out there’s enormous variation: in some metro areas, old homes tend to actually filter up, becoming occupied by wealthier households over time. In others, old homes tend to filter down. In nearly every metro, though, there is a lot of diversity across neighborhoods—places exist where filtering works in either direction.

The study deals with owner-occupied homes with mortgages funded by Freddie Mac itself. So it’s not directly designed to address Jacobs’s “old buildings” issue, but it does illuminate, I think, a basic truth about our cities that bears on it. There’s a ton of other interesting stuff in the study, much of it summarized in a series of tweets by Len Kiefer.

The basic truth I’m talking about is illustrated by these two images from the Freddie Mac study: