The Philadelphia Fed's new study on gentrification made a big splash recently with the finding that between 2000 and 2014, the city lost one-fifth of its affordable rental units--23,628 in total.   

Fed researchers defined affordability as units renting for $750 or less, based on an estimate of what the average renter can afford. Jake Blumgart has a deep dive into the findings on where this is happening and why, but there's one word that's oddly absent from this discussion: zoning.

If it's a problem that low-priced rental units are disappearing, one natural follow-up question is how and where more apartments can be created. And when you look at the Philadelphia zoning map, one of the things you'll notice is that there are just very few places where you're allowed to build apartments in general.

Kevin Hunter, a staffer at the Philadelphia City Planning Commission, made a couple of charts last year that illustrate the point by showing the percentage of zoned land area that each of the City's zoning categories cover:

Taken together, the zoning categories under which you can build the most common types of apartment buildings cover just 17.7% of Philadelphia's land area. That means that on 82.3% of Philly's land, it's illegal to build an apartment building. On Twitter, the Planning Commission claims that 28% of non-industrial, non-park land is zoned for multi-family housing, but this an odd framing that elides the question of whether all the current industrial zoning is appropriate (it's not.)

The categories that allow you to build an apartment building (RM or RMX) cover just 10.3% of Philadelphia. Commercial mixed-use (CMX) zoning covers just 7.3% of the city, and industrial-residential mixed-use (IRMX) zoning covers 0.1%. Single-family residential and industrial zoning cover the vast majority of our land, and parks and open spaces cover about 13%.

Several neighborhoods have been remapped since last year, so the exact percentages could be off somewhat, but the point stands that only on a small portion of Philadelphia's land can you build any type of apartment building, let alone an inexpensive one.

This is happening against a backdrop of an impossibly tight 3% rental vacancy rate citywide. While the current volume of construction may seem like a lot to many people, it's mostly catch-up growth after several years of underbuilding during the recession, and it hasn't been enough to move the vacancy rate. We're on track to build slightly fewer dwellings over the next few years than the population forecasts predict we'll need to keep up with demand.

Philly doesn't simply have a shortage of low-cost apartment units, but an overall shortage of apartments in general, and one of the main things threatening our affordability edge is our stingy low-density zoning.

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