Daniel Herriges wrote:

Examine building permits issued in Portland, Oregon, over the last three years and you'll notice something strange: the city seems to be acquiring a bunch of new 19-unit apartment buildings. Not 15. Not 25. Not 20. Specifically 19.

Why 19? The answer is no mystery to planners and housing advocates in the region. In December 2016, Portland enacted an Inclusionary Housing policy which requires the developers of all apartment buildings of 20 units or more to set aside a portion of their units for low- and moderate-income housing.

20 units or more. Get it?

In a recent Twitter thread about the phenomenon, commenters pointed out similar spikes in new 49-unit buildings in Minneapolis, 29-unit buildings in Newark, New Jersey, and even an international parallel, the proliferation of 9-unit apartment buildings in London. All are in response to regulations that begin at cutoffs of, respectively, 50, 30, and 10 units.

...

The 19-unit cluster is a side effect of a bigger problem. What it implies is not just that Portland is missing out on some 22- or 26-unit buildings, but more importantly that the inclusionary zoning policy is costly enough for builders to be worth trying to evade. And this means it's almost certainly depressing new housing construction more broadly