The one little rule that decides where Austin’s towers build parking

Not every tower in downtown Austin looks exactly the same, but there is one defining characteristic that describes almost all of them: parking. Most towers rest on top of what they call in the industry a parking plinth, the tower base where folks store their cars. (Plinth is a Swedish word meaning ugly thing.) Here’s a typical example, the Seaholm Tower in southwest downtown.

On the ground floor, there’s a restaurant. Above that, the area with small and sporadic windows is the parking garage. Above that, the area with balconies is where the condos live. Simple, effective, but not always super sightly, at least to my tastes. Why not build parking underground, freeing up aboveground levels for more homes? For one thing, building underground car parking is very expensive. The exact difference varies by site but I’ve seen estimates that moving a parking space underground can add $10K to the cost of the space — and the further you have to dig, the more expensive it gets. So perhaps underground parking is reserved for the most expensive buildings?

No, even in Austin’s newest luxury towers you see aboveground parking:

The Independent (aka the Jenga Tower) under construction. The open floors are parking, the one with windows above are where the residences start.
The Fifth and West Tower’s parking structure (behind the vet’s office) is much wider than the tower above it, which is constrained by capital view corridors.
The parking plinth at the Seven Apartments is again much wider than the building itself. The larger the footprint of the parking structure, the less expensive it is per space and the fewer stories a car needs to climb to reach its space. This reduces incentives to build a taller, thinner structure on the same footprint as the tower itself.

Parking underground is just too expensive for Austin, or so I thought, until Sid Kapur pointed out to me that there is somewhere in Austin building underground parking:

[tweet 938849429121060865]

Yes, of course, West Campus (aka Bizarro Austin) is building underground parking. See James Rambin’s writeup at sister site Austin Towers of projects like Skyloft and Aspen West Campus with four stories of underground parking. So why do developers building student housing decide to put parking underground? Are students more discerning aesthetic connoisseurs? Sorry, students, but I doubt it. I have an alternate theory.

There are many different ways that zoning codes can limit the amount of space that can be built. In downtown, the binding constraint preventing even larger buildings is something called Floor Area Ratio or FAR: roughly, the square footage of climate-controlled space in a building divided by its footprint. In West Campus, the binding constraint on the size of a building is regulations on maximum height. Crucially, parking counts toward a building’s height but doesn’t count toward its FAR. If a developer in West Campus moved their parking from below-ground to above-ground, they would have to remove apartments in order to fit it in, costing them a lot of money in lost rent. Developers downtown, though, don’t have a height limit so building a parking plinth costs less than putting parking underground, doesn’t use up the building’s FAR, and even makes the residential units more valuable by giving them better views.

If my theory is true, it makes some predictions: portions of northern downtown that are slated to be rezoned with height-constrained zoning categories (CC-40 and CC-60) will likely see underground parking, while the FAR-constrained central business district will continue to see parking plinths. Indeed, the condo building I live in downtown was built pre-CodeNEXT but it had height limits imposed as part of the rezoning process and consequently built most of its parking underground: