While I'm on the subject of duplexes, Austin has always struck me as having an unusually large number of duplexes. I finally got around to looking up the numbers. Duplex units make up 5.3% of Austin's housing stock. By comparison, they make up just 1.8% of Houston's stock, 1.7% of Dallas's stock, and 2.2% of San Antonio's stock.* Austin thus has more than three times as many duplexes as Houston or Dallas (relative to the total housing stock), and more than twice as many as San Antonio.
I'm not sure. One possibilty is that it is simply easier to build duplexes here. Dallas has a special duplex district, while Austin allows duplexes in most single-family districts. On the other hand, developers can build whatever they want in Houston, and they apparently don't want to build duplexes.
Perhaps Austin has so many duplexes because so much of its residential land is zoned for single-family; duplexes are the only option for smuggling in some multi-family.
Dunno. But it's a weird stat because I doubt housing preferences vary much among the four cities' residents.
*Source: 2006 Census estimates