Duplexes and the new land development code
Once upon a time, it was pretty easy to build a duplex in Austin. Duplexes could be built in any single-family district. Duplexes could be built on any size single-family lot. Duplexes did not have to share a common wall, but could be attached by a common carport or breezeway. Duplexes could be arranged side by side on the lot, or front to back, as the owner pleased.
And Austin used to build a lot of duplexes. Because duplexes were liberally allowed for a large portion of Austin’s history, Austin has proportionally more duplexes than other large Texas cities.
Things began to change in 1984. When the city adopted its new land development code in 1984, it created new single-family residential districts (SF-1 and SF-2) that expressly prohibit duplexes. Indeed, the only point of the SF-2 district is to ban duplexes–SF-2 and SF-3 regulations are otherwise identical. The 1984 code also increased the minimum lot size for duplexes from 5,750 s.f., the minimum single-family lot size, to 7,000 s.f., eliminating duplexes as an option on many of the smaller lots in the central city.
The real assault on the duplex began in the 2000s, though. In 2003, the maximum height of a duplex was dropped from 35′ (the maximum height of a detached single-family home) to two stories and 30′.
In 2006, the McMansion ordinance imposed a slew of height and bulk restrictions on central-city dwellings. The ordinance imposed new setback rules. It prescribed a complicated, angled building envelope. It required the sides of buildings to be articulated. And, perhaps most importantly, it limited buildings to a maximum floor-to-area ratio of 0.4:1, severely restricting the size of duplex units.
The McMansion ordinance also adopted a “common wall” requirement for duplexes. Henceforth, duplex units would be required to share a common wall extending 50% of the depth of the building, and duplexes could no longer be separated by carports, breezeways or other open building elements. This common wall requirement essentially required that duplexes be massed into one large structure on the center of the lot; the rest of the McMansion ordinance regulations severely curbed the height and bulk of that structure.
In 2008, the rules were changed again to require that the common wall between duplex units be measured from the front to back of the building. This effectively prohibited front-and-rear duplexes. Duplexes now could sit only side by side. Here is an example of a nice front-and-rear duplex around the corner from me that violates he common wall requirement. The front unit presents to the street as a detached single-family home. The second unit is tucked in the rear of the lot, separated from the front unit by an open carport:
This is the relevant context for evaluating staff’s proposed revisions to the duplex regulations. They are very good changes. But they mostly just strip away the anti-duplex rules that have accumulated like barnacles over the last few decades. If these rules make it into law, duplexes will become about as easy to build as they were in 1983. That’s genuine progress.
Here are the six key changes to duplex rules in the draft land development code:
1. Duplexes would be allowed in all single family zones. As noted above, the land development code currently does not allow duplexes in SF-1 or SF-2 districts. There’s not very much land zoned SF-1, but there’s a fair amount zoned SF-2. The proposed code would allow duplexes in all single family zones, except for one zone reserved for very small lots. This change, although welcome, might not result in a surge in duplex units because many SF-2 neighborhoods have deed restrictions prohibiting two units per lot. Zoning does not override deed restrictions.
2. Duplexes could be bigger. The code currently limits duplexes to a 0.4:1 floor -to-area ratio, the same limit that applies to detached single-family homes. Consequently, a 7,000 s.f. lot can yield one 2,800 s.f. home or two 1,400 s.f. duplexes. 1,400 s.f. is pretty small for the price that new central-city housing fetches these days. As a result, a lot of infill builders are choosing simply to build larger single-family homes. The new rules would bump up the allowed floor-to-area ratio for duplexes to 0.6:1 but leave the ratio unchanged for detached single-family homes. This would allow two 2,100 s.f. duplexes on a 7,000 s.f. lot versus a single 2,800 detached home. It would provide a tremendous incentive to build duplexes rather than detached homes, at least in pricey central-city neighborhoods. This change would still leave the rules more restrictive than the rules in place before 2006, when the cap on floor-to-area ratio was imposed.
3. The minimum lot size for duplexes would be reduced to 5,000 square feet. Zilker, Bouldin, and other central city neighborhoods have many lots smaller than the current 7,000 s.f. minimum for duplex lots.
4. The common wall requirement would be eliminated. The draft code would allow duplex units to be connected by a carport, breezeway or other structure. Duplexes would no longer be required to share a common wall. This would re-legalize front-and-back duplexes. We also would see smaller, semi-attached structures rather than one centrally located hulking structure.
5. Duplexes could be three stories. Duplexes are currently limited to two stories and 30′. (The height limit for single-family homes is 35′ outside of the central core and 32′ in the urban core, where the McMansion ordinance applies.) The new code would allow a height of 35′ more than 10′ from the side property line. If you are willing to build a flat roof or a shallow gable, this would allow three stories and make it more feasible to hit the 0.6 cap on floor-to-area ratio.
6. Parking would be reduced. The draft code would reduce parking to one space per unit–or none, if the unit is within a quarter mile of a transit corridor. This is an improvement even over the 1983 rules. No one will build a duplex without any parking, but they might build units with just one space.
These changes would spur a boom in duplex construction. Duplexes would be much easier to build. And duplexes would become a more attractive option than single family homes in many cases. They are a good step toward encouraging missing middle housing.
There are other things Council can and should do. Here, briefly, are two:
1. Extend the new duplex rules to NCCDs. Most land directly north of the University of Texas is included in a neighborhood conservation combining district (“NCCD”). Council has directed that NCCDs not be remapped with new zoning districts. But it has left open the door for some of the new rules to apply. Council should direct that the new duplex rules apply in NCCDs.
2. Allow accessory dwelling units with duplexes. The current draft allows an accessory dwelling with a duplex only when an existing, older dwelling is preserved. I understand the desire to preserve older dwellings, but this can be accomplished by exempting them from the cap on floor-to-area ratio. Let’s allow all duplexes to have ADUs. ADUs can be unobtrusively tucked behind a duplex just as they can be unobtrusively tucked behind a detached single family home. Allowing ADUs with duplexes would be a gentle increase in density. It would also be a genuine advance on the rules in place in 1983.
