A detached ADU, often called a backyard cottage in Austin, is a separate dwelling on the same lot as your house. It offers the most privacy of any ADU type. Because it needs its own foundation, roof and utility lines, it is usually the costliest to build.
Can I build a backyard cottage on my Austin lot?
On many lots, yes. Under the HOME amendments, which the city began accepting applications for on February 5, 2024, lots zoned SF-1, SF-2 and SF-3 can hold up to three housing units. The old rules that treated a backyard unit as a "secondary" dwelling were removed, so a cottage is now one of the units in a two-unit or three-unit use.
How big can it be?
Austin no longer sets a single square-foot cap for the second unit. Size is driven by the lot: the city's HOME page lists 40% building coverage for two-unit and three-unit uses, plus impervious cover limits and setbacks. Inside the McMansion boundary, every enclosed space, including garages and lofts, counts toward floor area. Very small units of 400 sq ft or less, not counting a loft, fall under the city's tiny home definition.
Do I need to add parking?
Not under city code. Austin removed its minimum parking requirements in 2023. Your deed restrictions or HOA rules may still say something about parking, so we check those too.
What foundation does a cottage need?
Many Central Texas lots sit on clay soils that swell when wet and shrink in drought. We plan the foundation with an engineer, based on soil conditions at your site, rather than reusing a design from another lot. Lots west of MoPac can bring limestone and slope instead, which changes excavation and drainage.
What does a detached ADU cost in Austin?
As of October 2026, one published Austin guide puts site-built detached ADUs at about $175,000 to $400,000 and prefab or modular units at about $120,000 to $250,000. Utility trenching to the back of the lot and electrical service upgrades are common extras. See the Austin ADU cost guide.
What does the answer depend on?
- Coverage and setbacks. What remains after the house, driveway and patios are counted.
- Trees. Protected and heritage trees and their root zones limit where you can dig.
- Floodplain and drainage. Some lots near creeks carry floodplain rules.
- Utilities. The distance to Austin Water lines and whether Austin Energy service at the house can carry another unit.
- Private restrictions. Deed restrictions can forbid a second dwelling even where zoning allows it.