An attached ADU is a self-contained unit joined to the main house. Under Austin's HOME amendments, a house plus an attached unit is treated as a two-unit use, so the same coverage, setback and address rules apply as for a detached cottage. It can reuse nearby utility lines, but the shared wall must meet fire separation rules.
When does an attached unit make sense?
- The backyard is too shallow for a cottage after rear setbacks.
- You want an aging parent next door with a short, step-free path between units.
- Water, sewer and electrical service are already on that side of the house.
- You plan to sell the property as one home later and want the addition to read as part of it.
Does it need its own address?
Yes. The city's HOME guidance says each dwelling in a two-unit or three-unit use must have a unique address. We add the address request to the permit package.
What does the shared wall require?
Two dwellings under one roof need fire-rated separation between them under the residential building code, and each needs its own egress. Sound insulation is not always required, but we recommend it in the design.
How much does an attached ADU cost?
We did not find a reliable Austin-specific published range for attached ADUs as of October 2026. Cost tends to fall between a garage conversion and a new cottage, depending on how much new foundation and roof the addition needs. You get a written, itemized quote after the site visit. See the Austin cost guide for the ranges that are published.
What does it depend on?
- How the new roof ties into the existing one.
- Matching or contrasting with the house's exterior materials.
- Side setbacks, which are narrow on many older Austin lots.
- Floor-area limits inside the McMansion boundary.