An attached ADU is a self-contained dwelling built onto the main house. San Antonio applies the same size limit to attached and detached ADUs: up to 800 sq ft or 50% of the main house, whichever is larger, and no more than 1,600 sq ft. Height follows the zoning district rather than the 25-foot detached limit.
Why choose attached over detached?
- Shorter plumbing and electrical runs, since the house's service is next door.
- No need for backyard depth beyond the house's own setbacks.
- Easy, covered access between units for caregiving.
- One roofline that can match the existing home.
What does the shared wall need?
Building code treats the two units as separate dwellings, so the wall and any floor or ceiling between them must be fire-rated, and each unit needs its own way out. We also design for sound control, which tenants and family both appreciate.
Is there a bedroom limit?
No. The city's ADU page states there is no limit on bedrooms; the floor-area cap is what constrains the layout.
What does it cost?
One published San Antonio guide groups attached units with garage conversions at about $40,000 to $80,000 for construction, as of October 2026. A larger addition with a new foundation and roof will run above that. Read the cost guide for context; your number comes in a written, itemized quote.
Do I still need the affidavit?
Yes. The owner-occupancy affidavit applies to every ADU in the city. See who has to live on site.