As of October 2026, published Austin estimates put site-built detached ADUs at about $175,000 to $400,000 and garage conversions at about $80,000 to $150,000. Permit and impact fees add an estimated $6,000 to $16,000. Your lot, the unit size, utility runs and finishes decide the real number.
What does each type cost?
| ADU type | Published Austin range, October 2026 | What drives it |
|---|---|---|
| Site-built detached | About $175,000 to $400,000 | New foundation, structure, roof and utility lines |
| Prefab or modular detached | About $120,000 to $250,000 | Factory unit plus foundation, delivery, crane and hookups |
| Above-garage unit | About $150,000 to $300,000 | Whether the garage below is reused or rebuilt |
| Garage conversion | About $80,000 to $150,000 | Slab, framing and roof condition |
| Interior conversion | About $60,000 to $120,000 | Existing space inside the house |
| Attached addition | No reliable Austin-specific range found | New foundation and roof tie-in |
These ranges come from one Austin real estate guide and are not our prices. A separate national cost publisher, using RSMeans city cost indices, models a 600 sq ft detached unit in Austin at about $84,000 (basic) to $175,000 (high-end) for construction alone. The gap between the two shows how much depends on scope and finishes.
It depends: what moves an Austin ADU price?
- Foundation. Expansive clay soils in much of the area, or rock and slope in the west, change foundation design.
- Utility runs. Long trenches from the street to the back of the lot, and electrical service upgrades.
- Trees. Protected trees can push the footprint or require special foundation methods near roots.
- Size and layout. Kitchens and bathrooms cost the same whether the unit is 450 or 900 sq ft.
- Finishes. Builder-grade and custom selections can differ by tens of thousands of dollars.
What costs sit outside construction?
- Design, engineering and surveys.
- City permit and review fees, and utility impact or connection fees.
- HOA architectural review fees, where an HOA exists.
- Higher property taxes once the new unit is appraised.
How should I compare quotes?
Ask each builder for a written, itemized quote, and check whether it includes design, permit fees, utility work to the unit, and appliances. Ask how allowances work and who pays for changes the city requests during review.
What if I want rental income to cover it?
Read how much rent an ADU can make in Austin before you set a budget.
Sources, as of October 2026:
- Neuhaus Realty Group, "Complete Guide to ADUs and Guest Houses in Austin (2026)," updated August 2026, for Austin cost ranges and fee estimates.
- CostToBuildHouse.com, "2026 Texas ADU Cost Guide," for RSMeans-based Austin construction estimates.