ADU Cost Estimator
Pick a type and size to get a ballpark planning range for Southern California in 2026. This isn't a quote. Real bids depend on your lot, access, finishes, utilities and city fees.
Estimated all-in range–
Ranges used
| Type | Planning range |
|---|---|
| Detached | $350–550/sq ft |
| Attached | $325–500/sq ft |
| Garage conversion | $200–350/sq ft |
| JADU | $50k–150k total |
| Prefab installed | $325–550/sq ft |
| Two-story / above-garage | $400–600/sq ft |
Sources and method
- UC Berkeley Terner Center, Implementing the Backyard Revolution (2021): a survey of California ADU owners. It reported a statewide median cost of about $250 per sq ft ($150,000 median total) for units permitted in 2018–2019, including design, labor, materials and permits..
- 2025–26 Southern California builder pricing guides, which commonly quote $280–375/sq ft for detached construction alone, before design, permits, fees and utilities.
- We added typical soft costs (design, engineering, permits, utility connections) and adjusted for construction inflation since the Terner survey. All figures are rounded planning ranges, not quotes.
See our full SoCal ADU cost guide and financing options, then compare builders.
FAQ
How much does an ADU cost in Southern California?
For 2026 planning, most detached ADUs run roughly $350 to $550 per square foot all-in, including design, permits and utilities. Garage conversions run about $200 to $350 per square foot, and JADUs about $50,000 to $150,000.
Are permit and impact fees included?
The ranges include typical soft costs, but fees vary by city. ADUs of 750 square feet or less pay no impact fees under state law.
Why add a contingency?
Unknowns like soil, sewer laterals, electrical panel upgrades and fire-department requirements commonly add 10 to 15 percent.