Short answer. A detached ADU in California runs $250,000 to $500,000 in 2026. Attached ADUs run $200,000 to $400,000. JADUs run $80,000 to $180,000. The per-square-foot range is $350 to $600 for most builds, higher on small ADUs and lower on prefab.
California ADU rules opened the door for tens of thousands of homeowners to add a second unit on a single-family lot. The rule changes solved the legal question. The cost question still trips up almost every project. This guide gives the real 2026 numbers we are quoting in active builds across Los Angeles, the San Fernando Valley, and the wider state.
ADU cost in California by type
| Type | 2026 cost range | Typical size | Per sq ft |
|---|---|---|---|
| Detached new build | $250,000 to $500,000 | 600 to 1,200 sq ft | $400 to $600 |
| Attached new build | $200,000 to $400,000 | 500 to 1,000 sq ft | $350 to $500 |
| Garage conversion to ADU | $120,000 to $220,000 | 400 to 600 sq ft | $300 to $450 |
| JADU (within home) | $80,000 to $180,000 | up to 500 sq ft | $200 to $400 |
| Prefab or modular | $180,000 to $350,000 | 400 to 900 sq ft | $300 to $450 with site work |
What California ADU law says about size and setbacks
California Government Code Section 65852.2 is the statewide rule that governs ADUs. Detached ADUs are allowed up to 1,200 square feet by right. Setbacks are limited to 4 feet on the side and rear. Owner occupancy cannot be required on most permits issued through 2025. Local cities can add modest requirements but cannot block ADUs that comply with the state rule.
Soft costs to plan for
- Architectural design and structural engineering. $8,000 to $25,000.
- Title 24 energy reports. $500 to $1,500.
- Plan check and permit fees. $4,000 to $12,000 depending on city.
- Utility connections. $2,000 to $20,000 if a separate electric meter or water service is needed.
- School district fees on ADUs over 750 square feet. Varies by district.
- Geotechnical and survey work on hillside or irregular lots. $3,000 to $8,000.
What drives the cost up
- Hillside or sloped lots that require grading, retaining walls, and deeper foundations.
- Long utility runs from the main panel or sewer line to the new structure.
- Two-story designs that need stairs, fire-rated assemblies, and structural upgrades.
- Premium finishes pulled from main-home remodeling rather than ADU-appropriate spec.
- Owner-driven design changes mid-build. Every change order on an ADU compounds because the trades are tightly stacked.
How to lock in an accurate California ADU number
Get a site visit before any quote. Insist on a fixed-fee design phase that produces complete construction drawings and a defined material spec before construction pricing. Then bid the project from drawings, not from a square-foot rule of thumb. That sequence is the single biggest predictor of a project that finishes on budget.
For a Los Angeles specific breakdown, read ADU cost in Los Angeles. For contractor screening, run the CSLB license lookup. For the build process, see the ADU service page.
