Prefabricated and modular ADUs are factory-built, then trucked and craned onto a prepared site. Faster install than custom builds (weeks vs. months on-site) and often 10-25% cheaper.
All-in design-build estimates for a typical 600 sqft prefab adu. Real builder quotes vary 20-40% with lot conditions and finishes — get at least three before committing.
| Region | All-in cost, low to high |
|---|---|
| Bay Area | $105,000–$280,800typical $175,200$175–$468 per sqft |
| Los Angeles | $81,000–$216,000typical $135,000$135–$360 per sqft |
| San Diego | $76,800–$205,200typical $127,800$128–$342 per sqft |
| Central Coast | $84,600–$226,800typical $141,600$141–$378 per sqft |
| Sacramento | $68,400–$183,600typical $114,600$114–$306 per sqft |
| Inland Empire | $68,400–$183,600typical $114,600$114–$306 per sqft |
| Central Valley | $60,600–$162,000typical $100,800$101–$270 per sqft |
California has preempted most local restrictions on ADUs. These are the universal rules; your city may be more permissive.
From contractor estimates and 2026 builder pricing pages.
Cities ranked by total ADU permits 2013–2022 (all types). Type-specific data isn't published, but high-permit cities tend to be the most accommodating across all ADU types.
Statewide, a prefab adu typically runs $60,600–$280,800+ all-in (design + permits + construction + soft costs). The Central Valley anchors the low end and the Bay Area the high end. See the per-region cost cards above for your area.
Generally yes. California state law requires cities to approve ADUs that meet the statutory conditions on lots with a proposed or existing single-family home in a residential or mixed-use zone, with limited local discretion. Cities can set objective standards within state limits but cannot ban qualifying ADUs outright.
Most CA ADU projects take 12-18 months from initial design to certificate of occupancy. Under state law a city has 60 days to approve or deny a completed ADU application on a lot with an existing dwelling (ministerial review). Construction is the long pole — 6-12 months for new builds, 4-8 months for conversions.
Yes — California requires a B-license general contractor (or specialty B-2 residential remodelling) for ADU construction. Verify any contractor against the CSLB Instant License Check before signing. Our city pages list ADU-specialist B-license builders by area.
Most CA homeowners use a HELOC, cash-out refinance, or construction loan. CalHFA's ADU Grant Program (when funded) offers up to $40k for pre-development costs to income-qualified homeowners. Some lenders now offer ADU-specific loan products that include projected rental income in qualifying calculations.
Yes — the ADU's added value is reassessed at current market rates and added to your property's assessed value. The main house's existing assessment is preserved (Prop 13 applies). Practically, a $250k ADU adds roughly $2,500–$3,000/year in property tax in most CA counties.