A standalone backyard structure built from scratch — the most flexible ADU option, with the highest privacy and resale value, and the highest cost.
All-in design-build estimates for a typical 750 sqft detached 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 | $175,500–$468,000typical $292,500$234–$624 per sqft |
| Los Angeles | $135,000–$360,000typical $225,000$180–$480 per sqft |
| San Diego | $128,250–$342,000typical $213,750$171–$456 per sqft |
| Central Coast | $141,750–$378,000typical $236,250$189–$504 per sqft |
| Sacramento | $114,750–$306,000typical $191,250$153–$408 per sqft |
| Inland Empire | $114,750–$306,000typical $191,250$153–$408 per sqft |
| Central Valley | $101,250–$270,000typical $168,750$135–$360 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 detached adu typically runs $101,250–$468,000+ 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.