ADU Construction

The 2026 California ADU playbook (with real numbers)

California law now does much of the work for ADU applicants. When a house already stands on the lot, the city must approve or deny a complete ADU application within 60 days. For a new unit it can require a setback of no more than four feet from the side and rear lot lines, and a detached ADU can reach 1,200 square feet.

This guide breaks down what an ADU costs to build in California and Greater Seattle, the permit playbook, how to run the rental math, and how to find a builder who has done ADUs before, not only home additions.

Three paths: garage conversion, detached new build, prefab

Garage conversion is the cheapest path. HomeGuide puts a garage-to-apartment conversion at $20,000 to $100,000+ nationally. You reuse the slab, walls and roof structure. New work focuses on framing, insulation, drywall, kitchen, bath, electrical, and heating and cooling. In Los Angeles, conversions inside existing walls declared a median $40,000 on the permit from 2022 through 2025. Read the Los Angeles ADU guide for the full permit data.

Detached new-build is the most flexible option. Cost vs Value 2025 prices a 660-square-foot one-bedroom unit on a slab at $166,406 nationally, $178,536 in Los Angeles, $203,481 in San Francisco and $210,944 in Seattle. Foundation, utility runs and finish level move the number most.

Prefab ADUs are built in a factory and set on a prepared site. Among California makers that publish a full installed price, a prefab ADU runs $155,000 to $498,800 all-in, or $256 to $776 per square foot. Abodu, the one company that itemizes site work, prices installation at $64,000 to $105,000 depending on the model. Compare makers in the prefab ADU guide.

The rental income math

Start with rent for comparable units in your neighborhood, not a statewide average. Take a year of that rent and subtract a vacancy allowance, management if you hire it, maintenance, added insurance and the property tax on the new unit. Divide what is left by the full project cost, including design, permits and site work, to get your cash yield before financing.

Run it twice: once with the builder's bid and once with a cushion for what turns up in the ground. In Seattle, add King County's sewer capacity charge of $46.01 a month for 15 years. A primary home with a legal, rentable unit also carries resale value that the yield math leaves out.

When a house already stands on the lot, California gives the city 60 days from a complete application to approve or deny an ADU permit.

Permit timeline: California vs Washington

California: state law requires the permitting agency to approve or deny a complete ADU application within 60 days. The clock starts when the application is deemed complete, so plan-set quality matters. In Los Angeles, the median garage conversion waited 90 days from application to issue, and one in four waited 181 days or more.

Washington: in Seattle a lot can hold up to two ADUs, attached or detached. A detached unit built from a pre-approved ADUniverse plan can get its permit in 2 to 6 weeks in most cases, and custom designs take longer. See the Seattle ADU guide. Bellevue and other Eastside cities run their own permit processes.

Find a builder with relevant ADU experience

Ask for recent projects similar to your proposed unit, references you can contact and examples of local permit coordination. Confirm who handles design, site work, utility connections and inspection closeout. Compare complete scopes rather than the building price alone.