Building an ADU in California: cost, plans, who designs it

The demand, and one caveat about the numbers

Cotality's 1 July 2026 analysis puts California at about 41,000 ADU permits, or 52 ADUs for every 100 conventional residential permits. Washington was next at about 2,200 and Florida at 2,000. In Los Angeles and San Francisco, Cotality reports, more ADU permits were issued in 2025 than permits for all other housing types combined. The caveat: Cotality's state table does not say which year it covers, so we quote the figures as Cotality published them and not as “2025 permits”.

The architects' panel points the other way nationally. In the AIA Home Design Trends Survey for the first quarter of 2026 (report dated 20 May 2026, a panel of hundreds of residential firms), the net score for accessory dwellings and rental units fell to 25 from 62 a year earlier. That is a national momentum measure, not a count. The permit data say California is where the building is happening.

What it costs

The modelled figure and the marketplace average are closer here than for most rooms, $166,406 against $180,000, which we read as both describing a new detached unit. The garage conversion average is lower because the slab, the roof and some walls already exist; whether they can be kept is the first thing a structural engineer checks. Neither figure is California-specific; get a local price on a drawn scope.

On the 41% recoup: Cost vs Value measures resale value only. It does not model rental income or housing a relative, so it answers a narrower question than most owners are asking. In the same report a basement remodel recouped 71% and a minor kitchen remodel 113% (Zonda via JLC, September 2025). The case for an ADU rests on the use, and that is not in the model.

Permit-ready plans: what to ask

Google's autocomplete on 10 October 2026 shows “adu permit ready plans” as a live search. We did not research individual city or county programmes for this guide, so what follows is the set of questions to put to a plan provider or planning counter, not a description of any one programme.

• Ask what has been pre-checked. A pre-approved plan has usually been reviewed for the building itself. Ask the planning counter whether that review is accepted as it stands, and what remains to be reviewed for your application.
• Ask what the plan cannot know: your lot. Setbacks, the distance to the main house, utility connections, fire access, slope, soil and drainage are site questions. The California Architects Board tells owners to consult the local building department about snow, wind, seismic, flood and soil conditions when deciding whether a project needs a licensed architect or engineer (CAB, Consumer's Guide).
• Ask who prepares the site plan, the foundation design for your soil and the utility and drainage drawings, and who stamps them. That is usually where an engineer or architect enters a stock-plan project.
• Ask what changes are allowed, and specifically whether mirroring the plan, moving a window or changing the roof takes it out of its pre-approval.
• Ask what the design fee covers. Angi's marketplace estimate puts design at 10 to 15% of total ADU cost (2026, read from search-result text). A stock plan saves some of that; the site work is still design.

Who may design an ADU in California

California does not require a licensed architect for most single-family wood-frame work. The California Architects Board's Consumer's Guide sets out the exemption: unlicensed persons may design single-family wood-frame dwellings of not more than two storeys and a basement, wood-frame multiple dwellings of up to four units with the same height limit, garages and appurtenant structures, and non-structural, non-seismic interior alterations (Business and Professions Code 5537, 5538 and 6737.1, as summarised by CAB). On our reading, a detached wood-frame ADU of one or two storeys sits inside the exemption; confirm it with your building department.

The exemption stops at safety. The same guide says an unlicensed person “may not design any component that changes or affects the safety of any building, including but not limited to structural or seismic components”. Foundations, the frame, a sloping lot and a seismic or fire hazard zone are where that line sits, and CAB tells owners to ask the local building department whether the project requires a licensed architect or engineer. Check with your local building department; local amendments and requirements for stamped drawings vary.

Cost vs Value's $166,406 and Angi's 10 to 15% design share together put design somewhere around $17,000 to $25,000 on a typical new unit, as arithmetic on two different sources rather than a quoted fee. Whether that buys an architect, a designer with an engineer, or a stock plan plus site engineering is the decision to make. Our guide to what an architect costs for a renovation sets out how fees are structured.