ADU construction
How long does it take to build an ADU?
By the HyreADU research desk. Last checked .
The short answer
How long it takes to build an ADU depends mostly on permit review and on the build method. Expect five phases in sequence: design, permit review, site work, construction and finishes.
California requires a decision within 60 days of a complete application, but measured times from application to issuance are longer: a median of 162 days in Los Angeles and 445 in San Francisco in our analysis.
| Phase | What controls it |
|---|---|
| Design | How quickly you decide, surveys, engineering |
| Permit review | Completeness of your set and your city’s workload; 60-day decision on a complete application in California (Gov. Code § 66317) |
| Site work | Soil, slope, access, weather, utility trenches |
| Construction | Method: site-built, panelized, modular |
| Finishes and final | Inspections, utility meters, lead times on fixtures |
Why permit review takes longer than 60 days
The 60-day clock in Gov. Code § 66317 starts from a complete application. The measured times in our permit times research run from the first application date to issuance, including every round of corrections.
Los Angeles showed a median of 162 days across 27,188 issued ADU permits, San Francisco 445 days across 1,713, and unincorporated Marin County 174.5 days across 472. Those measure a longer span than the statute.
How to shorten the timeline
- Check feasibility before design, so drawings are not redone.
- Submit a complete set: energy forms, structural calculations and site plan. The ADU permit checklist shows what is missing.
- Use a current-cycle pre-approved plan for a detached ADU in California, which carries a 30-day decision (Gov. Code § 65852.27).
- Answer correction comments in one round.
- Order long-lead items such as windows and electrical gear early.
Plan your own timeline
Type your own weeks for each phase into the ADU timeline planner. Our construction duration research covers how long building takes after the permit.
A mistake to avoid
The mistake to avoid is quoting the construction time as the project time. A builder who says “four months” usually means time on site after the permit. Design and review come first and can take longer than the building itself.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Does prefab make it faster?
It shortens time on site, not permit review.
What if my city misses the 60 days?
California treats a complete application as deemed approved if the city does not act in 60 days. Ask your city how it applies this.
Are garage conversions faster?
Often on site, but corrections on old structures can slow review.
How long is the final inspection stage?
It depends on inspection scheduling and utility meter set-up in your area.
Sources
- Gov. Code § 66317 (ministerial review, completeness and the 60-day decision). Checked September 5, 2026.
- Gov. Code § 65852.27 (preapproved ADU plan programs, AB 1332). Checked September 5, 2026.
- LADBS, Building Permits Issued from 2020 to Present (open data). Checked September 5, 2026.
- DataSF, Building Permits. Checked September 5, 2026.
- Marin County, Building Permit (open data). Checked September 5, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.