San Mateo County · ADU permit data · Updated 2026-10-20
7.0months median, application to building permit, 2023–2025
That is 16% longer than the Bay Area median of 6.0 months. One in four applicants waited more than 11.1 months.
ADUs permitted in 2023–2025, 67 matched on parcel number. Includes time owners spent revising plans.
Source: California HCD Annual Progress Reports, filed by San Mateo County
Fast quarter, median and slow quarter, in months
Unincorporated San Mateo County 4.4–11.1 mo
San Mateo County
Bay Area 3.8–10.5 mo
Median Fast quarter to slow quarter
Median wait
Where Uninc. San Mateo Co. sits among 52 ranked cities
Uninc. San Mateo Co. Other ranked cities Bay Area 6.0
Unincorporated San Mateo County
7.0months
median wait, 2023–2025
State law gives the city 60 days to act once an application is complete. Counted from first submission, this share of ADUs took longer. Ask for the completeness letter, which is due within 15 business days, and keep its date.
Source: HCD Annual Progress Reports, 2023–2025; Gov. Code 66317
Unincorporated San Mateo County
90%
of matched ADUs
One applicant in four took longer than this. Plan on it if the city sends your plans back for another round of corrections.
Source: HCD Annual Progress Reports, 2023–2025, 75th percentile
Unincorporated San Mateo County
11.1months
One applicant in four had a building permit within this time from first submission.
Source: HCD Annual Progress Reports, 2023–2025, 25th percentile
Unincorporated San Mateo County
4.4months
Source: units by calendar year, as reported by San Mateo County to California HCD
Unincorporated San Mateo County, units per year
ADU permits a year, 2023–2025 average
ADU permits a year per 1,000 houses, 109 places
Uninc. San Mateo Co. Other Bay Area places
Unincorporated San Mateo County
56a year
ADU permits, 2023–2025 average
Of every home Unincorporated San Mateo County permitted in 2023–2025, this share was an ADU.
Source: HCD Annual Progress Reports, 2023–2025
Unincorporated San Mateo County
33%
Permits a year for every 1,000 detached houses, so small and large cities can be compared.
Source: HCD Annual Progress Reports; DOF housing estimates
Unincorporated San Mateo County
3.0
rank 39 of 92
Yearly ADU permits in 2023–2025, up 44% from 2019–2021.
Source: HCD Annual Progress Reports
Unincorporated San Mateo County
+44%
Median time from building permit to certificate of occupancy for ADUs finished in 2023–2025. Add it to the permit wait for a full timeline.
Source: HCD Annual Progress Reports, 2023–2025
Unincorporated San Mateo County
10.9months
median of 45
One ADU in four took longer than this to reach a certificate of occupancy after the permit.
Source: HCD Annual Progress Reports, 2023–2025, 75th percentile
Unincorporated San Mateo County
16.7months
Median Fast quarter to slow quarter Bay Area median, 6.0 months Number is the rank out of 52 cities
Every page in San Mateo County: median wait where we can measure it, otherwise ADU permits a year
Timeline tool
Pick a start date and see the fast, likely and slow dates for Unincorporated San Mateo County, from the numbers on this page.
Estimate my permit datesNext step
Answer a few questions about your lot and your plans. We point you to the right kind of help for an ADU in Unincorporated San Mateo County: an architect, a permit-drawing team, or a design-build firm. About two minutes, no call list.
Find the right first callStart with the city's own rules: ADU rules and process.
Source: Bay Area ADU Permit Clock 2026, our analysis of the California HCD Housing Element Annual Progress Reports (data.ca.gov) filed by San Mateo County, and DOF housing estimates. Wait times are matched on parcel number and include applicant revision time. The 60-day rule is in Government Code section 66317. How we measured it · See all 109 jurisdictions
For ADUs permitted in 2023–2025, the median time from application to building permit in unincorporated San Mateo County was 7.0 months (213 days), based on 67 permits we could match. A quarter took longer than 11.1 months.
Unincorporated San Mateo County issued 168 ADU building permits in 2023–2025, about 56 a year. ADUs were 33% of all new homes permitted there.
The median time from building permit to certificate of occupancy was 10.9 months for ADUs completed in 2023–2025.