San Mateo County · ADU permit data · Updated 2026-10-20
5.3months median, application to building permit, 2023–2025
That is 12% shorter than the Bay Area median of 6.0 months. One in four applicants waited more than 8.5 months.
ADUs permitted in 2023–2025, 80 matched on parcel number. Includes time owners spent revising plans.
Source: California HCD Annual Progress Reports, filed by the City of Hillsborough
Fast quarter, median and slow quarter, in months
Hillsborough 2.8–8.5 mo
San Mateo County
Bay Area 3.8–10.5 mo
Median Fast quarter to slow quarter
Median wait
Where Hillsborough sits among 52 ranked cities
Hillsborough Other ranked cities Bay Area 6.0
Hillsborough
5.3months
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
Hillsborough
89%
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
Hillsborough
8.5months
One applicant in four had a building permit within this time from first submission.
Source: HCD Annual Progress Reports, 2023–2025, 25th percentile
Hillsborough
2.8months
Source: units by calendar year, as reported by the City of Hillsborough to California HCD
Hillsborough, units per year
ADU permits a year, 2023–2025 average
ADU permits a year per 1,000 houses, 109 places
Hillsborough Other Bay Area places
Hillsborough
49a year
ADU permits, 2023–2025 average
Of every home Hillsborough permitted in 2023–2025, this share was an ADU.
Source: HCD Annual Progress Reports, 2023–2025
Hillsborough
97%
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
Hillsborough
12.7
rank 1 of 92
Yearly ADU permits in 2023–2025, up 92% from 2019–2021.
Source: HCD Annual Progress Reports
Hillsborough
+92%
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
Hillsborough
18.2months
median of 40
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
Hillsborough
30.9months
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
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Source: Bay Area ADU Permit Clock 2026, our analysis of the California HCD Housing Element Annual Progress Reports (data.ca.gov) filed by the City of Hillsborough, 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 Hillsborough was 5.3 months (162 days), based on 80 permits we could match. A quarter took longer than 8.5 months.
Hillsborough issued 148 ADU building permits in 2023–2025, about 49 a year. ADUs were 97% of all new homes permitted there.
The median time from building permit to certificate of occupancy was 18.2 months for ADUs completed in 2023–2025.