ADU Rules in San Mateo County, California
We track 2 cities in San Mateo County. Each city writes its own ADU ordinance on top of California law, so size limits, fees, and design rules differ from one city to the next. Among the cities we track, Daly City has the lowest starting permit fees ($10,000 – $20,000).
Cities
2
Max Detached Size
1,200 sq ft
Build Costs From
$210,000
Pre-approved Plans
1 of 2 cities
San Mateo County Cities Compared
| City | Max detached | Height | Permit fees | Build cost |
|---|---|---|---|---|
| Daly City | 1,200 sq ft | 18 ft detached (up to 20 ft to match the main home's roof pitch) | $10,000 – $20,000 | $210,000 – $420,000 |
| San Mateo | 1,200 sq ft | 18 ft to top plate line and 24 ft to roof peak (detached) | $11,000 – $22,000 | $220,000 – $430,000 |
Daly City ADU rules
Daly City has a pre-reviewed detached ADU plan program, but you must hire the designer who drew the plan and cannot modify it, and site-specific engineering is still required. Many parcels are on hillsides and need geotechnical review. On tight lots, garage conversions and attached units are often the only practical option.
San Mateo ADU rules
San Mateo allows detached ADUs up to 18 ft to the top plate and 24 ft to the roof peak, enough for a loft-style unit. Up to 800 sq ft of ADU space is exempt from the lot's floor area limit, but larger detached ADUs must fit within the zone's remaining FAR. Compliant ADUs get a ministerial building permit with no public notice, and the owner must record a deed restriction against selling the ADU separately.
Rules that apply in every California city are summarized in our California ADU law guide. See all California cities or compare every city side by side.