San Mateo permitted 610 ADUs from 2013 to 2025 — among the most active markets in California. Here's what it costs, what's allowed, and who builds them.
All-in design-build estimates (architecture + permits + construction + soft costs). Real quotes vary by lot conditions, finishes and contractor — get at least three before signing.
Per-square-foot ranges reflect 2026 pricing in the bay area market. Permit fees ($5–25k), site work ($10–40k) and high-end finishes can push totals 20–40% higher. See cost methodology for the full regression model and the 8 California builder-pricing sources it's calibrated against.
California state law sets a floor that every city must meet. San Mateo may allow more than the minimums shown — confirm with the San Mateo planning department before finalising plans. See how we verify city-specific ordinance data.
| Rule | Standard |
|---|---|
| ADUs allowed on single-family lot | Yes — required by state law |
| Max units per lot | 1 ADU + 1 JADU on single-family; up to 2 detached ADUs + JADU on multifamily |
| Owner-occupancy required | Not allowed to require for ADUs (state preempts) |
| Side setback (detached) | 4 feet |
| Rear setback (detached) | 4 feet |
| Max height | 16 ft — 18 ft if within 0.5 mi of major transit |
| Off-street parking | None required if within 0.5 mi of public transit (state law applies in most metros) |
| Max ADU size | 850 sqft for studio/1BR; 1,000 sqft for 2+BR (city may allow more) |
| Min lot size | No minimum required by state |
| Permit timeline (ministerial) | 60 days from complete application (state law) |
San Mateo adopted its current ADU ordinance in 2018. Read the full ordinance text (PDF).
Annual ADU permits issued, 2013–2025. Sources: Big West Coast ADU Dataset for 2013–2017 and the California HCD Annual Progress Report (September 2026 release) for 2018–2025.
Active CA-licensed B-class general contractors with ADU-specialist branding, sourced from the CSLB Public Data Portal snapshot from April 28, 2026. Click any license number to verify current status at the CSLB Instant License Check — status can change at any time. See builder-directory methodology.
Most San Mateo ADU projects fall in a $100,000–$400,000+ range, all-in (design + permits + construction). The big drivers are size, type (detached costs more than a garage conversion), and finish level. See the per-type cost table above for typical ranges in this market.
Yes — every ADU in California requires a building permit. San Mateo processes ADU applications under the state's 60-day ministerial review rule (no public hearings, no discretionary review for code-compliant designs). Permit fees vary but typically run $5,000–$25,000 depending on size and impact fees.
State law guarantees a minimum allowance of 850 sqft (studio/1BR) or 1,000 sqft (2+BR). Many California cities allow larger detached ADUs — check the San Mateo planning department or the ordinance link in our rules table above for the local maximum.
No, not for an ADU. AB-976 (effective 2023) extended the state's ban on owner-occupancy requirements for ADUs through January 1, 2030 — California cities may not require an owner to live on the property as a condition of permitting an ADU during that window. JADUs are different: owner-occupancy of either the primary residence or the JADU itself is required by state law throughout the JADU's life.
A JADU (Junior ADU) is a unit ≤500 sqft created within an existing single-family home, sharing a wall with the main residence. JADUs are cheaper and faster to build, but they require owner-occupancy of either the main house or the JADU. Most homeowners build a separate ADU for rental flexibility.
According to the California ADU permit dataset compiled by BuildinganADU.com, San Mateo permitted 151 ADUs in 2025, the peak year on record (2013–2022). California-wide ADU permits have grown roughly 20× over that period — homeowner demand is outpacing every other housing-supply tool the state has tried.
UC Berkeley's Center for Community Innovation graded San Mateo's ordinance C in their 2020 ADU Scorecard, scoring the 2018 ordinance text. The mean grade across California was C+, so anything above that ranked as relatively homeowner-friendly at the time. This is historical, not current law: state-level updates since 2020 (AB-2221, AB-976, AB-1033, SB-477, SB-1211 and others) have raised the statewide floor and many cities have rewritten their local ordinances since the 2020 grading. Always check the current ordinance text linked above for what applies to a project you're about to start.