Permit guides

ADU Permits in San Mateo: a Homeowner's Guide

What you can build, which permits you need, and how the review actually works — in plain language.

Last updated October 2026. Rules change; always confirm current requirements with the city or county before you build.

Who handles your permit

ADU permits in the city go through the Community Development Department at 330 W. 20th Avenue, San Mateo, CA 94403 — (650) 522-7172. Applications and plan resubmittals run through the city's CSS online portal as digital PDFs, not paper plan sets. The San Mateo Consolidated Fire Department weighs in on fire-sprinkler determinations. Don't confuse the city with San Mateo County: if your property is in an unincorporated pocket, the county planning office runs a separate process with its own rules.

What you can build

California state law sets the baseline, and San Mateo's ordinance follows it:

1,200 sq ftMax for a detached ADU

16 ftHeight limit for most detached ADUs

4 ftMinimum rear and side setbacks for detached ADUs

1 + 1One ADU plus one JADU allowed per single-family lot

750 sq ftADUs below this size are exempt from impact fees

60 daysState review clock, once the application is deemed complete

Pre-approved ADU plans gallery

The city runs an ADU Plans Gallery (cityofsanmateo.aduaccelerator.org) with 17 pre-reviewed designs, from 340 to 1,110 square feet — studios, one- and two-bedroom layouts, mostly prefab plus one site-built. Use a gallery plan unaltered and you pay a reduced $1,455.43 permit fee.

Eligibility is the catch: the lot must be residential, outside a high-hazard FEMA flood zone and the wildland-urban interface, on a slope under about 15%, and free of easements and open code-enforcement cases. Even with a gallery plan, review still checks site placement, setbacks, utilities, and drainage for your specific property.

Pre-approved doesn't mean permit-free. You still need a building permit, and reviewers still verify site placement, foundation conditions, utility connections, and fire access for your specific property.

What goes into the submittal

Site plan with zoning, lot size, floor-area ratio, setbacks, and easements marked
Floor plans, exterior elevations, foundation and framing details
Structural calculations and Title 24 energy compliance
Utility connection plans — water, sewer, electrical
Fire-sprinkler determination per the Consolidated Fire Department's standard (no sprinklers for the ADU unless the main house requires them)
Arborist report and tree-protection plan if work is near protected or heritage trees (Municipal Code Chapter 13.40)
Proof of ownership — a grant deed or a preliminary title report under a year old

Where applications usually get stuck

Flood zone, WUI, or steep lots. The pre-approved shortcut doesn't apply here, and full review gets harder — confirm eligibility before committing to a design.
Heritage and protected trees. San Mateo guards its tree canopy closely; removals or nearby work trigger arborist review.
Unmarked easements that shrink where the ADU can legally sit.
Incomplete submittals. The state's 60-day clock starts only when the application is deemed complete — missing structural details or energy forms stop it.
Garage conversions that need foundation, egress, ceiling-height, and fire-separation upgrades.

Not sure where to start? That's the part we do. We look at your property, map the requirements, coordinate the plans and specialists, and manage the submittal and corrections — one point of contact.

Tell us what you need help with.

Not sure where to start? That's what we're here for.

Send your project

Erik — (916) 540-8960 · Vall — (916) 956-8106
office@pcg-engineering.pro