San Mateo / ADU construction
ADU Construction in San Mateo
San Mateo sets no fixed ADU size cap. Floor area ratio does the limiting.
Under San Mateo's ADU ordinance, updated March 4, 2026, the first 800 square feet of an ADU do not count toward the lot's floor area ratio. We design and build ADUs here from the zoning check to final, with the permit submitted through the city's online portal.
SMMC CHAPTER 27.19
Four ADU types, four sets of limits.
San Mateo's development standards table, revised August 31, 2026, sets different limits for each ADU type on a single-family lot. One of each can be combined on the same lot.
| Detached | Attached | Conversion | Junior ADU | |
|---|---|---|---|---|
| Maximum size | 800 sq ft FAR-exempt, more within FAR | 800 sq ft FAR-exempt, more within FAR | Existing space | 500 sq ft |
| Height | 18 ft plate, 24 ft peak | 24 ft plate, 32 ft peak | Existing structure | Within the house |
| Side and rear setbacks | 4 ft | 4 ft | Existing | Zoning district |
| Daylight plane | 12 ft at 45 degrees, unless state exempt | 12 ft at 45 degrees, unless state exempt | Not applicable | Zoning district |
| Owner occupancy | Not required | Not required | Not required | Required if bathroom is shared |
| Parking | None required | None required | None required | None required |
Source: San Mateo: ADU resources and development standards chart ↗
SAN MATEO ADU TYPES
800 square feet free, the rest from FAR.
San Mateo does not set a maximum ADU size. Up to 800 square feet is exempt from floor area ratio; anything larger must fit the lot's remaining FAR and stay smaller than the main home. A detached ADU can reach 18 feet to the top plate and 24 feet to the peak, so a two-story ADU is possible when it fits the daylight plane.
Detached ADU
18 feet to top plate and 24 feet to peak, 4 foot side and rear setbacks, inside a 12 foot, 45 degree daylight plane unless it is a state exempt ADU of 800 square feet or less.
Attached ADU
24 feet to top plate and 32 feet to peak. It must stay smaller than the main home unless that home is 800 square feet or less.
Conversion ADU
Garages and accessory structures converted in place keep their setbacks and have no size limit, with up to 150 square feet added for an entrance.
Junior ADU
Up to 500 square feet within the walls of the house or an attached garage. Detached garages cannot become a JADU, and a deed restriction is recorded.
Pre-approved plans
Designs in the city's ADU Plans Gallery are pre-approved. The site-specific permit fee is $1,455.43, and the design cannot be changed.
Two-story details
Second-story windows within 5 feet of a neighbor use obscured glass, decks are capped at 50 square feet and cannot face neighbors, and rooftop decks are prohibited.
SAN MATEO AT A GLANCE
Who handles a San Mateo ADU.
Planning sets the standards, the Building Division issues the permit through the Customer Self-Service portal, and San Mateo Consolidated Fire and Public Works review their parts. A compliant ADU needs no public hearing.
- 68%
- of San Mateo homes built before 1980
- 800
- square feet of ADU area exempt from floor area ratio
- $2,831.57
- flat city permit fee for a new ADU
Planning
ADU standards and counter help
The code consultation counter is open Monday to Friday, 8:30 to 11:30am, with afternoon consultations by appointment. Email planning@cityofsanmateo.org.
(650) 522-7212ADU information ↗Permits
Building Division and CSS portal
Residential permits, including ADUs, are submitted, paid for and resubmitted through the Customer Self-Service portal. Email building.info@cityofsanmateo.org.
(650) 522-7172Online Permit Center ↗Fire
San Mateo Consolidated Fire
Fire review is part of ADU permitting. Contact fire@smcfire.org for fire code questions on access and sprinklers.
(650) 522-7940Utilities
PG&E plus WestLight Energy
PG&E delivers power and gas; WestLight Energy, the new name of Peninsula Clean Energy since July 2026, supplies the electricity. WestLight's appliance rebates exclude new construction, including ADUs.
WestLight appliance rebates ↗
BEFORE YOU DESIGN
What to check on a San Mateo lot.
Check first
- Zoning district, with the city's Property Zone Look-Up Tool
- FEMA flood zone, on the current 2024 map
- Heritage trees: oaks 10 inches or more, other species 15 inches, at 54 inches high
- Remaining floor area if the ADU will exceed 800 square feet
A pre-approved plan fits when
- The design comes unchanged from the city's ADU Plans Gallery
- The designer prepares the site plan and site-specific details
- The lot meets the setbacks for that footprint
- The Phase 2 fee of $1,455.43 is paid with the submittal
Expect a longer path when
- The ADU misses an objective standard and needs Zoning Administrator review
- An existing unit was built without permits
- The lot sits in a FEMA special flood hazard area
- A heritage tree stands in the footprint or the utility trench
Planning the work
How a San Mateo ADU gets built.
Site check
Free visit. Zoning district, remaining FAR, FEMA flood zone, heritage trees and utility routes.
Design
Custom plans or a pre-approved gallery design. A compliant ADU needs no planning application or public notice.
Permit
Submittal through the Customer Self-Service portal, the flat ADU fee, then building, fire and public works review.
Build and final
Foundation to final, with each inspection requested online by 3pm the day before.
WHAT SLOWS ADUS IN SAN MATEO
Problems San Mateo owners run into, and how we plan around them.
From the city's ADU, flood map and tree permit pages, the ADU ordinance.
A heritage tree stands where the ADU or the utility trench should go.
Trees are measured at the site visit; heritage trees need a city permit to remove or prune, so the footprint and trench route are drawn around them.
The lot was in a FEMA flood zone, and nobody is sure if it still is.
We check the October 2024 map revision, which removed many North Central and North Shoreview lots from the high-risk zone, but not every parcel.
A second-story ADU runs into the daylight plane and privacy rules.
We test the 12 foot, 45 degree plane early, or keep the unit at 800 square feet or less as a state exempt ADU where it fits.
An existing unpermitted unit draws code enforcement.
Units built before 2020 can use the AB 2533 path; we pre-inspect against the city's health and safety checklist before applying.
Counter help is only available in the morning.
Questions are batched for the 8:30 to 11:30am consultation hours, or booked as afternoon appointments in advance.
Sources: cityofsanmateo.org ADU, FEMA flood map and tree permit pages; SMMC 27.19.
How big can an ADU be in San Mateo?
There is no fixed maximum. Up to 800 square feet of ADU living area is exempt from the lot's floor area ratio; a larger ADU must fit the remaining FAR and be smaller than the primary home, unless that home is 800 square feet or less. Junior ADUs are capped at 500 square feet.
How tall can a detached ADU be in San Mateo?
18 feet to the top plate and 24 feet to the roof peak, measured from existing grade, under SMMC 27.19.060. Attached ADUs can reach 24 feet to the plate and 32 feet to the peak, and nonexempt ADUs must fit a daylight plane that starts 12 feet up at the side and rear lot lines.
What are the ADU permit fees in San Mateo?
$2,831.57 for a new ADU, $2,031.12 for a remodel-type ADU and $1,454.02 for a junior ADU, charged as flat per-unit fees according to the city's ADU page. A pre-approved gallery design is permitted with a site-specific fee of $1,455.43.
Do I need a planning application for an ADU in San Mateo?
No, if the ADU meets the ordinance's objective standards. Approval is ministerial with only a building permit; an ADU that misses a standard can seek discretionary review by the Zoning Administrator, with public notice and the right to appeal.
Can I legalize an unpermitted ADU in San Mateo?
Yes, if it was built before January 1, 2020. Under state law AB 2533 the city will not deny a permit only because the unit misses current standards, but it requires a signed construction-date affidavit with evidence and inspects for substandard conditions under Health and Safety Code 17920.3.
Does an ADU in San Mateo need parking?
No. No off-street parking is required for an ADU or JADU, and none is needed to replace a converted or demolished garage. Up to 2 voluntary stalls per unit are allowed, including tandem stalls, and the old driveway and curb cut can stay.
