Building an ADU in San Mateo

What you can build in San Mateo

San Mateo allows ADUs and junior ADUs (JADUs) in every zoning district that allows homes. The city says the approval process for a compliant ADU or JADU is "ministerial, with only a building permit and related permits required." You don't need a planning application or public notice.

• Single-family lot: you can combine one attached or conversion ADU, one new detached ADU, and one junior ADU inside the walls of the house.
• Multifamily lot with existing buildings: up to eight new detached ADUs, but no more than the number of existing homes on the lot. You can also convert non-living space (such as storage rooms) into ADUs: at least one, and up to 25% of the existing units.
• Multifamily lot with a new building: up to two new detached ADUs.

Every ADU and JADU needs its own exterior entrance. The city assigns each one a separate address before the permit is issued.

If your ADU can't meet every standard (for example height, setbacks or daylight plane), you can ask the Zoning Administrator for discretionary review instead. That route requires public notice, and the decision can be appealed. It can't be used to add floor area beyond what the lot allows. [1][2][3][4][5][6]

Size, height and setbacks

San Mateo's code sets no fixed maximum size for a new ADU. Up to 800 sq ft of ADU living area is exempt from your lot's floor area ratio (FAR). If you want a bigger ADU, the extra area must fit within the FAR left on your lot. An ADU also can't be larger than the main home. If the main home is 800 sq ft or less, you can still build an 800 sq ft "state exempt" ADU. Converting space inside existing walls has no size limit, and you may add up to 150 sq ft for a way in and out.

HCD, the state housing agency, says a city's size limits must allow an ADU of at least 850 sq ft, or 1,000 sq ft with more than one bedroom. In its October 2024 letter to San Mateo, HCD said the city "must also allow, at minimum," those sizes. If FAR or the size of your main home would hold your ADU below that, ask the Planning Division how the current code applies.

Height. A detached ADU can be 18 feet to the top plate line and 24 feet to the roof peak, measured from existing grade. An attached ADU can be 24 feet to the top plate line and 32 feet to the roof peak.

Setbacks. New construction needs 4 feet from side and rear lot lines, including an ADU built over an existing garage. No setback applies when you convert a garage or accessory building in the same spot, at the same size and height. A detached ADU in the rear third of the lot that meets the city's accessory building rules also needs no setback. Over 800 sq ft, a new ADU must meet the zone's front setback. A state exempt ADU (an ADU created under Gov. Code 66323, such as a new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks) needs no front setback.

Design rules for larger ADUs. ADUs that aren't state exempt must stay within a daylight plane that starts 12 feet up at the side and rear lot lines and rises at 45 degrees. Second-story windows within 5 feet of a neighbor's lot line need obscured glass up to 5 feet above the floor. Rooftop decks are not allowed, and a second-story deck or balcony can be no more than 50 sq ft. [1][3][7][8][9][10]

Parking

San Mateo requires no off-street parking for any ADU or JADU. If you convert or tear down a garage, carport or parking space to build the ADU, you don't have to replace that parking for the main home.

You can add parking voluntarily: up to two spaces per ADU or JADU, on the same lot, including tandem spaces or uncovered spaces in setback areas. If you convert a garage, you can keep the existing driveway and curb cut. The driveway for a single-car garage can be widened to 20 feet. [1][3]

Fees

The city charges flat permit fees per unit: $2,831.57 for a new ADU, $2,031.12 for an ADU remodel, and $1,454.02 for a JADU. Other permitting fees follow the city's Comprehensive Fee Schedule.

San Mateo's code says "no impact fees shall be charged for ADUs 750 square feet in size or smaller." Above 750 sq ft, impact fees are charged in proportion to the size of the main home. School district fees may also apply. HCD says school districts may charge fees on ADUs larger than 500 sq ft, so check with your school district.

Utility connections: a conversion ADU needs no new water, sewer or power connection and pays no connection fee or capacity charge. For a new ADU separate from existing buildings, the city may require new utility connections and will charge a connection fee or capacity charge proportionate to the ADU's burden.

See our guides to ADU property tax, ADU costs and financing. [1][2][4][10]

How long it takes

State law sets the clock. The city must tell you whether your application is complete within 15 business days. When there's already a home on the lot, it must approve or deny a complete application within 60 days. If it denies the application, it must give you a full set of written comments on how to fix it. See our ADU permit process guide.

San Mateo's code says a ministerial ADU or JADU permit, once issued, "may not be appealed." [10][11]

Garage conversions and junior ADUs

You can convert an attached or detached garage, or another accessory building, into an ADU. A conversion in the same location and footprint needs no setback and has no size cap. You don't have to replace the lost parking. See our garage conversion guide.

A junior ADU must be inside the walls of an existing or proposed single-family home. That can include an attached garage, but not a detached garage or carport. It can be up to 500 sq ft and needs:

• its own exterior entrance (plus an inside door to the house if it shares a bathroom or utility controls such as the thermostat);
• an efficiency kitchen with a cooking appliance, sink, refrigerator, cabinets and counter;
• a smoke alarm connected to the main home's alarms, and a carbon monoxide detector.

If the JADU shares a bathroom with the house, the owner must live in the JADU or the main home. No owner occupancy is required if it has its own bathroom. City staff draft a deed restriction for you to sign and record before the permit is issued. It says the JADU can't be sold separately and records its size and features. JADUs are not allowed on multifamily lots. [1][3][8][12]

Renting and selling

You may rent an ADU or JADU, but not for less than 30 consecutive days, so short-term rentals are out. San Mateo does not require the owner to live on the property for an ADU. Owner occupancy applies only to a JADU that shares a bathroom with the main home.

An ADU can't be sold separately from the main home "except as allowed by Government Code Section 66341." HCD describes that exception as narrow: it applies when a qualified nonprofit built or developed the property and sells the ADU to a qualified buyer. [8][10][12][13]

Legalizing an unpermitted ADU

If your ADU or JADU was built without permits before January 1, 2020, the city follows state law (Gov. Code 66332). It "will not deny a permit for an unpermitted ADU/JADU built before 2020 solely because it does not meet building standards or state or local standards for ADUs/JADUs." It can still deny a permit if the unit is "substandard" under Health and Safety Code 17920.3.

You sign an affidavit that the unit was built before 2020 and add proof such as dated photos, receipts, utility bills or assessor records. The city then inspects the unit for health and safety items, such as a working toilet, shower or tub, kitchen sink and hot and cold water, safe wiring, plumbing and heat, no mold or pests, and adequate exits. You can hire a licensed contractor for a confidential inspection first. The city says you can't be charged impact fees or connection fees unless new utility work is needed for health and safety. Start with the city's AB 2533 handout. [14]

Lot splits and duplexes (SB 9)

San Mateo applies state law SB 9 through Chapter 27.21, the Two-Unit Development Overlay for R1 single-family zones. It allows up to two primary homes on one lot, and a lot that uses it is capped at four units in total. SB 9 homes need 4-foot side and rear setbacks. A detached SB 9 home can be 16 feet to the top plate and 24 feet to the roof peak. Each SB 9 home needs one parking space unless the lot is within a half mile walk of a major transit stop or high-quality transit corridor, or within one block of a car-share spot. Units can't be rented for less than 30 days. Some lots don't qualify, such as historic properties, hazard areas, and lots where the project would demolish or alter housing rented by a tenant in the last three years. See our SB 9 lot split guide. [15]

Where San Mateo and state law differ

In October 2024, HCD reviewed San Mateo's 2022 ADU ordinance and found it did not comply with state ADU law in several ways. Its findings included the ban on ADUs in front setbacks, size limits, height measurement and parking exemptions. The city responded in November 2024 and adopted an updated ordinance, which took effect March 4, 2026. The new code removed the front-setback ban for state exempt ADUs, raised detached ADU height to 18 feet at the plate line, and dropped ADU parking.

One point to watch: HCD's letter said the city must allow ADUs of at least 850 sq ft (1,000 sq ft with two or more bedrooms). The 2026 code still ties ADUs over 800 sq ft to the lot's FAR and caps an ADU at the size of the main home. If those limits would keep your ADU smaller than the state minimums, raise it with the Planning Division and read our California ADU law overview. [1][3][9][10][16]

Permit steps in San Mateo

1. Check your zoning and lot conditions

   Look up your zoning district with the city's Property Zone Look-Up Tool. Check whether you are in a FEMA flood zone and whether protected (heritage) tree rules apply. [2]

2. Review the ADU standards

   Read the Single Family ADU and JADU Handout and the ADU Development Standards Table. The Planning counter offers code consultations Monday to Friday, 8:30 to 11:30 a.m., with afternoons by appointment. [1][2]

3. Prepare plans or pick a pre-approved design

   Have plans drawn for your lot, or choose a design from the ADU Plans Gallery and ask its designer to add a site plan. Pre-approved designs can't be changed. [2][17]

4. Apply online

   Register on the city's Customer Self-Service (CSS) Portal, then choose Apply and upload your plans and documents. Include a request for a separate address. [4][18]

5. Pay fees and answer review comments

   Pay the application fees online; the project then goes to Building staff for review. You get comments and upload resubmittals through the portal. [18]

6. Record the JADU deed restriction (if building a JADU)

   City staff draft the deed restriction. You sign and record it before the permit is issued. [1]

7. Get your permit and build

   The city notifies you when plans are ready and the permit is issued. Then construction can start, with city inspections along the way. [18]

Pre-approved and sample plans

• ADU Plans Gallery (pre-approved ADU plans)

  Detached ADU designs pre-approved by the city; you pick one, the designer adds site details, and the city reviews only the site-specific part for a reduced fee. Not available in high-hazard flood zones, wildfire (WUI) areas, on steep slopes or with active code cases. [2][17]

Who to call

• Planning Division

  ADU zoning standards and code consultations (planning@cityofsanmateo.org)

  (650) 522-7212

  San Mateo City Hall, 330 West 20th Avenue, San Mateo, CA 94403

  [2]

• Building Division

  Building permits, plan review and inspections (building.info@cityofsanmateo.org)

  (650) 522-7172

  [2]

• Public Works Department

  Utility connections and public right-of-way (publicworks@cityofsanmateo.org)

  (650) 522-7300

  [2]

• San Mateo Consolidated Fire

  Fire sprinkler and fire access questions (fire@smcfire.org)

  (650) 522-7940

  [1][2]

Questions people ask