San Mateo Zoning and ADU Guide for Homeowners

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Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team.

San Mateo reviews most ADUs through a ministerial building permit, with no public hearing, under Chapter 27.19 of its municipal code, and the city’s handout was last updated in March 2026. This guide covers the rules for a home in the City of San Mateo, what they mean for a sale, and who to call to confirm details for your lot. It applies to the city, not to unincorporated San Mateo County.

San Mateo ADU rules at a glance

Allowed per single-family lot	One attached or conversion ADU, one detached ADU and one JADU
Review path	Ministerial building permit
Side and rear setbacks	4 feet
Height, attached	24 ft to the plate, 32 ft to the peak
Height, detached	18 ft to the plate, 24 ft to the peak
JADU max size	500 sq ft
Parking	Not required; no replacement parking
Short-term rentals	Not allowed (under 30 days)
Planning	(650) 522-7212, planning@cityofsanmateo.org, 330 W 20th Ave

How San Mateo regulates ADUs

San Mateo’s ADU ordinance is Chapter 27.19 of the municipal code, effective April 22, 2022, and the city’s handout was updated March 4, 2026. A compliant ADU is approved through a building permit with no discretionary hearing.

The city also offers an alternative discretionary path through the Zoning Administrator under Section 27.19.045 for projects that do not meet every standard. That path involves public notice and can be appealed, so it takes longer than ministerial review.

What you can build

On a single-family lot, the city allows one attached or conversion ADU, one detached ADU and one junior ADU. Each ADU needs a separate entrance.

An ADU generally must be smaller than the primary home unless the primary home is under 800 square feet, and up to 800 square feet of ADU area is exempt from the floor area ratio calculation.

A junior ADU is capped at 500 square feet, must be created within the existing single-family home and requires a deed restriction and an efficiency kitchen. If the JADU shares a bathroom with the main house, owner occupancy is required.

The JADU does not trigger the fire wall or noise separation requirements that apply to some other units.

Height, setbacks and design standards

Attached ADUs may reach 24 feet at the plate and 32 feet at the peak, and detached ADUs 18 feet at the plate and 24 feet at the peak. Side and rear setbacks are 4 feet.

ADUs of 800 square feet or less that qualify for state exemptions need no front setback and are exempt from the daylight plane, which otherwise starts at 12 feet and slopes inward at 45 degrees.

Architectural standards include obscured second-story windows within 5 feet of a lot line, a 5-foot setback for stairs and a limit of 50 square feet for decks.

Parking, utilities and fire

San Mateo does not require parking for ADUs and does not require replacement of parking lost to a garage conversion. If you choose to provide parking, the handout allows up to two stalls, permits tandem layouts and permits driveway parking up to 20 feet.

Detached ADUs may need their own utility connections, and the city assigns separate addresses. Whether fire sprinklers are required depends on the main house and the Fire Marshal. New detached ADUs that are not manufactured homes must include solar. Fees follow the city’s fee schedule, plus school district fees.

Rental and ownership rules

ADUs cannot be rented for fewer than 30 days, so short-term rentals are out. An ADU cannot be sold separately from the main property. For sellers, this means a San Mateo ADU adds value through the whole property sale, and a buyer takes both units together.

If your unit was built without a permit

Garage conversions and backyard units that predate the current ordinance are common in San Mateo. If you suspect a unit was built without permits, start with Planning at (650) 522-7212 and ask whether it could qualify under today’s standards. Our San Mateo code enforcement guide explains how complaints and corrections work, and our statewide guide to unpermitted work explains seller disclosure duties. A short checklist:

Find any permit or inspection records for the unit.
Check the unit against the height, setback and entrance standards above.
Ask Planning whether it could be approved and what it would cost.
Collect leases and utility information if the unit is rented.
Compare the cost of legalizing it with a no-repairs sale.

Selling a San Mateo house with an ADU

A permitted ADU is a selling point, but it adds documents buyers check, including permits, final inspection and the JADU deed restriction. An unpermitted unit must be disclosed, and it can complicate lender financing.

Cash buyers purchase as-is, which can save time if you do not want to fund permit work or manage tenants during a listing. San Mateo County property taxes and transfer taxes also come into play; see our San Mateo County property tax guide.

How to sell a house in San Mateo

List every unit and addition on the property and note which are permitted.
Gather permits, deed restrictions and leases.
Request a no-obligation cash offer for the house as it stands.
Choose your closing date.
Close with no repairs or showings required.

Related local pages: sell as-is in San Mateo, sell fast, cash home buyers and the cash offer process.

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Frequently asked questions

How many ADUs can I have on a San Mateo single-family lot?

One attached or conversion ADU, one detached ADU and one JADU.

Does San Mateo require parking for an ADU?

No. Parking is not required and replacement parking is not required for garage conversions.

Can I rent my ADU on a short-term basis?

No. Rentals under 30 days are not allowed, and the ADU cannot be sold separately.

How big can a JADU be?

Up to 500 square feet, within the existing single-family home.

Do detached ADUs need solar?

The city’s handout says new detached ADUs that are not manufactured homes must include solar.

Can I sell a San Mateo house with an unpermitted unit?

Yes. You can disclose it and sell as-is, including to a cash buyer.

More guides

Sources

This guide is general information from public sources, not legal advice. Confirm current rules with the city.