Two Detached ADUs in San Mateo (AB 956, 2027)
Plan Two ADUs in San Mateo: Free Site Assessment
San Mateo ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires cities, including San Mateo, to approve up to two detached ADUs on a lot with a single-family home. San Mateo updated its ADU rules in early 2026, raising detached heights so standard two-story ADUs can be approved by building permit alone. Here is how the new law and those rules fit together.
- Units allowed now: one attached or conversion ADU, one detached ADU and one JADU on a single-family lot (Municipal Code Chapter 27.19, Ordinance 2026-02, effective March 4, 2026).
- Size: up to 800 sq ft of ADU living area is exempt from floor area limits. An ADU cannot be larger than the main house, except that an 800 sq ft state-exempt ADU is allowed when the house is 800 sq ft or less. A JADU can be up to 500 sq ft.
- Height: a detached ADU can be 18 ft to the top plate and 24 ft to the peak. An attached ADU can be 24 ft to the plate and 32 ft to the peak.
- Setbacks: 4 ft side and rear, including ADUs over garages. No separation from the main house is required on single-family lots.
- Parking: no off-street parking is required for ADUs or JADUs, and no replacement parking.
- Process: a compliant ADU gets a ministerial building permit that cannot be appealed. Other designs go to Zoning Administrator review. The state 60-day rule applies.
State review of San Mateo's ADU rules
On October 18, 2024, the state housing department (HCD) sent San Mateo a letter with 17 findings. The city replied on November 15, 2024, accepting most of them. It disputed a 650 sq ft JADU size and an interior-access rule, and it proposed a higher plate height for two-story units. Ordinance 2026-02 resolved the letter, setting the JADU at 500 sq ft. Where local rules are stricter than state law, state law applies.
Recent Axiom work
Professional photos from a recent Axiom Renovation full remodel and addition in Los Gatos.
What changes on January 1, 2027
AB 956 raises the number of detached ADUs the city must approve on a single-family lot to two, both new construction. The city may limit each of the two to 800 sq ft of living space and apply its height limits, and side and rear setbacks can be no more than 4 ft. Where two detached ADUs are built, the city does not have to approve a JADU on the same lot. The law also limits how homeowners associations can restrict these units.
San Mateo now allows one detached ADU, so it will need to update Chapter 27.19 to allow a second. The 800 sq ft cap matches the floor area exemption the city already uses. The 18 ft plate and 24 ft peak limits can stay. We will confirm how the city combines the two-unit option with its attached-ADU and JADU rules.
Planning two detached ADUs on a San Mateo lot
- Trees: heritage tree rules apply. We check protected trees before placing two units.
- Flood zones: the city links the FEMA flood zone map from its ADU page. We check it for every lot.
- Fire: San Mateo Consolidated Fire reviews access. Sprinklers in an ADU are generally needed only if the main house has them, but long driveways or weak hydrant flow may need mitigation.
- Solar: new detached ADUs need solar under the Energy Code.
- Utilities: new or separate connections may be required. Two ADUs usually need a load calculation and often a service upgrade.
Building ADUs in San Mateo with Axiom
Read our San Mateo ADU guide, see our San Mateo service area, or start with the AB 956 overview. Nearby, see the guides for Hillsborough and Belmont.
Process
Our ADU Process
Step 1
Initial Consultation
We begin with a thorough initial consultation to understand your needs, assess your space, and discuss your vision, budget, and timeline.
Step 2
Planning and Design
At Axiom Renovation, we understand that the planning and design phase of an ADU project is crucial to its success. Our dedicated project managers will work closely with you to understand your goals, design ideas and budget for the project.
Step 3
Material Selection
We guide you through selecting high-quality materials that fit your design and budget, ensuring durability and aesthetic appeal.
Step 4
Demolition and Preparation
Careful demolition and preparation work are performed to ready the space for construction, minimizing disruptions and ensuring a clean work environment.
Step 5
Construction and Installation
Our skilled craftsmen handle the construction phase, including obtaining permits, scheduling appointments, and the actual construction and installation with precision, building the new space according to the approved design.
Step 6
Electrical and Plumbing Work
Licensed professionals conduct all electrical and plumbing work, ensuring everything is up to code and seamlessly integrated into the new ADU.
Step 7
Painting and Finishing
We pay close attention to detail during the painting and finishing stage, ensuring all surfaces are properly prepared and beautifully finished.
Step 8
Fixture and Appliance Installation
We expertly install all fixtures and appliances, ensuring they function correctly and fit perfectly with the overall design.
Step 9
Final Touches and Clean-Up
Final touches are added, and a thorough clean-up is performed, leaving your new space pristine and ready for use.
Step 10
Final Walkthrough and Client Satisfaction
We conduct a final walkthrough with you to ensure every detail meets your expectations and address any remaining concerns, ensuring your complete satisfaction.
San Mateo Two-ADU FAQ
Can I build two detached ADUs in San Mateo?
From January 1, 2027, AB 956 requires the city to approve up to two detached, new-construction ADUs on a lot with a single-family home, if the project meets objective standards. Today San Mateo allows one detached ADU, one attached or conversion ADU and one JADU.
How large can each ADU be in San Mateo?
Up to 800 sq ft of ADU living area is exempt from floor area limits today. Under AB 956, the city may limit each of two detached ADUs to 800 sq ft of living space.
Can a detached ADU in San Mateo be two stories?
Yes. Since March 2026 a detached ADU can be 18 ft to the top plate and 24 ft to the peak, which allows a standard two-story design by building permit.
Do I need parking for an ADU in San Mateo?
No. San Mateo requires no off-street parking for ADUs or JADUs and no replacement parking.
Do I need solar on a new San Mateo ADU?
Yes. New detached ADUs must include solar under the California Energy Code.
Who builds two-ADU projects in San Mateo?
Axiom Renovation designs, permits and builds ADUs in San Mateo and is planning two-unit projects for 2027. Call 408-702-2022, Monday to Friday, 9 am to 5 pm.
Sources: AB 956 (Quirk-Silva), signed September 29, 2026, effective January 1, 2027, as summarized by Brownstein Hyatt Farber Schreck and California YIMBY. This page is general information, not legal advice. Local rules change; we confirm current requirements with the city for every project.