Two Detached ADUs in Millbrae (AB 956, 2027)
Plan Two ADUs in Millbrae: Free Site Assessment
Millbrae ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires cities, including Millbrae, to approve up to two detached ADUs on a lot with a single-family home. Millbrae’s code keeps detached ADUs to one story and generally in the rear half of the lot, so fitting two units takes careful planning. Here is how the new law and the city’s current rules fit together.
- Units allowed now: the city’s ADU code (Title 10, Article XXXII, Ordinance 782 as amended by Ordinance 804) allows one ADU and one JADU on a single-family lot.
- Size: the code allows 150 sq ft minimum, and up to 850 sq ft for a studio or one-bedroom or 1,000 sq ft with more than one bedroom. An attached ADU can be no more than half the home’s floor area. A JADU can be up to 500 sq ft.
- Height: 16 ft for a detached ADU, plus 2 ft to match the roof pitch. Detached ADUs are one story, with a 9 ft plate height and up to 36 inches of crawlspace.
- Setbacks and placement: 4 ft side and rear, and 5 ft from other buildings. Detached ADUs generally go in the rear half of the lot.
- Parking: one space per ADU, which can be tandem or in a setback. State exemptions apply, and no replacement parking is needed.
- Process: ministerial approval within 60 days when a home already exists on the lot.
We did not find a city portal or pre-approved plan program for Millbrae, so we confirm the current submittal steps with the city on each project.
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.
Millbrae’s code allows one ADU per lot, so the city will need to update Article XXXII. Its 850 and 1,000 sq ft limits are above the 800 sq ft each unit may be capped at. Its one-story and 16 ft rules can stay, since the law lets cities apply height limits. The 5 ft separation and rear-half placement will matter most when two units share a backyard. We will confirm the city’s new rules as it adopts them.
Recent Axiom work
Professional photos from a recent Axiom Renovation full remodel and addition in Los Gatos.
Planning two detached ADUs on a Millbrae lot
- Backyard layout: two one-story units, each 5 ft from other buildings and 4 ft from side and rear lines, need a site plan that works before design starts.
- Sprinklers: the code does not require sprinklers in an ADU if the main house does not need them.
- Electrical: two ADUs usually need a load calculation and often a service upgrade.
- Rentals: ADUs and JADUs must be rented for 30 days or more. Short-term rentals are not allowed.
Building ADUs in Millbrae with Axiom
See our Millbrae ADU builder page and Millbrae service area, or read the AB 956 overview. Nearby, see the guides for San Bruno and Burlingame.
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.
Millbrae Two-ADU FAQ
Can I build two detached ADUs in Millbrae?
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. Until then, Millbrae’s code allows one ADU and one JADU.
How large can each ADU be in Millbrae?
Today the city’s code allows up to 850 sq ft for a studio or one-bedroom and 1,000 sq ft with more than one bedroom. Under AB 956, the city may limit each of two detached ADUs to 800 sq ft.
Can a detached ADU in Millbrae be two stories?
Not under the current code. Detached ADUs are limited to one story and 16 ft, plus 2 ft to match the roof pitch. AB 956 lets cities keep height limits.
How long does ADU approval take in Millbrae?
When a home already exists on the lot, the city must act on a complete ADU application within 60 days. If an ADU is submitted with a new house, it is acted on after the house is approved.
Where can a detached ADU go on a Millbrae lot?
Detached ADUs generally go in the rear half of the lot, 4 ft from side and rear lines and 5 ft from other buildings. That spacing shapes how two units fit.
Who builds two-ADU projects in Millbrae?
Axiom Renovation designs, permits and builds ADUs in Millbrae 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.