Two Detached ADUs in Atherton (AB 956, 2027)
Plan Two ADUs in Atherton: Free Site Assessment
Atherton ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires the Town of Atherton to approve up to two detached ADUs on a lot with a single-family home. Today the Town allows a second detached ADU only on lots of 2 acres or more, and only when it is deed-restricted for lower-income renters. Here is how the new law and the Town’s current ADU rules fit together.
- Units allowed now: one ADU plus one JADU within existing or proposed space, or one detached new-construction ADU combined with a JADU (Municipal Code Ch. 17.52, as amended by Ord. 665 in 2024 and Ord. 675 in 2025).
- Bonus unit: one extra detached ADU on lots of 2 acres or more, if deed-restricted for lower-income households and rented on terms of at least 9 months.
- Size: a new ADU may be 150 to 1,200 sq ft. A JADU may be up to 500 sq ft. The first 800 sq ft of an ADU is excluded from floor area.
- Height: 16 ft for a detached ADU, or 18 ft within a half mile of transit, plus 2 ft for a roof pitch that matches the house.
- Setbacks: 4 ft side and rear for an ADU of 800 sq ft or less, A detached ADU over 800 sq ft follows accessory-building setbacks and separation.
- Parking: one space per ADU, tandem allowed, with transit and conversion exemptions.
- Pre-approved plans: the Town lists vendor plans that have been pre-approved. Owners apply through the Building Department ePlan Submittal Portal with a designer authorization form.
State review of Atherton's ADU rules
In a letter dated December 14, 2021, the state housing department (HCD) found Atherton’s 2020 ordinance out of compliance. It flagged setbacks over 4 ft, an 11 ft sidewall height limit and limits on where ADUs could go. The Town has amended its code since, in 2024 and 2025. 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 requires the Town to approve up to two detached new-construction ADUs on a single-family lot. The Town may cap each at 800 sq ft of living space and apply height limits; side and rear setbacks can be no more than 4 ft. Where two detached ADUs are built, a JADU need not be approved, and HOA rules cannot block or unreasonably restrict the units. In Atherton, this means two detached units no longer depend on a 2-acre lot or a deed restriction. The Town will need to update Chapter 17.52, and we confirm current rules as it does.
Planning two detached ADUs on an Atherton lot
- Heritage trees: some ADUs are exempt from the Heritage Tree Ordinance under state law. Tree removal for an ADU can happen only after the building permit issues, and pre-approved ADUs need a Tree Protection Plan inspected first.
- Fire review: an approval letter from the Menlo Park Fire Protection District is required. An ADU needs sprinklers only if the main house requires them, and an ADU does not trigger sprinklers in the house.
- Water: CalWater is the water provider named in the Town’s pre-approved ADU process.
- Occupancy order: an ADU certificate of occupancy issues only after the main house has its own.
Building ADUs in Atherton with Axiom
See our Atherton ADU builder page and Atherton service area, read the AB 956 overview, or compare the rules in neighboring Menlo Park and Redwood City.
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.
Atherton Two-ADU FAQ
Can I build two detached ADUs in Atherton?
From January 1, 2027, AB 956 requires the Town 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, a second detached ADU is allowed only on lots of 2 acres or more with an affordability deed restriction.
How large can each ADU be in Atherton?
Today a new ADU in Atherton can be up to 1,200 sq ft, and the first 800 sq ft is excluded from floor area. Under AB 956, the Town may limit each of two detached ADUs to 800 sq ft of living space.
How tall can a detached ADU be in Atherton?
16 ft, or 18 ft within a half mile of transit, plus 2 ft for a roof pitch that matches the main house. ADUs are limited to two stories, not counting basements.
Do I need a fire district letter for an Atherton ADU?
Yes. The Town requires an approval letter from the Menlo Park Fire Protection District. Sprinklers are required in the ADU only if the main house requires them.
When can I remove a heritage tree for an ADU in Atherton?
Only after the building permit issues. Some ADUs are exempt from the Heritage Tree Ordinance under state law, and pre-approved ADUs need a Tree Protection Plan inspected before the permit.
Who builds two-ADU projects in Atherton?
Axiom Renovation designs, permits and builds ADUs in Atherton 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.