Two Detached ADUs in Hillsborough (AB 956, 2027)
Plan Two ADUs in Hillsborough: Free Site Assessment
Hillsborough ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires cities and towns, including Hillsborough, to approve up to two detached ADUs on a lot with a single-family home. Hillsborough’s ADU rules were reviewed by the state in 2024, and several of them must yield to state law. Here is what we know and how the new law applies.
- Code: Hillsborough’s ADU rules are in Municipal Code Chapter 17.52. The state reviewed Ordinance 787, adopted December 12, 2022. The town may have amended it since, so we confirm the current text on each project.
- Units allowed: Ordinance 787 allowed one ADU and one JADU on a single-family lot. State law also allows a conversion ADU plus a detached ADU plus a JADU.
- Height: generally 16 ft for a detached ADU.
- Front setback: the ordinance set a 25 ft front setback. It cannot be used to block an 800 sq ft ADU with 4 ft side and rear setbacks.
- Process: ministerial review within 60 days.
State review of Hillsborough's ADU rules
On May 30, 2024, the state housing department (HCD) sent Hillsborough a letter with 11 findings on Ordinance 787. HCD said the town must allow the state’s unit combinations, and it flagged the 30-day JADU rental minimum, owner occupancy for ADUs permitted after 2025, deed restriction, income reporting and neighbor-notice steps, an incomplete maximum-size rule, the detached height and the 25 ft front setback. It also asked the town to say it “shall approve or deny” within 60 days. 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 town must approve on a single-family lot to two, both new construction. The town 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 town does not have to approve a JADU on the same lot. The law also limits how homeowners associations can restrict these units.
Hillsborough will need to update Chapter 17.52 to allow two detached units. The town may cap each at 800 sq ft and keep a height limit. Its front setback still cannot stop a qualifying unit. We will confirm the town’s new rules as it adopts them.
Planning two detached ADUs on a Hillsborough lot
- Site layout: we map setbacks, slopes and access first, then place two units where utilities can reach both.
- Fire review: we confirm fire hazard mapping and fire access for each unit with the fire authority before design.
- Electrical: two ADUs usually need a load calculation and often a service upgrade.
- Town steps: we confirm with the town which of the steps HCD flagged, such as deed restrictions or neighbor notice, still apply.
Building ADUs in Hillsborough with Axiom
See our Hillsborough ADU builder page and Hillsborough service area, or read the AB 956 overview. Nearby, see the guides for Burlingame and San Mateo.
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.
Hillsborough Two-ADU FAQ
Can I build two detached ADUs in Hillsborough?
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.
How large can each ADU be in Hillsborough?
Under AB 956, the town may limit each of two detached ADUs to 800 sq ft of living space. HCD found the town’s current maximum-size rule incomplete, so we confirm the size limit with the town on each project.
How tall can a detached ADU be in Hillsborough?
Ordinance 787 set a detached ADU height of generally 16 ft. AB 956 lets the town apply height limits to the two units.
How long does ADU approval take in Hillsborough?
State law requires the town to approve or deny a complete ADU application within 60 days.
Does Hillsborough’s 25 ft front setback apply to ADUs?
Not if it would block an 800 sq ft ADU with 4 ft side and rear setbacks. HCD flagged this in its 2024 letter, and state law controls.
Who builds two-ADU projects in Hillsborough?
Axiom Renovation designs, permits and builds ADUs in Hillsborough 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.