Two Detached ADUs in Alamo (AB 956, 2027)
Plan Two ADUs in Alamo: Free Site Assessment
Alamo ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires Contra Costa County, which issues ADU and building permits in unincorporated Alamo, to approve up to two detached ADUs on a lot with a single-family home. The county rewrote its ADU code in May 2025.
- Code: County Ordinance Code Chapter 82-24, amended in its entirety by Ordinance No. 2025-07, adopted May 13, 2025.
- Units allowed now: the county’s ADU code allows an internal conversion ADU with a JADU, or one detached new-construction ADU, which may be combined with a JADU.
- Detached ADU size: 1,000 sq ft generally and 1,200 sq ft on lots of 12,000 sq ft or more. JADUs are 500 sq ft.
- Height: the state’s 16, 18 and 25 ft limits apply to any part of an ADU in a front, side or rear yard.
- Setbacks: 4 ft side and rear; none for conversions or same-footprint rebuilds.
- Parking: one space, with transit and other exemptions.
- Process: a ministerial ADU application to the Department of Conservation and Development first, then the building permit through the county’s ePermits portal. Review is ministerial, and state law sets a 60-day decision deadline.
- Pre-approved plans: the county Pre-Approved ADU (PADU) program.
State review of the county's ADU rules
On December 24, 2024, HCD sent the county seven findings on Ordinance No. 2023-13. They covered recent state law changes, the unit allowance (a converted ADU, a detached ADU and a JADU together), a subjective standard and ADU deed restrictions. The county answered with Ordinance No. 2025-07. 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, new-construction ADUs the county must approve from one to two. The county 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 are built, the county need not approve a JADU. HOA protections also apply.
The county’s code still lists one detached new-construction ADU, so AB 956 adds a second detached unit for Alamo owners. Today’s 1,000 and 1,200 sq ft caps are above the 800 sq ft limit allowed for each of two units. Contra Costa County will need to update Chapter 82-24 again, and we confirm the current rules with county planners as it does.
Planning two detached ADUs on an Alamo lot
- Septic: sites on septic need Environmental Health approval, and private sewage or water systems need health officer approval before an ADU is established. Elsewhere, Central San serves Alamo for sewer.
- Soils: the county’s ADU submittal checklist requires a soils or geotechnical report or questionnaire.
- Trees: county code protects native trees at about 6.5 inches in diameter. A March 2025 draft ordinance would widen protection; its status is not yet known.
- Fire: San Ramon Valley Fire Protection District. County guidance says an ADU needs sprinklers only if the main house requires them.
- All-electric: ADUs may need to meet the county’s All-Electric Building Ordinance.
- Williamson Act: a contract may bar ADUs.
Building ADUs in Alamo with Axiom
Axiom Renovation designs, permits and builds ADUs. For two units we start with a site study of fit, utilities and review path. See our Alamo ADU builder page, our Alamo service area page, the AB 956 overview, and guides for nearby Danville and Walnut Creek.
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.
Alamo Two-ADU FAQ
Can I build two detached ADUs in Alamo?
From January 1, 2027, AB 956 requires Contra Costa County to approve up to two detached, new-construction ADUs on a lot with a single-family home in Alamo, if the project meets objective standards. Today the county code lists one detached ADU.
Who issues ADU permits in Alamo?
Contra Costa County. Alamo is unincorporated, so the county Department of Conservation and Development reviews the ADU application and issues the building permit through its ePermits portal.
How large can each ADU be in Alamo?
Today a detached ADU can be 1,000 sq ft, or 1,200 sq ft on lots of 12,000 sq ft or more. Under AB 956, the county may limit each of two detached ADUs to 800 sq ft of living space.
Can I build an ADU in Alamo if my home is on septic?
Often, yes, but county Environmental Health must approve the septic system first. Two units add load to the system, so we confirm capacity early.
Do Alamo ADUs need fire sprinklers?
County guidance says an ADU needs sprinklers only if the main house requires them. San Ramon Valley Fire Protection District signs off before final.
Who builds two-ADU projects in Alamo?
Axiom Renovation designs, permits and builds ADUs in Alamo 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 county for every project.