Two Detached ADUs in Union City (AB 956, 2027)
Plan Two ADUs in Union City: Free Site Assessment
Union City ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires cities, including Union City, to approve up to two detached ADUs on a lot with a single-family home. Union City replaced its ADU ordinance in March 2025, but the checklist on the city website still shows older rules. Here is what applies now and what changes.
- Units allowed now: the city’s ADU code (UCMC Ch. 18.34, Ordinance 2025-002, March 25, 2025) allows one JADU plus up to two ADUs: either one new ADU (attached or detached), or a detached ADU of 800 sq ft or less plus an ADU within existing or proposed space.
- Sizes: the code allows a new detached ADU up to 1,200 sq ft and a new attached ADU up to 1,000 sq ft. JADUs are up to 500 sq ft; conversions have no limit.
- Height: 16 ft for a detached ADU; 18 ft near transit or on lots with a multistory multifamily building, and up to 20 ft with roof pitch near transit. Attached ADUs: 30 ft or the district limit, up to two stories.
- Setbacks: 4 ft side and rear. Front setback follows the district unless the ADU is 800 sq ft or less.
- Parking: none for an ADU or JADU, and no replacement parking.
- Process: apply through the Building Division. The city decides within 60 days of a complete application, or the permit is deemed approved.
- Pre-approved plan: one plan in the Pre-Approved ADU Program, 700 sq ft with one bath.
- Older checklist: the online ADU checklist dated January 2023 lists older sizes and a 5 ft separation. Do not rely on it.
State review of Union City's ADU rules
On December 15, 2022, HCD reviewed Union City’s earlier ordinance and raised 10 issues. They included the 5 ft separation between buildings where it blocks an 800 sq ft ADU, corner-lot setbacks, how floor area was counted, height, subjective design terms and JADU rules. The city later replaced that ordinance in 2025. We found no newer HCD letter. 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 Union City must approve on a single-family lot from one to two, both new construction. Each may be limited to 800 sq ft of living space, local height limits can still apply, and side and rear setbacks can be no more than 4 ft. Where two detached ADUs are built, a JADU on the same lot does not have to be approved. The law also includes protections for owners in homeowners associations.
Union City already allows two ADUs in one combination: a small detached unit plus a conversion. AB 956 lets both ADUs be new detached buildings. The city’s 1,200 sq ft detached limit is above the 800 sq ft each unit may be held to in a two-unit project, and its 700 sq ft pre-approved plan already fits under that size. Union City will need to update its ordinance, and we will confirm the current rules as it does.
Planning two detached ADUs on a Union City lot
- Utilities: water is Alameda County Water District, sewer is Union Sanitary District and power is PG&E. The city’s 2023 checklist requires ADU utility connections to be underground; we confirm this under the current code.
- Electrical: two ADUs usually need a load calculation and often a service upgrade.
- Separation: HCD objected to the old 5 ft separation rule. With two units on one lot, we confirm spacing with the city early.
- Sprinklers: an ADU needs sprinklers only if the main house requires them.
- Use and sale: owner occupancy applies only to a JADU. Rentals must be longer than 30 days, and separate sale is allowed only under Gov. Code §66341.
Building ADUs in Union City with Axiom
See our Union City ADU builder page and our Union City service area, read the AB 956 overview, or compare neighboring Hayward and Fremont.
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.
Union City Two-ADU FAQ
Can I build two detached ADUs in Union City?
From January 1, 2027, AB 956 requires Union City to approve up to two detached, new-construction ADUs on a single-family lot that meets objective standards. Today the city’s code allows two ADUs only as a detached unit of 800 sq ft or less plus an ADU within existing or proposed space.
How large can each ADU be in Union City?
The city’s ADU code allows a new detached ADU up to 1,200 sq ft and a new attached ADU up to 1,000 sq ft. Under AB 956, the city may limit each of two detached ADUs to 800 sq ft of living space.
How tall can a detached ADU be in Union City?
16 ft in most cases. Near transit or on lots with a multistory multifamily building it can be 18 ft, and up to 20 ft with roof pitch near transit.
How long does Union City take to approve an ADU?
The city must decide within 60 days of a complete application, or the permit is deemed approved. Applications go through the Building Division.
Is the Union City ADU checklist online up to date?
No. The checklist dated January 2023 shows rules from before the 2025 ordinance, including older sizes and a 5 ft separation. We work from the current code and confirm details with the city.
Who builds two-ADU projects in Union City?
Axiom Renovation designs, permits and builds ADUs in Union City 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; Union City Pre-Approved ADU Program; HCD letter, December 15, 2022. This page is general information, not legal advice. Local rules change; we confirm current requirements with the city for every project.