ADU rules in Union City

Detached ADUs, including converted detached garages and other accessory structures, are permitted on any lot with an existing or proposed single-family or multifamily dwelling in areas zoned to allow single-family or multifamily residential use (UCMC 18.34.030(A)(2), (B)); the MMU, VMU, SEMU-R and CSMU mixed-use districts and the 511 Area District list ADUs consistent with Chapter 18.34 as permitted uses. On a single-family lot the code allows, besides a JADU, either (A) one attached or detached new-construction ADU or (B) one detached new-construction ADU of up to 800 sq ft plus one ADU converted from existing space (18.34.030(C)(1)(a)). A lot with an existing multifamily dwelling may add up to eight detached ADUs, never more than the number of existing units, and a lot with a proposed multifamily dwelling two (18.34.030(C)(1)(b)(ii)), matching Gov. Code 66323(a)(4). New detached units need 4 ft side and rear setbacks and the district front setback, which is waived for units of 800 sq ft or less (18.34.030(C)(4)).

ADUs attached to or located within the proposed or existing primary dwelling, including attached garages and storage areas, are allowed on the same lots as detached units (UCMC 18.34.030(B)). A conversion of existing floor area has no size or height limit and no setback requirement, and under the (B) combination a conversion may expand up to 150 sq ft beyond an existing accessory structure for ingress and egress; new attached additions are capped at 1,000 sq ft and at 30 ft or the district height limit, never more than two stories (18.34.030(C)(1)(a)(i)(B), (C)(2)-(4)). On multifamily lots at least one ADU, and up to 25 percent of the existing unit count, may be created in non-livable space such as storage rooms, attics, basements or garages (18.34.030(C)(1)(b)(i)). A JADU of up to 500 sq ft may be created within a single-family residence, one per single-family lot (18.34.040(A)-(C)).

Ord. 2025-002 caps ADUs built from new floor area at 1,000 sq ft attached and 1,200 sq ft detached, leaving garages, carports, covered porches and patios, breezeways and other accessory structures out of the count, while ADUs converted from existing floor area have no size or height limit (UCMC 18.34.030(C)(2)-(3)). The flat 1,000 sq ft attached cap takes the place of the 50 percent of the primary dwelling measure in Gov. Code 66314(d)(4), and both caps clear the 850 and 1,000 sq ft interior-livable-space floors of 66321(b)(2), though the code does not say how the square footage is measured (see caveats). Detached units may be 16 ft tall, or 18 ft within one-half mile walking distance of a major transit stop or high-quality transit corridor (plus 2 ft for a roof pitch matching the main house, 20 ft in all) or on a lot with a multistory multifamily dwelling; attached units may reach 30 ft or the district height limit, never more than two stories (18.34.030(C)(3)). Side and rear setbacks are 4 ft and the district front setback is waived for ADUs of 800 sq ft or less (18.34.030(C)(4)); JADUs are limited to 500 sq ft of floor area (18.34.040(C)). The 850/1,000 sq ft caps and 18 ft detached limit of the repealed 2021-2022 chapter, still printed in the city's 2023 checklist, no longer apply.

Chapter 18.34 imposes no owner-occupancy requirement on ADUs, consistent with Gov. Code 66315, which bars owner-occupant conditions for ADUs. For JADUs the codified text still requires the owner to live in the remaining portion of the house or in the JADU, excusing only government agencies, land trusts and housing organizations (UCMC 18.34.040(D)), alongside a recorded deed restriction against separate sale (18.34.040(E)). Since 1-1-2026 Gov. Code 66333(b), as amended by AB 1154, permits an owner-occupancy requirement only where the JADU shares sanitation facilities with the house, so a JADU with its own bathroom carries none despite the 2025 local text, which predates that amendment.

ADUs are not subject to the city's minimum parking requirements anywhere in the city, and no replacement parking is required when a garage, carport, covered parking structure or uncovered space is demolished for, or converted to, an ADU; a demolition permit for a detached garage replaced by an ADU is issued together with the ADU permit (UCMC 18.34.030(C)(5)). JADUs need no additional or replacement spaces, even when converted from an attached garage (18.34.040(H)). The citywide exemption goes beyond the location-based exemptions of Gov. Code 66322(a), as 66314(b)(2) allows, and it supersedes the 2023 checklist's rule that a JADU must replace converted garage parking.

Every ADU and JADU, including conversions of existing space, needs only a building permit, and applications are processed ministerially without discretionary review or a hearing (UCMC 18.34.020(A), (B)(1)). On a lot with an existing dwelling the city must approve or deny within 60 days of a complete application or the application is deemed approved, a denial must list the deficiencies and how to cure them, and an application filed with a new home may be held until the home is decided (18.34.020(B)(2)-(4)); Gov. Code 66317(a)(2) adds a written completeness determination within 15 business days that the local text omits. Chapter 18.34 takes precedence over any conflicting Title 18 provision (18.34.010), which matters because older sections still describe administrative site development review for accessory structures over 450 sq ft and a conditional use permit for any development in the Hillside (-H) district (see caveats), and Gov. Code 66317(a)(1) bars special use permits for ADUs in any case. No ADU item appeared on a Planning Commission or City Council agenda from July 2025 through September 2026. Pre-approved detached ADU plans are offered through the city's AB 1332 program.

ADUs may be rented but not for a term of less than 30 days (UCMC 18.34.030(J)), and may not be sold or conveyed separately from the primary dwelling except through a qualified nonprofit under Gov. Code 66341 (18.34.030(H)); Union City has not opted into the 66342 ADU-condominium regime, and a JADU's recorded deed restriction bars its separate sale (18.34.040(E)). Gov. Code 66315 lets a city require terms of 30 days or longer for ordinary ADUs, while 66323(e) and 66333(g) require terms longer than 30 days for state-exempt units and JADUs, which the local text does not address. The city has no short-term rental permit program: the ADU clause is the only zoning limit, and short-term rentals otherwise appear only as a business-tax category (5.04.300) and under the transient occupancy tax (Chapter 3.18). A rented ADU is a rental unit under the Residential Landlord and Tenant Relations chapter, which requires a business license and per-unit registration with the city and allows evictions only for listed causes (5.50.030, 5.50.040, 5.50.090), and tenants may seek conciliation and mediation of rent increases above 7 percent in 12 months under the Rent Review Ordinance (5.55.040).