Alameda County

Union City ADU rules

Research summary of local ADU rules on file for Union City in Alameda County. Not a determination for a specific lot.

Last checked October 26, 2026.

City or county ADU page

Union City Planning department. No ADU-specific page was found.

Union City rules on file

Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.

Which rules apply
Union City's ADU rules are in Municipal Code Chapter 18.34, which Ordinance 2025-002 (March 25, 2025) replaced. Its owner-occupancy rule for junior ADUs is left out of this summary because it may conflict with state law; state rules apply to that item.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Secs. 18.34.010-18.34.040, accessed 2026-10-25.

Review and timing
ADU and junior ADU applications need a building permit and are reviewed ministerially, with no hearing. On a lot with an existing home the City acts within 60 days of a complete application; if it does not, the application is deemed to be granted. With a new home, the City may wait to act until it acts on the new home.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.020(A)-(B), accessed 2026-10-25.

Entrance and address
Each ADU and junior ADU needs its own exterior entrance and a separate address from the main home.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.020(C)-(D), accessed 2026-10-25.

Where ADUs are allowed
On lots with an existing or proposed single-family or multifamily home in areas zoned for residential use. ADUs do not count toward density.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(A), accessed 2026-10-25.

How many units
Single-family lots: a junior ADU plus either one new attached or detached ADU, or one detached ADU up to 800 sq ft and one ADU inside the home or an existing accessory structure. Multifamily lots: at least one and up to 25% of existing units as conversions of non-livable space, plus up to eight detached ADUs with an existing building (not more than the existing units) or two with a proposed building.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(C)(1), accessed 2026-10-25.

Size limits
Converted space has no size limit. New attached ADUs: up to 1,000 sq ft. New detached ADUs: up to 1,200 sq ft. Garages, carports, covered porches and breezeways are not counted.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(C)(2), accessed 2026-10-25.

Height
Converted space has no height limit. New attached ADU: 30 ft or the zone limit, and no more than two stories. New detached ADU: 16 ft, or 18 ft near major transit or with a multistory multifamily building (up to 20 ft near transit to match the main roof pitch).
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(C)(3), accessed 2026-10-25.

Setbacks
No setbacks for ADUs in existing space or rebuilt in the same place and size. New ADUs: 4 ft side and rear; front setback per the zone unless the ADU is 800 sq ft or less.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(C)(4), accessed 2026-10-25.

Parking
ADUs are not subject to the City's minimum parking rules, and replacement parking is not required when a garage or parking area is converted or removed for an ADU. Junior ADUs need no parking.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Secs. 18.34.030(C)(5), 18.34.040(H), accessed 2026-10-25.

Historic resources
Exterior changes to structures on the California Register of Historical Resources must follow the Secretary of the Interior's Standards.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(C)(6), accessed 2026-10-25.

Fees and sprinklers
Impact fees apply only to ADUs of 750 sq ft or more, charged in proportion to the main home. Fire sprinklers are needed only if the main home needs them.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(E)-(F), accessed 2026-10-25.

Sale and rentals
ADUs may not be sold separately except as Gov. Code 66341 allows, and may not be rented for less than 30 days.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.030(H), (J), accessed 2026-10-25.

Junior ADUs
One junior ADU per single-family lot, up to 500 sq ft, with an efficiency kitchen and, if it has no bathroom of its own, an interior door to the home. A deed restriction against separate sale must be recorded.
Source: Union City Municipal Code Ch. 18.34 Accessory Dwelling Units (eCode360; Ord. 2025-002, March 25, 2025), Sec. 18.34.040(A)-(G), accessed 2026-10-25.

Where to ask
Union City Planning Division (Economic and Community Development).
Source: City of Union City, Economic and Community Development page, accessed 2026-10-25.

California state rules on file

The statewide baseline recorded from state law and the HCD ADU Handbook.

Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.

Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.

Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.

Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.

Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.

Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.

Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.

State baseline on the directory → · State ADU rules on the homepage →

Can I build an ADU in Union City?

State law sets a baseline for ADUs in Union City, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with the City of Union City Planning Division, or run a free address summary.

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