Alameda County

Unincorporated Alameda County ADU rules

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

Last checked October 26, 2026.

City or county ADU page

Unincorporated Alameda County ADU page

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Unincorporated Alameda County 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
Unincorporated Alameda County's ADU rules are in Zoning Ordinance Chapter 17.55, as amended by Ordinance O-2026-1 (adopted January 8, 2026). Three items are left out of this summary because they may conflict with state law; state rules apply to those items. Cities in Alameda County have their own rules.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Ch. 17.55 (Ord. O-2026-1), accessed 2026-10-25.

Review
The County reviews building permits for ADUs and junior ADUs, and allowed combinations, ministerially without a hearing in agricultural, residential and mixed-use districts. Lots on septic systems also need Environmental Health review.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Secs. 17.55.020, 17.55.100, accessed 2026-10-25.

Where ADUs are allowed
ADUs and junior ADUs are allowed in all districts that allow single-family, multifamily or mixed-use homes, on a lot with an existing or proposed home; junior ADUs need a single-family home. ADUs up to 1,200 sq ft in agricultural areas also follow the County's Policy for Secondary Units in Rural Residential and Agricultural Areas.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Secs. 17.55.030, 17.55.040, accessed 2026-10-25.

How many units (single-family)
Up to one junior ADU, one new-construction ADU and one converted ADU for each single-family home. Either ADU may be attached or detached, but no more than one may be attached.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.050(1), accessed 2026-10-25.

Multifamily lots
At least one ADU, and up to 25% of the existing units, inside non-livable parts of an existing multifamily building. Junior ADUs are not allowed with multifamily buildings.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Secs. 17.55.050(2)(a), 17.55.060(2), accessed 2026-10-25.

Setbacks and separation
4 ft side and rear setbacks for new attached and detached ADUs (not for conversions). ADUs over 800 sq ft (other than those protected by Gov. Code 66323) need the main home's front setback and 6 ft from other buildings.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.070(1)-(2), accessed 2026-10-25.

Height
Detached: 16 ft; 18 ft within a half mile of major transit or with a multistory multifamily building; 20 ft near transit to match the main roof pitch. Attached: 25 ft or the zone limit, whichever is lower. Two stories maximum.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.070(3)-(4), accessed 2026-10-25.

Detached ADU size
New detached ADUs (other than those protected by Gov. Code 66323): up to 850 sq ft with 0 or 1 bedroom, or 1,200 sq ft with 2 or more bedrooms. Converted space has no size limit unless it is expanded by more than 150 sq ft.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.080(2)(a), (c), accessed 2026-10-25.

Junior ADUs
Up to 500 sq ft inside an existing or proposed single-family home, with an efficiency kitchen and a separate entrance (and an interior door if the bathroom is shared). The owner must live on site only if the bathroom is shared, and a deed restriction is recorded.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Secs. 17.55.060(4), 17.55.080(3), 17.55.130(3), accessed 2026-10-25.

Parking
No parking for junior ADUs. One on-site space per ADU (may be in front or street-side setbacks) except for ADUs protected by Gov. Code 66323 and ADUs near transit, in historic districts, inside the home or an accessory structure, where on-street permits are not offered, near car share, with no bedrooms, or built with a new home that meets one of these.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.090, accessed 2026-10-25.

Rentals, sale and owner occupancy
ADUs and junior ADUs may not be sold separately (except as Gov. Code 66341 allows) and rentals must be longer than 30 days. No owner occupancy for ADUs.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.130(1)-(2), accessed 2026-10-25.

Park fees
No park dedication fees for ADUs under 750 sq ft or for junior ADUs.
Source: Alameda County Ordinance O-2026-1 amending Zoning Ordinance Ch. 17.55 Accessory Dwelling Unit and Junior Accessory Dwelling Unit Ordinance (adopted Jan. 8, 2026), Sec. 17.55.140, accessed 2026-10-25.

Where to ask
Alameda County Planning Department. The County's ADU (secondary units) page has its ADU information and contacts.
Source: Alameda County Planning, Accessory Dwelling Units 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 Unincorporated Alameda County?

State law sets a baseline for ADUs in Unincorporated Alameda County, 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 Alameda County Planning Department, or run a free address summary.

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