Alameda County Municipal Code Ch. 17.55 Accessory Dwelling Units and Junior Accessory Dwelling Units

The purpose of this chapter is to comply with state law, which authorizes cities and counties to set standards for the development of accessory dwelling units and junior accessory dwelling units to increase the supply of small and affordable housing while ensuring that they remain compatible with existing primary dwelling units, neighborhoods, and rural areas.

(Ord. No. 2026-1, § 5, 1-8-26)

Notwithstanding the requirements otherwise established in this title, the county shall ministerially approve an application for a building permit within an agricultural, residential, or mixed-use zoning district to create any allowable accessory dwelling unit or junior accessory dwelling unit or any allowable combination of units without discretionary review or a hearing.

Ministerial approval of previously unpermitted accessory dwelling units and junior accessory dwelling units is further detailed in Section 17.55.180, consistent with the requirements under state law.

(Ord. No. 2026-1, § 5, 1-8-26)

A. Accessory Dwelling Units. Will be permitted to be established as a permanent housing option on any property that either contains or is constructed concurrently with a primary dwelling unit, and when established in compliance with state and local ordinances.

B. Junior Accessory Dwelling Units. Will be permitted to be established as a permanent housing option on any property within a zoning district where a single-family dwelling is permitted and that either contains or is constructed concurrently with a single-family dwelling, and when established in compliance with state and local ordinance.

(Ord. No. 2026-1, § 5, 1-8-26)

Accessory dwelling units shall be permitted in all zoning districts that permit single-family, multifamily, or mixed-use dwellings where the lot contains an existing or proposed dwelling. Junior accessory dwelling units shall be permitted in all zoning districts that permit single-family dwellings where the lot contains an existing or proposed single-family dwelling.

(Ord. No. 2026-1, § 5, 1-8-26)

Accessory dwellings units and junior accessory dwelling units shall not be counted when calculating the maximum permitted density requirements of a property; however, they may be counted to meet minimum density requirements.

1. Single-Family Dwellings. The maximum number of junior accessory dwelling units on a building site shall be one. The maximum number of accessory dwelling units on a building site shall be one by new construction and one by conversion; this applies to each single-family dwelling on a property. No more than one accessory dwelling unit may be attached.

2. Multifamily Dwellings. The maximum number of accessory dwelling units on a building site shall be as follows:

a. Within portions of an existing multifamily dwelling, the county shall allow at least one conversion to an accessory dwelling unit and shall allow up to twenty-five (25) percent of the number of existing multifamily dwelling units converted into accessory dwelling units (rounded down); and

b. On a lot with an existing multifamily dwelling, there shall not be more than eight detached accessory dwelling units. The number of accessory dwelling units allowable shall not exceed the number of existing units on the lot.

c. For development on a lot with a proposed multifamily dwelling, not more than two detached accessory dwelling units.

(Ord. No. 2026-1, § 5, 1-8-26)

A. For Single-family dwellings:

1. Accessory dwelling units may be attached to the single-family dwelling, detached from the single-family dwelling, or may involve the conversion of floor area of an existing structure.

2. Junior Accessory Dwelling Units.

a. Shall be contained within the exterior walls of an existing or proposed single-family dwelling; and

b. May share bathroom facilities with the primary dwelling unit.

B. For multifamily dwellings:

1. Accessory dwelling units are allowed within the portions of existing multifamily dwellings that are not currently used as livable space, including, but not limited to, storage rooms, boiler rooms, passageways, attics, basements, or garages, if each unit complies with state building standards for dwellings.

2. Accessory dwelling units are allowed as detached units.

3. Junior accessory dwelling units are prohibited.

C. For mixed-use dwellings: the conversion of non-livable spaces within mixed-use dwelling properties shall be limited to the residential portion of the property, so long as each unit complies with state building standards for dwellings.

D. For all projects:

1. Shall have at least an efficiency kitchen including a cooking facility with appliances, a food preparation counter, and storage cabinets of reasonable size in relation to the unit;

2. Shall have a separate entrance from the primary dwelling unit. Access to the public right-of-way may be provided through the rear yard of the primary residence or dedicated pathway. For purposes of this section, "access to the public right-of-way" or "dedicated pathway" do not share the same meaning as the statutory definition of "passageway," under Government Code Section 66313, and such access is not required to be unobstructed clear to the sky. Internal connection to the primary dwelling unit is optional, except for junior accessory dwelling unit sharing a bathroom with a primary dwelling unit which must have both an internal connection and separate entrance.

(Ord. No. 2026-1, § 5, 1-8-26)

A. Required yards (setbacks).

1. Side, Street Side, and Rear. Minimum required four-foot side and rear yard for both attached and detached accessory dwelling units. This shall not apply to development of accessory dwelling units by conversion of existing structures.

2. Front. Minimum is the same as for primary dwelling unit for accessory dwelling units which measure over eight hundred (800) square feet in floor area, except for accessory dwelling units to be developed pursuant to Government Code Section 66323.

B. Distance Separation. Six-foot separation minimum from any other building for accessory dwelling units measuring over eight hundred (800) square feet in floor area, except for accessory dwelling units to be developed pursuant to Government Code Section 66323.

C. Height.

1. Up to sixteen (16) feet: A detached accessory dwelling unit on a lot with an existing or proposed single family or multifamily dwelling unit.

2. Up to eighteen (18) feet:

a. In addition to subsection (C)(1), if located within one-half mile walking distance of a "major transit stop" or a "high-quality transit corridor," as defined in Section 21155 of the Public Resources Code; or

b. For a detached accessory dwelling unit on a lot with an existing or proposed multi-family, multi-story dwelling.

3. Up to twenty (20) feet if, in addition to subsection (C)(2)(a), the 20-foot height is necessary to accommodate a roof pitch on the accessory dwelling unit that is aligned with the roof pitch of the primary dwelling unit.

4. Up to twenty-five (25) feet or the height limitation in the applicable zoning district that applies to the primary dwelling unit, whichever is lower, for an attached accessory dwelling unit.

D. Stories. Maximum of two stories.

E. Accessory dwelling units measuring over eight hundred (800) square feet in floor area are subject to the "Residential Design Standards and Guidelines for the Unincorporated Communities of West Alameda County," as amended, as incorporated in Title 17, except when the "Residential Design Standards and Guidelines for the Unincorporated Communities of West Alameda County," as amended, as incorporated in Title 17, conflict with Government Code Sections 66314, 66321, 66323, and any other state accessory dwelling unit law.

(Ord. No. 2026-1, § 5, 1-8-26)

A. Floor Area Calculation. The size of the unit shall be measured as the floor area within the inside perimeter of the exterior walls of the unit. Carports, covered porches and patios, chimneys, exterior stairwells, and mechanical rooms are not counted toward the determination of floor area.

B. Accessory Dwelling Units.

1. The following development standards apply to newly constructed detached accessory dwelling units:

a. If the detached accessory dwelling unit is to contain zero or one bedrooms, then limited to eight hundred fifty (850) square feet maximum of interior livable space, except for accessory dwelling units to be developed pursuant to Government Code Section 66323.

b. If the detached accessory dwelling unit is to contain two or more bedrooms, then limited to one thousand two hundred (1,200) square feet maximum of interior livable space, except for accessory dwelling units to be developed pursuant to Government Code Section 66323.

2. The following development standards apply to newly constructed attached accessory dwelling units:

a. Maximum of fifty (50) percent floor area of the existing primary dwelling unit, with a cap of eight hundred fifty (850) square feet for accessory dwelling units with up to one bedroom and a cap of one thousand (1,000) square feet for accessory dwelling units with two or more bedrooms. Where fifty (50) percent of the floor area does not provide for eight hundred fifty (850) square feet of interior livable space, an accessory dwelling unit of up to eight hundred fifty (850) square feet of interior livable space is allowed.

3. The conversion of an existing accessory structure or a portion of the existing single-family dwelling to an ADU shall not be subject to ADU size requirements. Should the accessory structure or existing single-family dwelling be expanded beyond one hundred fifty (150) square feet to create an ADU, then the ADU shall be subject to the size maximums listed herein.

C. Junior Accessory Dwelling Units.

1. A junior accessory dwelling unit shall measure no more than five hundred (500) square feet in interior livable space, contained within the exterior walls of a proposed or existing single-family dwelling.

(Ord. No. 2026-1, § 5, 1-8-26)

Junior accessory dwelling units are not required to provide additional parking.

Except for accessory dwelling units to be developed pursuant to Government Code Section 66323, accessory dwelling units are required to provide one on-site parking space for each accessory dwelling unit, and, notwithstanding other development regulations, may otherwise be located within front yard or street side yard setbacks. On-site parking is not required in the following instances:

1. Site is located within one-half mile walking distance of public transit;

2. Site is located within an architecturally or historically significant property or district;

3. When the accessory dwelling unit is part of the proposed or existing primary dwelling unit or an accessory structure;

4. When on-street parking permits are required but not offered to the occupants; or

5. When there is a car share vehicle located within one block.

6. When the accessory dwelling unit contains no bedrooms.

7. When a permit application for an accessory dwelling unit is submitted with a permit application to create a new single-family dwelling or a new multifamily dwelling on the same lot, provided that the accessory dwelling unit or the parcel satisfies any other criteria listed above.

(Ord. No. 2026-1, § 5, 1-8-26)

Accessory dwelling units and junior accessory dwelling units shall be allowed to be established either simultaneously with or after the construction of a primary dwelling unit that is located on the same lot of record and under one common ownership. Applications for accessory dwelling units and junior accessory dwelling units shall be subject to applicable building codes and require approval of the county building inspection department. Applications for accessory dwelling units and junior accessory dwelling units on parcels served by private septic systems (on-site wastewater treatment systems) shall require approval of the county department of environmental health.

A demolition permit for a detached garage that is to be replaced with an accessory dwelling unit must be reviewed with the application for the accessory dwelling unit and issued at the same time. Applicants shall not be required to provide written notice or post a placard for the demolition of a detached garage that is to be replaced with an accessory dwelling unit, unless the property is located within an architecturally and historically significant historic district.

(Ord. No. 2026-1, § 5, 1-8-26)

Accessory dwelling units established by conversion of an existing legal structure can be demolished and reconstructed within the same building volume at the same footprint location, and with no floor area or height limit.

(Ord. No. 2026-1, § 5, 1-8-26)

A junior accessory dwelling unit must be located within a single-family dwelling and may not include an expansion beyond the physical dimensions of the single-family dwelling, except as provided below.

An accessory dwelling unit or junior accessory dwelling unit located within a single-family dwelling or accessory structure may include an expansion of not more than one hundred fifty (150) square feet beyond the same physical dimensions as the existing structure. This expansion shall be limited to accommodate ingress and egress (for example, a covered front porch).

(Ord. No. 2026-1, § 5, 1-8-26)

A. For accessory dwelling units and junior accessory dwelling units:

1. The units shall not be sold, transferred or owned separately from the primary dwelling unit on the property, except an accessory dwelling unit may be sold, transferred or owned separately when owned by certain nonprofit corporations pursuant to California Government Code Section 66341 and the criteria established therein, as that section may be amended; and

2. The units may be occupied by or rented to a separate household living independently from the occupant(s) of the primary dwelling unit (or vice versa) provided that the terms for separate occupancy of the accessory unit and/or primary dwelling unit shall be longer than thirty (30) days.

B. For accessory dwelling units only: the owner of the property shall not be required to reside on the property in either the primary dwelling unit or accessory dwelling unit, except when the property is held pursuant to a recorded tenancy in common agreement, pursuant to California Government Code Section 66341 and the criteria established therein, as that section may be amended.

C. For junior accessory dwelling units only:

1. The property owner shall reside on the property in either the primary dwelling unit or the junior accessory dwelling unit if the junior accessory dwelling has shared sanitation facilities with the existing structure. Owner occupancy is not required if the junior accessory dwelling has separate sanitation facilities or if the owner is a governmental agency, land trust, or housing organization.

2. The owner shall record a deed restriction to run with the land, and which shall be recorded with the Alameda County Recorder's Office with a conformed copy filed with the Alameda County Planning Department, and shall include both of the following terms:

a. A prohibition on the sale of the junior accessory dwelling unit separate from the sale of the primary dwelling unit, including a statement that the deed restriction may be enforced against future purchasers; and

b. A restriction on the size and attributes of the junior accessory dwelling unit that conforms with [Chapter] 17.55.

3. The rental term of a junior accessory dwelling unit shall be for a term longer than thirty (30) days.

D. The foregoing restrictions shall be binding upon any successor in ownership of the property. Failure to comply with any of the foregoing restrictions shall be considered a violation of this title.

(Ord. No. 2026-1, § 5, 1-8-26)

No park dedication impact fees are required for accessory dwelling units measuring less than seven hundred fifty (750) square feet in interior livable space and no park dedication impact fees shall be required for junior accessory dwelling units.

(Ord. No. 2026-1, § 5, 1-8-26)

Pursuant Government Code sections 66322(b) and 66336, the county shall not require the correction of the following as a condition of an accessory dwelling unit or junior accessory dwelling unit permit approval:

1. Nonconforming zoning conditions prior to the issuance of a permit for accessory dwelling units or junior accessory dwelling units, except to comply with the California Building Code where the structure is not in compliance with current California Building Code standards, and that are not affected by the construction of the accessory dwelling unit or junior accessory dwelling unit.

2. Existing unpermitted structures not affecting proposed accessory dwelling unit, unless the existing unpermitted structure presents a threat to public health or safety or affects the construction of the accessory dwelling unit.

3. Existing building standards violations on the primary dwelling unit, provided that correcting the violation is not necessary to protect health and safety and is not affected by the construction of the accessory dwelling unit or junior accessory dwelling unit.

(Ord. No. 2026-1, § 5, 1-8-26)

Pursuant to 66310 et seq., applications for accessory dwelling units and junior accessory dwelling units shall be subject to all applicable state laws, including the California Building Standards Code, and the regulations below.

The accessory dwelling unit and/or junior accessory dwelling unit shall:

1. Be placed on a permanent foundation;

2. Provide side and rear setbacks that are sufficient for fire and safety; and

3. Comply with the requirements of Government Code Section 66310.

The construction of an accessory dwelling unit shall not constitute a Group R occupancy change under the local building code, as described in Section 310 of the California Building Code (Title 24 of the California Code of Regulations), unless the building official makes a written finding based on substantial evidence in the record that the construction of the accessory dwelling unit could have a specific, adverse impact on public health and safety. Nothing in this paragraph shall be interpreted to prevent the county from changing the occupancy code of a space that was uninhabitable space or was only permitted for nonresidential use and was subsequently converted for residential use pursuant to accessory dwelling unit regulations (Article 2 of California Government Code Title 7, Division 1, Chapter 13).

(Ord. No. 2026-1, § 5, 1-8-26)

A junior accessory dwelling unit shall not be considered a separate or new unit for purposes of any fire or life protection ordinance or for purposes of providing water, wastewater or power services. Primary dwelling units containing a junior accessory dwelling unit may be subject to the same requirements as applied to primary dwelling units not containing a junior accessory dwelling unit.

(Ord. No. 2026-1, § 5, 1-8-26)

The county shall ministerially approve any application to legalize a previously unpermitted accessory dwelling unit or junior accessory dwelling units that is proposed consistent with the requirements and timeframe of eligibility under state law, including California Government Code Section 66311.7, as may be amended over time.

1. Permitting.

a. The county shall not deny a permit for an unpermitted accessory dwelling unit or junior accessory dwelling UNIT, as an Accessory dwelling unit or junior accessory dwelling unit defined in this chapter, constructed before the cut-off date established by state law, due to either:

i. Violation of building standards pursuant to Article 1 (commencing with Section 17960) of Chapter 5 of Part 1.5 of Division 13 of the Health and Safety Code; or

ii. Noncompliance with development standards for accessory dwelling units and junior accessory dwelling units established under state law (Article 2 and Article 3 of Division 1 of Title 17 in the California Government Code) or under Title 17 of the Alameda County Municipal Code.

b. The county may deny a permit for an unpermitted accessory dwelling units or junior accessory dwelling units if the county makes a finding that correcting the violation is necessary to comply with the standards specified in Section 17920.3 of the Health and Safety Code.

c. Approval of an unpermitted accessory dwelling units or junior accessory dwelling units shall not apply to a building that is deemed substandard pursuant to Section 17920.3 of the Health and Safety Code.

d. Permitting of unpermitted accessory dwelling units and junior accessory dwelling units that are not consistent with the requirements and timeframe of eligibility under state law shall be considered under the standard review procedures and requirements otherwise established in this title.

2. Inspection. Upon receiving an application to permit a previously unpermitted accessory dwelling units or junior accessory dwelling units constructed before the cut-off date established by state law, an inspector from the county may inspect the unit for compliance with health and safety standards and provide recommendations to comply with health and safety standards necessary to obtain a permit. If the inspector finds noncompliance with health and safety standards, the county shall not penalize an applicant for having the unpermitted accessory dwelling unit or junior accessory dwelling unit and shall approve necessary permits to correct noncompliance with health and safety standards.

3. Information. The county shall inform the public about these provisions through public information resources, including permit checklists and the website, which shall include both of the following:

a. A checklist of the conditions specified in Section 17920.3 of the Health and Safety Code that would deem a building substandard.

b. Informing homeowners that, before submitting an application for a permit, the homeowner may obtain a confidential third-party code inspection from a licensed contractor to determine the unit's existing condition or potential scope of building improvements before submitting an application for a permit.

4. Impact Fees and Charges. A homeowner applying for a permit for a previously unpermitted accessory dwelling units or junior accessory dwelling units, constructed before the cut-off date established by state law, shall not be required to pay impact fees or connection or capacity charges except when utility infrastructure is required to comply with Section 17920.3 of the Health and Safety Code and when the fee is authorized for connections under this chapter (consistent with Government Code Section 66311.5(e)).

(Ord. No. 2026-1, § 5, 1-8-26)