ADU rules in Alameda County

Chapter 17.55, restated in full by Ord. No. O-2026-1 (adopted January 8, 2026, effective February 7, 2026), permits ADUs in every zoning district that allows single-family, multifamily or mixed-use dwellings where the lot has an existing or proposed dwelling, and each single-family dwelling may have one ADU by new construction and one by conversion, either of which may be detached (17.55.040, 17.55.050(1), 17.55.060(A)(1)). A lot with an existing multifamily building may add up to eight detached ADUs, never more than the number of existing units, and a lot with a proposed multifamily building up to two (17.55.050(2)(b)-(c)). In the A (Agricultural) district 17.06.030.L allows at most two ADUs per building site, and when two are built one must be detached new construction with 4-foot side and rear setbacks. This entry covers the unincorporated area only (Castro Valley, the Eden Area communities of Ashland, Cherryland, Hayward Acres and San Lorenzo, Fairview, Sunol and the East County rural area).

An ADU may be attached to the home as an addition, built within a proposed home, or converted from floor area of an existing structure such as a garage, basement or accessory building (with up to 150 sq ft of added space for ingress and egress), but no more than one ADU on a single-family lot may be attached (17.55.050(1), 17.55.060(A)(1), 17.55.120). One JADU of up to 500 sq ft is allowed inside the walls of an existing or proposed single-family dwelling in any district that permits single-family dwellings, the A district included; it may share the home's bathroom, in which case it needs both an interior connection and a separate entrance (17.55.030(B), 17.55.040, 17.55.060(A)(2), (D)(2)). On multifamily properties ADUs may be converted from non-livable space such as storage rooms, attics, basements or garages (at least one, up to 25 percent of the existing units), while JADUs are prohibited there (17.55.050(2)(a), 17.55.060(B)). Garage conversions into an ADU or JADU that meet Chapter 17.55 need no site development review (17.54.225(E)), and state law bars the one-to-one replacement parking that 17.52.955 demands of other garage conversions (Gov. Code 66314(d)(11)).

Section 17.55.080 measures floor area within the inside perimeter of the exterior walls and caps a newly built detached ADU at 850 sq ft of interior livable space with zero or one bedroom and 1,200 sq ft with two or more, and a newly built attached ADU at 50 percent of the existing primary dwelling, capped at 850 sq ft (up to one bedroom) or 1,000 sq ft (two or more), with 850 sq ft allowed wherever 50 percent would yield less. Converting an existing accessory structure or part of the home carries no size limit unless it adds more than 150 sq ft, ADUs built under the state-exempt pathway of Gov. Code 66323 are excluded from these caps, and a JADU may not exceed 500 sq ft (17.55.080(B)-(C)). Heights are 16 feet for a detached ADU, 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor or on a lot with a multistory multifamily building (20 feet near transit where needed to match the home's roof pitch), and 25 feet or the zone limit, whichever is lower, for an attached ADU, with a two-story maximum (17.55.070(C)-(D)). An ADU over 800 sq ft must meet the home's front setback, keep 6 feet from other buildings and follow the West County Residential Design Standards and Guidelines except where they conflict with Gov. Code 66314, 66321 or 66323, and 66323 units are exempt from the setback and separation rules (17.55.070(A)(2), (B), (E)). In the A district, when only one ADU is proposed, an ADU not subject to the Chapter 17.55 floor-area rules is capped at 1,200 sq ft of interior livable space unless converted from a previous use, and every new ADU over 1,000 sq ft must sit within the primary dwelling's building envelope (17.06.030.L.3-4).

Section 17.55.130(B) states that the owner need not live in either the primary dwelling or the ADU, the only exception being a property held under a recorded tenancy-in-common agreement for an ADU sold through a qualified nonprofit under Gov. Code 66341, whose buyer must occupy a unit as a principal residence; Gov. Code 66315 bars owner-occupancy conditions on ADUs. For JADUs, O-2026-1 adopted the post-AB 1154 rule: the owner must live in the home or the JADU only when the JADU shares sanitation facilities with the house, and not at all when it has its own bathroom or the owner is a government agency, land trust or housing organization (17.55.130(C)(1); Gov. Code 66333(b)). A JADU still needs a recorded deed restriction barring separate sale and fixing its size and attributes, with a conformed copy filed with the Planning Department (17.55.130(C)(2)). The owner-occupancy clause Municode still prints for the old SU combining district (17.30.110(F), 'The property must be owner occupied') is not operative: Ord. No. O-2024-32 deleted that article in September 2024 (see caveats).

Section 17.55.090 requires one on-site space for each ADU, which may sit in the front or street-side setback notwithstanding other rules, but exempts units built under Gov. Code 66323 and any ADU within half a mile walking distance of public transit, on an architecturally or historically significant property or district, forming part of the proposed or existing home or an accessory structure, where on-street permits are required but not offered, within one block of a car share vehicle, with no bedroom, or applied for together with a new single-family or multifamily dwelling when another exemption also applies. JADUs need no parking (17.55.090; Gov. Code 66334(a)), and required spaces may be tandem or in setback areas unless findings on topography or fire and life safety say otherwise (17.52.780, as amended by O-2026-1). No replacement parking may be required when a garage, carport or parking space is demolished or converted for an ADU (Gov. Code 66314(d)(11)), so in practice a space is owed only for a newly built ADU with a bedroom that is outside the state-exempt pathway and every exemption. State law caps any requirement at one space per unit or per bedroom, whichever is less (Gov. Code 66314(d)(10)), and the half-mile transit exemption applies notwithstanding any other law, a bus stop counting as public transit (Gov. Code 66313(m), 66322(a)); the Fairview Specific Plan's narrower transit exemption is flagged in the caveats.

Section 17.55.020 directs the County, notwithstanding any other requirement of Title 17, to ministerially approve a building permit for any allowable ADU, JADU or combination of units in an agricultural, residential or mixed-use district without discretionary review or a hearing, and 17.55.180 adds ministerial legalization of unpermitted units built before the state cut-off (Gov. Code 66311.7). Applications go through the Public Works Agency's online permit portal: Planning Department zoning verification first, then a building permit reviewed by the Building Inspection Department, the Fire Department and, for septic parcels, Environmental Health (17.55.100; Public Works ADU checklists dated 1/1/2026, which list 20 business days for the first building review). State law requires a completeness decision within 15 business days and approval or denial within 60 days of a complete application, with a written appeal to the Board or the Planning Commission (Gov. Code 66317). Garage conversions into ADUs or JADUs need no site development review (17.54.225(E)). The discretionary route that remains is for units that are not Chapter 17.55 ADUs: the larger 'secondary dwelling unit' on A-district parcels of 25 acres or more (up to 2,000 sq ft, or 2,500 sq ft on 100 acres or more), which needs site development review and, under 100 acres, a Planning Commission hearing (17.06.030.H).

Section 17.55.130(A) lets an ADU or JADU be occupied by or rented to a separate household but requires that the terms for separate occupancy of the accessory unit and/or the primary dwelling be longer than 30 days, and 17.55.130(C)(3) repeats the over-30-day term for JADUs, so neither the ADU nor the main house on a lot with one may be rented short term. The units may not be sold, transferred or owned separately from the primary dwelling except for an ADU sold through a qualified nonprofit under Gov. Code 66341; the County has not opted into the Gov. Code 66342 ADU-condominium regime, and the draft minutes of the December 1, 2025 Planning Commission hearing record that the sale prohibition stays for now, with staff planning to revisit it later. The General Ordinance Code has no short-term rental permit program for the unincorporated area (Municode searches for 'short-term rental', 'vacation rental' and 'home sharing' return no hits), and the 10 percent Hotel and Lodging Tax applies to stays of 30 days or less (Chapter 2.132). Long-term ADU tenancies fall under the county's mandatory rent-dispute mediation program (Chapter 3.68) and its just-cause eviction ordinance (Chapter 3.70), which adopts the Civil Code 1946.2(e) exemptions (see caveats). State law is consistent: Gov. Code 66315 permits a 30-day minimum, and 66323(e) and 66333(g) require terms longer than 30 days for state-exempt units and JADUs.