California’s ADU law uses different dimensional protections for different project types. The March 2026 handbook from the state Department of Housing and Community Development (HCD) explains categories for existing-space conversions, new detached ADUs on single-family lots, and detached units on multifamily properties. Identify the statutory category first; the measurements below do not combine into one statewide design envelope.
Single-family sites and existing-space conversions
Two different size protections can look contradictory unless the statutory route is identified. Under Government Code §66323(a)(2), a local agency must ministerially approve one new detached ADU on a qualifying single-family lot with side and rear setbacks no greater than four feet; that specific route may carry a limit of 800 sq ft of livable space. Separately, §66321(b)(2) says a local maximum-size ordinance generally cannot set a limit below 850 sq ft of interior livable space, or 1,000 sq ft for an ADU with more than one bedroom. Section 66321(b)(3) also bars size, coverage, floor-area, open-space, front-setback, and minimum-lot-size rules from preventing an ADU of at least 800 sq ft with four-foot side and rear setbacks when it meets the other local standards. Treat these as separate statutory provisions and confirm which pathway applies.
For detached ADUs, Government Code §66321(b)(4)(A)–(C) sets different minimum local height allowances: 16 or 18 ft depending on the project category, with two additional feet for an aligned roof pitch in the specified transit-related route. An attached ADU follows a separate rule: under §66321(b)(4)(D), a local height limit may not be lower than 25 ft or the primary dwelling’s local height limit, whichever is lower. That provision does not require a local agency to allow more than two stories. The applicable subsection and project facts determine which rule applies.
An ADU created from existing space in a primary residence or accessory structure follows a separate conversion category and is not subject to the same unit-size limit. That does not remove exterior-access, fire-safety, building-code, or other applicable restrictions. The conversion rule should not be transferred to a new detached building or an addition.
Multifamily sites and local development standards
The handbook describes up to two detached ADUs on a lot with a proposed multifamily dwelling. On a lot with an existing multifamily dwelling, it describes up to eight detached ADUs, capped by the number of existing units. These categories include four-foot side and rear setback conditions; the law also limits when a local agency may require smaller existing multifamily setbacks to be changed as part of an ADU application.
For covered categories, local lot-coverage, floor-area-ratio, open-space, or landscaping rules may not be used to prevent an ADU that qualifies for at least 800 sq ft of interior livable space and maintains four-foot rear and side setbacks. The project must meet the applicable statutory category and other standards; this protection does not erase unrelated zoning, hazard, building, or fire requirements.
Match the project to the current rule
Before relying on a number, determine whether the proposal is an existing-space conversion, new detached ADU on a single-family lot, or detached ADU on a proposed or existing multifamily site. Confirm the current Government Code text, local zoning and overlays, height condition, measurement definitions, fire and exterior-access rules, and application-date requirements. HCD’s handbook is official guidance rather than a parcel determination. A four-foot zoning setback does not establish fire separation or permit approval; ask the local agency to confirm how the statutory category applies.