ADU Size and Height Limits
The usable space in an existing garage depends on its measured interior, structure, and the proposed work. Size and height requirements are tied to the applicable rules and property, so verify them before buying a plan.
Requirements depend on your property and the agency reviewing it. Confirm current details with that agency before finalizing plans.
Questions to consider
- Measure the existing footprint and ceiling height
- For the City of San Diego, Bulletin 400 describes a junior ADU within a single dwelling or attached garage, with a kitchen or efficiency kitchen and a stated size range of 150 to 500 square feet.
- HCD says local size standards have different treatment for certain existing-space conversions and new ADUs; the category and full project matter.
- HCD says a compliant local ADU ordinance may not impose a detached ADU height limit below 16 feet on a lot with a proposed or existing single-family or multifamily dwelling; other specified situations have higher minimum allowances. This is a limit on local restrictions, not a promise that every project may be built to that height.
- City of San Diego Bulletin 400 says ADUs must comply with applicable base and overlay zone height limits; on lots permitting single-dwelling but not multiple-dwelling development, it limits certain detached and accessory-structure-attached ADUs to two stories.
- Distinguish converting existing space from expanding or building new space
- Discuss entries, openings, and usable circulation with the designer
- Ask the reviewing agency which size and height requirements apply
Official resources
Common questions
Can I enlarge the garage as part of an ADU?
An expansion may be possible depending on the proposal and applicable requirements. Ask the reviewing agency to evaluate the specific design.
Will a published floor plan fit my garage?
Not necessarily. Have the existing building measured and compare its dimensions, openings, and utilities with the proposed plan.
Does California set a single maximum ADU height?
No. HCD explains that local agencies may impose height limits under a compliant ordinance, subject to state minimum allowances. The City of San Diego also identifies zone and overlay limits. Ask the agency to apply the current rules to the property and proposed project, especially if changing the existing garage roof.
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