ADU Size and Height Limits
A garage conversion sits in a favourable corner of the size rules: converting existing space is not held to the square-footage cap that applies to a newly built ADU, which is why a large two-car garage can become a bigger unit than you might expect.
California ADU law was renumbered by SB 477 in March 2024. The sections cited here are the current ones, Gov. Code 66310 to 66342. Local overlays still apply to your parcel, so confirm your specifics with your city building department or San Diego County before finalising plans.
What the rules actually say
- Converting an existing accessory structure, or part of the primary residence, is not subject to the 1,200 square foot maximum that applies to a new attached or detached ADU
- A conversion may add up to 150 square feet beyond the existing structure, and that allowance is strictly for ingress and egress, such as a staircase (Gov. Code 66323(a)(1))
- A JADU runs from 150 to 500 square feet, inside the single-family residence or an attached garage
- Height: a local cap cannot go below 16 feet for a new detached ADU, or 18 feet within half a mile walking distance of a major transit stop or high-quality transit corridor (Gov. Code 66321(b)(4))
- An attached ADU is limited to 25 feet or the height allowed in the zone, whichever is lower
Common questions
How big can a garage conversion ADU be?
As big as the existing garage, in practice. The 1,200 square foot maximum applies to newly built attached and detached ADUs; converting an existing accessory structure or part of the primary residence is not subject to it. You may add up to 150 square feet beyond the existing footprint, but only to accommodate ingress and egress (Gov. Code 66323(a)(1)).
Is the extra 150 square feet a bonus I can use for living space?
No, and this is a common misreading. The statute allows the expansion "solely to accommodate ingress and egress", meaning access such as a staircase or doorway. It is not general bonus square footage.
Can an ADU be two storeys?
Height is where that gets decided, and it depends on your zone and how close you are to transit. State law sets floors a local cap cannot go below: 16 feet for a new detached ADU, or 18 feet within half a mile walking distance of a major transit stop or a high-quality transit corridor (Gov. Code 66321(b)(4)). Your building department applies the base zone height on top of that.
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