Prefab ADUs and ADU Kits
Prefabricated and kit ADUs are marketed on speed, and the single most useful thing to know before you shop is that they do not shorten or skip the permit. A prefab unit lands on your lot into the same plan check, the same inspections, and the same energy code as anything built on site.
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
- A building permit is required to create an ADU or JADU, and the City of San Diego states plainly that there are no exemptions
- Approval is ministerial either way, so a factory-built unit gets no procedural advantage from being factory-built (Gov. Code 66317)
- California Title 24 energy compliance, structural requirements, and local zoning all still apply to a delivered unit
- A prefab unit set on the lot is new construction, so it faces the 1,200 square foot cap and the setback rules that a garage conversion does not
- Converting the garage you already have and dropping a prefab unit in the yard are different projects with different rules, even though they are sold side by side
Common questions
What is a prefabricated ADU?
A prefabricated ADU is built in a factory as modules, trucked to your property, and set on a foundation. An ADU kit is the same idea sold as parts to assemble on site. Both are new construction on your lot, which is a different legal category from converting the garage that is already standing there.
Does a prefab ADU skip the permit process?
No. The City of San Diego states that a building permit is required to create an ADU or JADU and that there are no exemptions. The factory work is inspected at the factory, but siting, foundation, utility connections, and final inspection all still happen at your property under a local permit.
Is a prefab ADU cheaper than converting my garage?
That depends entirely on your lot and the unit, and anyone quoting you a firm comparison without seeing the property is guessing. The structural advantage of a conversion is that the foundation, walls, and roof already exist. The advantage of a prefab is schedule certainty. Get both priced against your actual site before deciding.
Do the same size rules apply to a prefab unit?
No, and this catches people out. A prefab unit placed in the yard is new construction, so the 1,200 square foot maximum and the four-foot side and rear setback rules apply to it. Converting an existing garage is not subject to that square-footage cap (Gov. Code 66323), which is often the bigger unit.
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