ADU comparisons
Garage conversion vs detached ADU: which route to take
By the HyreADU research desk. Last checked .
The short answer
Garage conversion vs detached ADU depends on the garage. A conversion reuses an existing slab, walls and roof, keeps its existing setbacks under California law, and needs no replacement parking.
A new detached ADU starts clean, can be laid out freely and built to current code from the ground up. If the slab or framing must come out, much of the conversion’s head start goes with it.
| Factor | Garage conversion | New detached ADU |
|---|---|---|
| Foundation | Reuses the garage slab if it can carry a home | New foundation |
| Setbacks in California | None required for an existing structure converted in place (§ 66314(d)(7)) | Up to 4 ft side and rear (§ 66323) |
| Replacement parking | Not required in California (§ 66314(d)(11)) | Not required if parking is not lost |
| Layout | Fixed by the garage footprint and ceiling height | Free within the size and height rules |
| Energy code | Existing walls and slab must be brought up to code for the new use | Built to current code from the start |
When a garage conversion wins
A conversion wins when the garage is sound, close to the sewer line, and sitting where a new building could not go. California lets an existing structure keep its location and size with no setback, so an old garage on the property line can become a home where new construction would need four feet.
Our garage conversion research shows how common the route is in Los Angeles. Ideas for the space are on garage conversion ideas.
When a new detached ADU wins
A new build wins when the garage slab has no vapor barrier or footings, the walls are too short for a habitable ceiling, or the footprint is too small for the rooms you want.
Rebuilding those parts of a garage can leave little of the original structure. A new ADU also lets you place windows and doors for privacy rather than where the garage door was.
Test the garage before you choose
A structural engineer or experienced builder can check the slab, footings and framing in one visit. Then run both routes through the garage conversion vs new build tool, which ends the comparison early when the garage fails a basic test, and add up conversion bids in the garage conversion cost calculator.
A mistake to avoid
The mistake to avoid is assuming the garage slab is usable. Many older garage slabs have no vapor barrier and shallow edges, and a building department may ask for engineering to show they can support a home. Pay for that check before the design, not after the plan check comments arrive.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Do I lose parking with a garage conversion?
You lose the garage space, but in California the city cannot make you replace that parking (Gov. Code § 66314(d)(11)).
Can I keep the garage door?
Sometimes, if the wall still meets energy and egress rules. See garage conversion with a garage door.
Is a conversion always faster?
On site it can be. Permit review still applies, and slab or framing repairs add time.
Can I convert a garage and build a detached ADU too?
California allows certain combinations on one lot under § 66323. Ask your city which apply.
Sources
- Gov. Code § 66314 (ADU standards: parking, setbacks, sprinklers). Checked September 5, 2026.
- Gov. Code § 66323 (ADUs a city must approve ministerially). Checked September 5, 2026.
- California Energy Commission, 2025 Building Energy Efficiency Standards (Title 24, Part 6). Checked September 5, 2026.
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