The core difference
A garage conversion ADU reuses an existing structure — the foundation, walls, and roof are already there, so you're primarily changing the interior and adding utility connections. A detached ADU is new construction from the ground up, built wherever your lot has usable space, independent of any existing structure.
Side-by-side comparison
| Factor | Garage Conversion | Detached ADU |
|---|---|---|
| Typical Cost | $115,000 – $210,000 | $170,000 – $420,000+ |
| Typical Timeline | 4–8 months | 8–15 months |
| Foundation & Framing | Reuses existing structure | Entirely new construction |
| Lot Impact | Uses space you already have | Uses backyard or side-yard space |
| Design Flexibility | Constrained by existing footprint | Most control over layout and size |
| Parking Impact | Removes existing garage parking | Generally no parking impact |
Figures reflect typical ranges across our Southern California service area — see our full ADU cost guide for the complete breakdown.
When a garage conversion tends to make more sense
- You have a garage you rarely use for parking — many homeowners use garages for storage rather than cars, making the trade-off straightforward.
- Budget and timeline are the priority. Reusing an existing structure generally costs less and moves faster than building from scratch.
- Your backyard doesn't have much usable space, but your garage does.
- Your city ties conversion size to the existing structure — some cities, like the City of Orange, have specific rules here worth understanding before you commit to this path.
When a detached ADU tends to make more sense
- You still need your garage for parking or storage and don't want to give it up.
- You have real backyard space that meets setback requirements and isn't otherwise being used.
- You want maximum design control over layout, size, and finishes, unconstrained by an existing structure's footprint.
- Rental appeal or long-term flexibility is the priority — a fully separate, purpose-built unit is often the most attractive to a prospective tenant, though actual outcomes depend on your specific property and market.
What actually decides it, in practice
For most homeowners, this comes down to whether the garage is more valuable to you as parking/storage or as a starting point for a unit, and whether your backyard genuinely has room for new construction once setbacks are applied. A free property review looks at your specific lot rather than asking you to guess.
Frequently asked questions
Generally yes, since it reuses an existing foundation and structure rather than requiring entirely new construction — but the gap can narrow if your garage needs significant structural work to meet current code. See our ADU cost guide for typical ranges.
Yes, generally — the garage space becomes living space. California ADU law generally doesn't require replacement parking for a garage conversion, but you'll need to plan for where you'll park going forward.
On many qualifying lots, yes — see our guide on how many ADUs one property can have for how a conversion ADU and a new detached ADU can potentially combine on the same lot.