Step 07 · Compare
Every comparison here ends in "it depends", and then says precisely what it depends on. A page that declares a winner is answering a question nobody asked.
The short answer
The main ADU decisions are detached vs attached, conversion vs new build, prefab vs site-built, and ADU vs home addition. None has a universal answer: each is decided by your lot, your existing structures, your budget structure and what the unit is for. The tables below set out which option suits which situation.
01
Start from the constraint rather than the preference. In most projects one factor — an existing garage, a small lot, a septic system, a tight budget, an ageing parent moving in — decides the comparison before the others are weighed.
Where the decision genuinely is close, our garage-conversion-versus-new-build tool runs it on your garage’s actual condition rather than on a rule of thumb.
| Garage conversion | New detached | |
|---|---|---|
| Foundation | Existing slab, if it is sound | New pour |
| Utilities | Often shorter runs | Full runs to the rear yard |
| Yard space | Preserved | Consumed |
| Design freedom | Constrained by the existing shell | Open |
| Main risk | Slab condition discovered after demolition | Site conditions and utility distance |
| Parking | Garage lost; in California no replacement required (§ 66314(d)(11)) | Usually unaffected |
The decisive unknown in a garage conversion is almost always the slab, and it is frequently not knowable until work starts. Price that risk into the contract rather than the hope.
| Prefab / modular | Site-built | |
|---|---|---|
| On-site duration | Compressed | Longer |
| Site work required | Same | Same |
| Design flexibility | Limited to the catalog and its variants | Open |
| Access requirement | Crane and delivery access needed | Not applicable |
| Payment structure | Often larger deposits earlier | Draws against progress |
| Which standard applies | Depends on whether it is HUD-code or factory-built modular | Standard building code |
The word "prefab" covers legally distinct products. Which standard the unit is built to affects where it may be placed and how it is financed and appraised — see the prefab standards research.
| ADU | Addition | |
|---|---|---|
| Separate entrance and facilities | Yes, by definition | Not usually |
| Can be let separately | Yes, subject to local rules | No |
| California ministerial approval path | Yes, under §§ 66317 and 66323 | No — ordinary discretionary review may apply |
| Suits multigenerational living | Independence with proximity | Shared household |
| Effect on primary dwelling | Separate structure or clearly divided | Changes the house itself |
The ministerial approval path is a genuine and frequently decisive advantage of the ADU route in California. An addition gets no such protection.
Go deeper
ResearchOriginal analysis
StatisticsSourced figures
ToolsRun your numbers
Usually, because the slab and some of the shell already exist. But the saving depends on the slab being sound and on the utilities being reachable. Where the slab has to be removed, the advantage can disappear entirely.
Not reliably. Prefab compresses the on-site schedule, which has real value, but the site work, foundation, utilities and permits are unchanged. Compare total delivered cost including site work, not the unit price.
If you need a separate dwelling that can be let or occupied independently, an ADU — and in California it carries a ministerial approval path an addition does not. If you need more space in the existing household, an addition is usually simpler.
Plan your project
Three short steps: the ADU you want, your home, and how to reach you. Nothing goes to a contractor unless you agree.
A straight answer: our ADU professional matching is still being built, so this is not a quote service, and we cannot tell you whether your lot qualifies. We can help you work out which questions to ask and what a thorough proposal should contain. Your planning department decides what is permitted at your address.
Get ADU Options
Tell us about the ADU you are planning and where you are. We will follow up about connecting you with contractors serving your area.