How this calculator works
Total = (sum of the eleven lines) × (1 + contingency ÷ 100). The tool lists every line you left blank.
What each input means
| Input | What it is actually asking |
|---|---|
| Slab | Garage slabs may lack a vapor barrier or footings. If an engineer says it must be replaced or a raised floor added, this line grows. |
| Door infill | The new wall, window or glass door where the garage door was. See keeping the garage door. |
| Plumbing | Includes cutting the slab and the run to the sewer lateral. |
| Contingency | A reserve you hold, not a line the builder spends. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
A bid with three blank lines
Type every line a bid shows. If slab, plumbing and door infill stay blank, the tool lists them. Ask the bidder whether they are excluded or hidden in another line.
Two bids side by side
Run each bid separately and compare line by line, then use the ADU bid comparison to check they price the same drawings.
What changes the result
The slab and plumbing lines swing most between garages. Get the slab checked before you collect bids, so every bidder prices the same fix.
When not to use this
Do not use the total as a quote. Only a bid on your drawings is a price.
Questions this calculator answers
- How much does a garage conversion cost?
- We publish no figure; it depends on the slab, plumbing route and finishes. Add the lines from your own bids.
- Do impact fees apply to a garage conversion?
- In California, not on an ADU of 750 sq ft or less of interior livable space (Gov. Code § 66311.5).
- Is parking replacement a cost line?
- Not in California for an ADU conversion (Gov. Code § 66314(d)(11)).
- What contingency should I use?
- Your choice. Older garages carry more unknowns.
Sources and methodology
Figures dated 27 October 2026. Last reviewed .
- Gov. Code § 66311.5 — ADU impact fees and connection charges (California Legislature, retrieved 2026-09-05)
- Gov. Code § 66314 — ADU standards, including replacement parking (California Legislature, retrieved 2026-09-05)
Related
- Garage conversion vs new build Whether the garage is worth converting.
- Garage conversion plans What the drawings show.
- ADU cost What decides the price.