If your garage is sound and close to the size you want, converting it usually costs less: as of October 2026, San Diego builders publish about $100,000 to $225,000 for conversions against roughly $250,000 to $460,000 for new detached units. A new ADU is the better call when you need more room, a better layout, or the garage is failing.
How do the two options compare?
| Factor | Garage conversion | New detached ADU |
|---|---|---|
| Published San Diego cost range, as of October 2026 | About $100,000 to $225,000 | About $250,000 to $460,000 |
| Size | Set by the existing footprint; conversions are not subject to the state's unit size limits | Cities must allow at least 850 sq ft (1,000 sq ft with more than one bedroom) |
| Setbacks | None required for the existing structure under state law | Up to 4 feet at the sides and rear |
| Height | Limited to the original structure | 16 feet base, more near qualifying transit |
| Parking | Lost garage spaces do not have to be replaced | Often exempt; see the parking rules |
| Solar | Not required for conversions | Generally required on a new detached unit |
Cost figures come from Realm (March 2026) and Lars Remodeling (October 2026); rules come from the HCD ADU Handbook. None of it is our pricing.
What makes a San Diego garage a good candidate?
- A usable slab. Cracked, sloped or thin slabs may need replacement, which erodes the savings.
- Sound framing and roof. Termite damage and dry rot are common in older wood-framed garages and must be repaired.
- Enough ceiling height for a finished room once insulation and drywall go in.
- Reasonable distance to the sewer line, so a new bathroom and kitchen can drain without major trenching.
- A legal, permitted structure; an unpermitted garage may need extra work first.
An attached garage can become a junior ADU of up to 500 sq ft, or an attached ADU, rather than a separate unit. See junior ADUs.
What do I give up with a conversion?
Covered parking and storage. As of October 2026, state law does not require you to replace parking lost when a garage becomes an ADU, but the cars still have to go somewhere, and some San Diego streets are already tight. Laundry, workbenches and bikes need a new home too.
When does a new ADU make more sense?
- You want two bedrooms, or a two-story unit.
- The garage sits where you would rather keep yard space.
- The structure is poor enough that repairs approach new-build cost.
- You want the privacy of a unit set apart from the main house.
Can I do both?
Possibly. The HCD Handbook describes a state-law combination of a converted ADU, a junior ADU and a new detached ADU on a single-family lot, subject to conditions. How that plays out under the City of San Diego's own code depends on the lot, so we confirm it per address.
Sources, as of October 2026:
- Realm, "The Real Cost to Build an ADU in San Diego" (March 11, 2026).
- Lars Remodeling & Design, "What Does It Cost to Build an ADU in San Diego in 2026?" (published August 13, 2026; updated October 30, 2026).
- California Department of Housing and Community Development (HCD), ADU Handbook, updated March 2026, with its addendum on law in effect January 1, 2026, hcd.ca.gov.