Garage Conversion ADU San Diego (2026) — Cost & Permits

Why there is no fixed price

The three plans we publish prices for are ours from the slab up. We drew them, we have built them, and nothing underneath is a surprise. A conversion is the opposite: you inherit a slab somebody poured in 1964, a frame somebody else framed, and a roof that has been doing its own thing for sixty years. What is behind the drywall decides the number, and nobody can know that until it is open.

Any builder who quotes you a firm conversion price before opening it up is either padding heavily to cover themselves or planning to come back with change orders. We would rather give you a range, tell you what moves it, and put a real number in writing after we have actually looked.

What it costs in San Diego

Garage conversion ADU, City of San Diego, 2026. Permit fees are the city's published figures and sit outside the construction cost.
Item	2026 figure
Typical range	$100,000 – $140,000
Two-car garage, 400–450 sq ft	$120,000 – $135,000
Difficult conditions	Up to about $200,000
Roughly, per square foot	About $300
Plan check fee	$3,512.92
Inspection fee	$2,228.29 for the first 500 sq ft
Development impact fees	None — every conversion is under 750 sq ft

The good news first: you keep the parking

State law prohibits a city from making you replace the parking spaces you lose by converting a garage. No new carport, no new driveway, no hunting for somewhere to put a replacement space. This is the single most common thing homeowners assume will kill the project, and it does not.

The five things that decide your number

1. The slab and footings — and this is the one

A garage slab was poured to park a car on, which means it very likely has no vapour barrier under it. Put a floor and a bed on top of that and the moisture coming up through the concrete has nowhere to go.

The retrofits that work are a liquid-applied vapour membrane, a poly sheet with a 1.5 to 2 inch topping slab over it, or a moisture-mitigation epoxy. Which one depends on what your slab is doing, and several jurisdictions want an Alternate Materials Request filed before they will accept a retrofit at all.

This is the most common inspection failure on a garage conversion in California, and it is the item we most often find missing from somebody else's quote. If a conversion bid you are holding does not mention the slab, that is not because the slab is fine.

Underneath the slab, the footings sometimes have to be retrofitted too. They were sized for a garage, and when the engineer runs the numbers for habitable space they can come up short. That is especially likely when the garage sits under living space — a soft-story condition, which is common across San Diego and Los Angeles. There the foundation, the footings and the slab often need to be beefed up before anything else goes in.

2. Ceiling height

Habitable space needs a finished clear ceiling height of seven feet. Bathrooms, laundry and toilet rooms can come down to six feet eight. That sounds generous until you remember you are adding a floor assembly, insulation and a ceiling into a space that was measured to clear a garage door. On a low garage the height is what rules the project out, and we check it first.

3. Fire separation, if it is attached

An attached garage sharing a wall or a floor with the house needs a one-hour rated assembly between the two — typically five-eighths Type X gypsum on both sides over wood studs. Every penetration through it, every pipe, cable and duct, has to be fire-stopped with a listed assembly. Detached garages skip this entirely, which is one reason a detached garage is usually the cheaper conversion.

4. Egress

Every sleeping room needs an emergency escape opening: at least 5.7 square feet clear, no less than 24 inches high and 20 inches wide, with the sill no more than 44 inches off the floor. An exterior door satisfies it. In practice this usually means cutting a new window into a wall that currently has none, which is structural work in a bearing wall.

5. Title 24

A garage is an uninsulated box and the energy code treats your conversion as new habitable space. That is a full thermal retrofit — roughly R-21 in the walls, R-30 to R-38 in the ceiling, windows at U-0.30 or better — and a CF1R compliance report to go with it. It is not optional and it is not cheap, and it is the other line people find missing from a low bid.

When a conversion is the right call, and when it is not

It is the right call when the garage is detached, the ceiling is tall enough, the slab is in reasonable condition, and the budget is the binding constraint. At $100,000 to $140,000 against $299,000 for the smallest new build, that is a serious difference and we will say so.

It is the wrong call when the ceiling is too low to recover, when you actually want the garage, or when you want two bedrooms — a two-car garage finishes at roughly 400 square feet, and that is a one-bedroom at best. It is also the wrong call if a predictable number matters more to you than the saving, because a conversion will not give you one until it is open.

We do both, so we have no stake in talking you into the more expensive one. If the garage is the better answer for your lot, that is what we will tell you on the site walk.

Sources

Garage conversion in San Diego — cost, permits and rules
Garage conversion to ADU San Diego — 2026 cost
Garage conversion ADU — 2026 California code and process
California garage conversion — parking rules and what cities cannot require
Our San Diego ADU rules page — sizes, setbacks and the 2026 law changes

Permit fees are the city's published figures as at October 2026 and they move. Nothing here is a determination about your garage; the only way to get one of those is for somebody to stand in it.

Garage to ADU conversion in San Diego.