How Much Does a Garage Conversion ADU Cost in San Diego? (2026 Guide)

If you are weighing an ADU project in San Diego, the first question is usually how much it costs to convert a garage to an ADU, and the second is whether building a new unit would be the smarter move. The honest answer is that a garage conversion is usually the cheaper path, often by a wide margin. This guide gives you a real 2026 planning number, breaks down what moves the price up or down, and covers the cases where building new is still the better call.

The Short Answer

A garage conversion reuses the most expensive parts of any building: the foundation, the framing, and the roof are already there and already paid for. A new-build ADU has to create all of that from scratch, plus trench new utilities across your yard and complete site work before a single wall goes up. Converting almost always wins on cost, and it usually wins on speed too.

Here is the comparison at a glance:

	Garage Conversion	New-Build ADU
Structure	Foundation, framing, and roof already exist	Built entirely from scratch
Construction cost	Roughly $350 per sq ft, contractor dependent	Meaningfully higher per sq ft
Utilities	Typically shared with the main house	New runs trenched across the lot
Site work	Minimal	Grading and drainage often required
Typical size	Studio or one-bedroom within the existing footprint	Larger layouts, two bedrooms possible
Build time	Shorter, unit earning sooner	Longer, more carrying cost

How Much Does It Cost to Convert a Garage to an ADU in San Diego?

For construction, the number we see most often from contractors in the San Diego area is roughly $350 per square foot for a garage conversion, though it varies with your contractor and the finishes you choose. As a rough planning figure, a typical two-car garage of around 400 square feet works out to construction in the neighborhood of $140,000. A one-car garage runs proportionally less.

Two clarifications on that number. First, it is the construction cost paid to your contractor, not a quote: your garage's condition, the utility work involved, and your finish level all move it. Second, it does not include design and permitting, which is a separate, much smaller line item that happens first. That part is our role, and we quote it as a fixed, itemized price at a free consultation, so it is one number you never have to guess at.

Garage conversion cost per square foot, by garage size

Because contractors price conversions per square foot, the size of your garage is the biggest single lever. Using the $350 per square foot benchmark, here is roughly where construction alone lands for the garage sizes we see most often in San Diego:

Garage	Typical size	Construction at about $350 per sq ft
One-car garage	220 to 260 sq ft	$77,000 to $91,000
Small attached garage	About 300 sq ft	About $105,000
Standard two-car garage	About 400 sq ft	About $140,000
Oversized or tandem garage	440 to 480 sq ft	$154,000 to $168,000

Treat these as planning ranges, not quotes. A garage in good structural condition with utilities close by can come in under the benchmark; one that needs a new slab or a long sewer run can go over it. Attached garages usually land toward the lower end because they share walls with the house and sit close to the existing water, sewer, and electrical service.

Here is a real number from one of our own projects. An 18 by 20 foot single-car garage, 360 square feet that sat mostly unused, was converted into a one-bedroom ADU with a full kitchen and bathroom. Construction came in at $130,000, roughly $360 per square foot and right in line with that benchmark. As always, the final figure depends heavily on the contractor you choose and the condition of the structure.

The other side of that math matters just as much: the finished unit now rents for $1,800 per month, about $21,600 per year. At that rate the construction cost pays for itself in roughly six years of gross rent, and every month after that is income from square footage that used to store boxes.

That example is our project Thirty-Nine, and its project page shows the full transformation, including before photos of the original garage.

What Moves the Price Up or Down

Two garages of the same size can land tens of thousands of dollars apart. These are the factors that decide where yours falls:

• Structural condition. A sound slab, framing, and roof are the whole reason conversions are cheap. A cracked or sloped slab, undersized headers, or a roof near the end of its life eat into that discount.
• Utilities. Adding a kitchen and bathroom means new plumbing, and older homes often need an electrical panel upgrade to carry a second unit. The distance from the garage to the existing sewer and water lines matters more than most homeowners expect.
• Kitchen and bathroom scope. These are the two most expensive rooms per square foot in any home. A compact kitchen and a single bath keep the budget tight; a full-size kitchen and a spa bath do not.
• Envelope work. Filling in the garage door opening, adding egress windows, insulating the walls and ceiling, and meeting California energy code all add scope that a storage garage never needed.
• Heating and cooling. A ductless mini-split is the usual answer and is relatively affordable; extending the main house system can cost more.
• Finish level. Flooring, cabinetry, tile, and fixtures are where the same floor plan can swing widely, and it is the lever you control most directly.

Why New-Build ADUs Cost More

A detached new-build ADU carries a meaningfully higher cost per square foot, because the project scope is simply bigger:

• Foundation, framing, and roof from scratch. These are the priciest phases of construction, and a conversion skips all three.
• Utility runs. Water, sewer, and electrical have to be trenched from the main house or the street to wherever the new unit sits on the lot.
• Site work. Grading, drainage, and sometimes retaining conditions on sloped San Diego lots add scope before construction even starts.
• Longer build time. More months of construction means more carrying cost before the unit starts earning rent or housing family.