How Much Does an ADU Cost in San Diego? (2026 Price Guide)

An ADU is the most common question we get, and the hardest one to answer with a single number. The gap between the cheapest and the most expensive version of the same idea is well over three hundred thousand dollars, and almost none of that gap is about finishes. It is about how much building you are creating from scratch.

The three ranges, plainly

There are three routes to an accessory dwelling unit, and they price very differently because they involve very different amounts of new construction.

A garage conversion is cheapest because the slab, three or four walls and the roof already exist. You are paying to make an existing shell habitable. A detached unit is the most expensive because you are paying for a complete small building: foundation, framing, envelope, roof, utilities run across the property, and its own everything.

What actually moves the number

Once people see those ranges the next question is always why a project lands at the top rather than the bottom. In San Diego it is usually one of five things, and finish level is the smallest of them.

• Utilities. How far the new unit sits from the existing electrical panel, sewer line and water supply. Trenching across a property is real money, and a panel upgrade is common when you add a second dwelling.
• Site conditions. Slope, access for machinery, soil, and whether a truck can get to the back of the lot at all. A tight lot with hand-dug footings costs more than an open one.
• Foundation type. A flat pad and a simple slab is the cheap case. Stem walls, piers or anything engineered for slope pushes it up.
• Bathroom and kitchen count. Every wet room adds plumbing, waterproofing, ventilation and inspection.
• Design and permitting complexity. Coastal zones, hillside overlays and historic districts all add review time and drawing work before anyone builds anything.

Where the money goes inside the build

It surprises people how little of an ADU budget is the part you can see. In a typical detached unit the structure, envelope and systems account for most of the cost, while cabinets, tile and fixtures are a comparatively small slice. This matters when you are trying to reduce a number: cutting the finish package saves less than you would hope, and changing the size, the foundation or the utility runs saves far more.

Garage conversion: the cheapest route, with conditions

If you already have a garage, converting it is almost always the lowest-cost path to a legal unit. But a garage is not a room yet, and the work to change that is not cosmetic.

• Ceiling height has to meet the minimum for habitable rooms, which some older garages do not.
• The roll-up door opening becomes a wall, so a window or door has to replace it for light and egress.
• Slab, walls and roof all get insulated and detailed for moisture.
• New circuits, proper lighting, heating and ventilation get added to code.

None of that is exotic, but it is why a conversion is a construction project rather than a decorating one, and why the range starts where it does.

What your estimate should look like

The single most useful thing you can do when comparing contractors is refuse to accept one number. A real estimate is a line-item document with allowances, so you can see what is assumed and where the risk sits. If a bid is a single figure on one page, you cannot tell whether it is cheap or simply incomplete, and the difference will surface later as a change order.

Ask for the allowance amounts specifically. Allowances are the placeholders for things not yet selected, like tile or fixtures. A bid can be made to look cheaper simply by setting those allowances unrealistically low.

Before you call anyone

Three pieces of information make a first conversation far more productive: roughly what size unit you want, whether it is for family or for rent, and whether you have a garage or a clear piece of yard to build on. With those three answers a contractor can tell you which of the three ranges above you are actually in, before anyone walks the property.