How Much Does an ADU Cost in San Diego? What Sets the Price

The three routes

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 published San Diego figures show

Published figures, not Construction Crafters prices. Each line names its source and year; the older surveys predate recent cost increases. Your project is priced only after a site visit.

What actually moves the number

• 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.

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.

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 routes fits, before anyone walks the property.