What an ADU really costs, and where the money goes

Ask three builders what an ADU costs and you will get three numbers, none of them comparable. One is a base price for a box. One includes the foundation but not the sewer line. One is "all in" but assumes your lot is flat. Before you call anyone, it helps to know what real owners actually paid.

The best statewide number we have

The largest public survey of ADU owners comes from California. Researchers at UC Berkeley's Center for Community Innovation surveyed owners across the state and found a median construction cost of $150,000, or $250 per square foot. That figure covers design, labor, materials and permits. It does not include land, because the owner already has it.

The spread matters as much as the median. In the same survey, 37 percent of ADUs cost less than $100,000 and 71 percent cost less than $200,000. The survey was published in April 2021 and reflects units built before then, so treat it as a floor for today's prices, not a quote.

Where you live changes the price

Region made a big difference in that survey:

• San Francisco Bay Area: median $177,500, or $329 per square foot
• Central Coast: median $140,000, or $223 per square foot
• Orange and San Diego counties: median $130,000, or $200 per square foot
• Los Angeles County: median $100,000

Los Angeles came in lowest partly because of what people built there. The researchers note that garage conversions made up 39 percent of ADUs in Los Angeles County, compared with 11 to 14 percent elsewhere, and conversions are the cheapest type to build.

What kind of ADU you build matters more

The same survey broke costs out by type. These are medians:

• Detached new unit: $180,000 ($300 per square foot)
• Addition to the main house: $150,000 ($250 per square foot)
• Garage conversion or expansion: $90,000 ($189 per square foot)
• Conversion of space inside the house: $100,000 ($173 per square foot)

Bedrooms move the number too. A studio had a median cost of $100,000, a one-bedroom $150,000 and a two-bedroom $212,500. The average new ADU in the sample was 615 square feet, and 61 percent had one bedroom.

Why small costs more per foot

A 400 square foot unit still needs a kitchen, a bathroom, a water heater, an electrical panel, a foundation, a permit and a set of drawings. Those costs do not shrink with the floor plan. The San Diego Housing Commission, which built five ADUs as a public pilot, put it plainly: ADUs include the same features as a house in a smaller footprint, so the cost per square foot can be more than 25 to 35 percent higher than a typical single-family home.

Their own numbers show it. The pilot's 224 square foot studio cost $521 per square foot. The 1,199 square foot three-bedroom cost $285 per square foot. The studio was cheaper overall, but far more expensive per foot. If a builder quotes you a low per-foot price, check the size it assumes.

Soft costs and the cushion you need

Soft costs are everything that is not the building itself: design, engineering, surveys, permits and fees. In the San Diego pilot, soft costs including contingency came to roughly 20 to 30 percent of the total. The highest soft costs were permits and city fees, especially on units over 500 square feet.

The same report recommends a contingency of 15 to 20 percent for an ADU, compared with about 10 percent for a typical new house. That is not padding. Small backyard projects run into old sewer lines, undersized electrical panels and tight access more often than you would expect.

Fees you may not have to pay

In California, state law limits what cities can charge. An ADU with 750 square feet of interior living space or less is exempt from impact fees, and larger units can only be charged in proportion to the size of the main house. School districts can only charge on units over 500 square feet. Other states handle this differently, and our state rules guide covers the basics for five of them. If you are near the 750 square foot line, ask your designer what the fee difference would be before you finalize the plan.

A quick budgeting checklist

• Get the survey median for your region as a sanity check, then add inflation since 2021.
• Ask every builder for a written list of what is excluded: foundation, utility trenching, panel upgrade, permits, fees, landscaping, fencing.
• Price the site work separately. It is where most surprises come from.
• Hold 15 to 20 percent as contingency.
• Compare quotes on total cost for the same scope, not on price per square foot.

Before you call a builder: answer six questions to pick a type, rough out a budget, and save builders to My Stack as you go.