ADU Cost Calculator
A 700 sq ft detached accessory dwelling unit costs about $159,600 nationally — $228/sq ft all in. The number people find surprising is the split: 18% of that is utility connections and site access, not the building. Converting an existing garage instead brings the same size down to roughly $93,592.
Most jurisdictions cap ADUs between 800 and 1,200 sq ft. Check your local limit before designing.
Utilities and site access are 13% of this estimate. They barely change with size, so at 700 sq ft they are the single biggest lever on your cost per square foot — a bigger unit spreads them further.
A standalone small house. Full foundation, four walls, its own roof.
Heads up: at 700 sq ft you are below 740 sq ft, the smallest house in the federal data our per-square-foot model is validated against. Treat the construction figure as an extrapolation and get a local bid.
Why bigger ADUs cost less per square foot
This is the one place on this site where cost per square foot genuinely does fall as the building grows — and it is worth being precise about why, because the usual explanation is wrong.
It is not that small buildings are inefficient to build. Census microdata shows contract price per square foot is essentially flat across house sizes. What changes for an ADU is everything outside the building: a sewer tie-in, a trench across the yard, an electrical service upgrade, and the cost of working around an existing house. Those are near-fixed, so they land harder on a small unit.
| Size | Construction | Utilities & access | Total | $/sq ft |
|---|---|---|---|---|
| 400 sq ft | $75,200 | $28,000 | $103,200 | $258 |
| 600 sq ft | $112,800 | $28,000 | $140,800 | $235 |
| 800 sq ft | $150,400 | $28,000 | $178,400 | $223 |
| 1,000 sq ft | $188,000 | $28,000 | $216,000 | $216 |
| 1,200 sq ft | $225,600 | $28,000 | $253,600 | $211 |
National figures, mid-range finishes, a typical $28,000 utility scenario. The construction column scales with area; the utility column does not. That is the whole effect.
ADU cost by state
A 700 sq ft mid-range ADU in the states where they are most commonly built, usually because state law explicitly permits them.
| State | Detached | $/sq ft | Garage conversion |
|---|---|---|---|
| California | $218,820 | $313 | $130,308 |
| Oregon | $175,392 | $251 | $103,383 |
| Washington | $183,288 | $262 | $108,279 |
| Colorado | $172,760 | $247 | $101,751 |
| Texas | $149,072 | $213 | $87,065 |
| Florida | $166,180 | $237 | $97,672 |
| New York | $222,768 | $318 | $132,756 |
| Massachusetts | $205,660 | $294 | $122,149 |
Detached assumes a typical $28,000 utility scenario; garage conversion assumes $12,000 because services are usually close. Construction costs from this site's per-state model — see methodology.
What this can and cannot tell you
The construction figure uses the same per-state model as the rest of the site, adjusted for what an ADU type actually builds — a garage conversion inherits a slab, walls and roof, so it carries a smaller share of the work than a detached unit.
Two honest limits. First, the utility presets are scenario ranges, not measurements — no public dataset tracks what it costs to trench a particular backyard, and this is the single largest swing factor, so replace it with a contractor estimate as soon as you have one. Second, below 740 sq ft the construction figure runs past the smallest house in the federal data our per-square-foot model is checked against, and no backyard ADUs appear in that sample at all.
Not included: land (you already own it), permit and impact fees, which some jurisdictions levy heavily on ADUs, and any required parking, fire separation or utility-capacity upgrades your city may impose.
Data last refreshed 2026-08-24. Full methodology.
ADU cost questions
How much does it cost to build an ADU in 2026?+
A 700 sq ft detached ADU with mid-range finishes costs roughly $159,600 nationally — about $228 per square foot all in. Of that, $28,000 is utility connections and site access rather than the building itself. Converting an existing garage instead brings the same size down to about $93,592.
Why is an ADU so expensive per square foot?+
Because a large share of the cost does not scale with size. Running water, sewer and power to a second structure, and getting crews and concrete past an existing house, cost roughly the same whether the unit is 400 sq ft or 1,000. In the national example those fixed items are 18% of a 700 sq ft ADU. Going from 700 to 1,000 sq ft raises the total only to $216,000 while dropping the rate to $216/sq ft.
Is converting a garage cheaper than building a detached ADU?+
Substantially, yes. The slab, walls and roof already exist, and utilities are usually close by. A 700 sq ft garage conversion runs about $93,592 against $159,600 for the equivalent detached build. The trade-off is that you lose the garage, and an existing slab may not meet code for habitable space without work.