ADU construction cost = floor area × cost per sq ft; a 600 sq ft detached ADU costs about $150,000 at US-average prices, or $250 per sq ft all in, with a range of $108,500 to $200,500. Converting an existing garage of the same size is far cheaper, about $63,000.
How the ADU construction cost is calculated
An accessory dwelling unit is a small house, so a detached ADU is priced like one: per square foot, split into stages. Published guides disagree on the price per square foot, and the calculator takes the whole spread:
| Source | Price per sq ft |
|---|---|
| HomeAdvisor, building an in-law suite (June 2026) | $100 to $200 |
| Fixr, stick-built cottage (2022) | $170 to $300 |
| Terner Center, median California ADU (2021 survey) | $250 |
| Portland detached ADUs, 2016 to 2019 (Building an ADU) | $305 average |
The calculator uses $100 to $300 with a typical of $192.50, the mean of the two guides' midpoints. It splits the total into stages with Fixr's cottage breakdown (1,500 sq ft): planning and site prep 8%, foundation 11%, framing and exterior 41%, plumbing, electrical and HVAC 15%, interior 25%. Finish level moves the price inside the range. A general contractor's fee of 10% to 20% of the job (HomeAdvisor) and a 10% contingency sit on top.
Detached cost = area × price per sq ft for the finish level, split into five stages + panel upgrade + permits + 10% contingency + contractor fee
Worked example: 600 sq ft detached, mid-range
| Line | Typical |
|---|---|
| Site work and planning | $9,200 |
| Foundation | $12,700 |
| Shell: framing and exterior | $47,500 |
| Mechanical: plumbing, electrical, HVAC | $17,300 |
| Interior finishes | $29,000 |
| Electrical panel upgrade at the main house | $1,600 |
| Permits | $1,600 |
| Contingency, 10% | $11,700 |
| General contractor fee | $19,600 |
| Total | $150,000 (range $108,500 to $200,500) |
The $250 per sq ft result matches the California median. Against HomeAdvisor's $100 to $200 per sq ft the same unit would be $60,000 to $120,000 before contingency and fee; Fixr's stick-built cottage range gives $102,000 to $180,000.
Cost by type of ADU
| 600 sq ft, mid-range | Typical | How it is priced |
|---|---|---|
| Detached, built new | $150,000 | per sq ft in five stages |
| Attached addition | $135,000 | room addition $80 to $210 per sq ft (HomeAdvisor) + plumbing, water heater, HVAC |
| Garage conversion | $63,000 | conversion $25 to $75 per sq ft (HomeAdvisor) + plumbing, water heater, HVAC |
Converting a garage skips the foundation, framing and roof, which is why it costs a fraction of a new build. It adds what a dwelling needs that a garage lacks: HomeAdvisor's in-law suite page gives rough-in plumbing at $6,500, a water heater at $1,400, and a new HVAC system at $5,000 to $12,500. A panel upgrade at the main house is $1,600. See the garage conversion cost calculator for the shell alone.
Cost by size
| Size | Detached, mid-range |
|---|---|
| 400 sq ft | $101,500 |
| 600 sq ft | $150,000 |
| 800 sq ft | $198,500 |
| 1,000 sq ft | $247,500 |
Small ADUs cost more per square foot than large ones in practice, because the kitchen, bath, utility hookups and permits are the same whatever the floor area. The calculator uses a flat rate per sq ft, so treat the smallest sizes as a floor.
What raises the price
- Utilities. A sewer or septic connection ($8,986 to $16,350 for a septic system, Fixr) and a separate meter can rival the foundation.
- Fees. Permit costs are $200 to $3,000 in HomeAdvisor's addition guide, but impact and utility connection charges are set by your city and can be much higher. They are not in the estimate.
- Region. Coastal markets run well above the US average: the calculator scales labor and materials by state.
- Design. Custom architectural plans add to the planning share; stock plans reduce it.
What to work out next
Size the concrete slab, the framing and the electrical load. To compare with a room added to the house see the home addition cost calculator, and for a full house see the house construction cost calculator. Every other estimate is in the project costs hub.
Zoning rules for ADUs (size caps, owner occupancy, parking) vary by city; check before you draw plans. These are dated estimates from published national data adjusted for your state, not a bid.