Free Estimate
Every lot is different, so every ADU price is different. This page explains what makes up the cost, which parts cost the most, where the lowest-cost options are, and how you get a written price for your own property.
A complete detached ADU is a small house: a foundation, framing, roof, and siding; its own water, sewer, and electrical connections run from the street; a kitchen and a full bathroom; heating and cooling; windows, doors, and finishes; and the architect, engineering, permits, and inspections that make it a legal dwelling. A unit for a parent is usually around 400 square feet with one bedroom. The list is the same on every lot. What changes from lot to lot is how much of each item your property needs, which is why the price is set after a site visit rather than from a chart.
| Cost driver | What it changes | Effect |
|---|---|---|
| Type of ADU | A garage or basement conversion reuses an existing shell. A detached unit needs a foundation, roof, and every wall built new. | Conversions usually cost the least; detached costs the most |
| Size | Every added square foot adds framing, roof, floor, siding, and finish. The kitchen and bath cost the same whether the unit is 400 or 700 square feet, so cost per square foot drops as size rises. | Large |
| Kitchen and bathroom | The wet rooms are the most expensive rooms in any home: plumbing, tile, cabinets, fixtures, and venting. | Large. A second bath adds a lot. |
| Utilities and site | Distance from the street to the unit for water and sewer, trenching, an electrical panel upgrade, slope, soil, and trees. | Medium to large, and different on every lot |
| Finishes | Flooring, counters, cabinets, tile, fixtures, and windows. | Medium. This is the lever you control most. |
| Permits and city fees | Plan review, building permits, and system development charges, which vary by city. | Small to medium. Included in our estimate. |
| Design and engineering | Architect's drawings and structural engineering. | Small. Included in our estimate. |
The bathroom and kitchen, followed by the site work. Two rooms hold nearly all the plumbing, tile, and cabinetry in the building. On a difficult lot, getting water and sewer from the street to the back of the property can cost as much as a bathroom. Joel checks both on the site visit, which is why the price range he gives you on day one is usually close to the final number.
A budget goes further in a conversion. A two-car garage can become a 500 to 600 square foot apartment for less than a detached unit of the same size, because the foundation, walls, and roof already exist. On a detached build, the budget goes further on a flat lot with the utilities close to where the unit will sit, and with a simple roofline and standard windows.
A conversion of an existing garage or basement, when the structure is sound and dry. The trade-off is that you lose the garage, and a basement unit needs enough ceiling height and a proper exit window to be legal.
A permitted second dwelling adds living space, appraised value, and the option of rental income. Most of the families we build for use the unit for a parent first and rent it later. Ask Joel about the rents units like yours are getting in your part of Gresham or Portland.
It depends on the type, the size, the finishes, and how much site and utility work your lot needs. Joel gives you a working range for your property on the site visit and a written price after design. Conversions cost less than detached units of the same size.
Convert a garage or basement that is already sound and dry. The shell exists, so you pay mainly for the interior, plumbing, and utilities.
Yes. GenTerra's estimate covers the architect, engineering, permit fees, and city charges along with construction, so the number you approve is the whole project.
Book a site visit. Joel looks at the lot, the utility runs, and the size the city allows, and gives you a working range that day and a written price after design.
A site visit gives you a working range the same day and a written price after design.