ADU and DADU costs, Greater Seattle Area
How Much Does It Cost to Build an ADU?
An ADU in the Greater Seattle Area costs $150,000+ for a garage or basement conversion, $250,000+ for an attached ADU, $325,000+ for a 300 to 600 sq ft backyard DADU and $425,000+ for a 600 to 1,000 sq ft DADU, at Volcano Builders' Basic finish level. Design, permit fees and utility connections are priced separately.
Below are our prices by type and finish level, what sits outside them, and how layout choices change the total. To have us check your lot and price your unit in writing, Book a Consultation.
What does each type of ADU cost to build?
Three things set the price of an ADU: whether it is converted, attached or detached, its size and number of baths, and the finish level. The table lists our published starting prices for each type at all four finish levels, with the typical time on site after design and permits. Each price includes a standard material allowance. For the difference between the two kinds of unit, see ADU vs DADU in Seattle, WA: What Is the Difference?
| Type and typical size | Basic | Mid-Range | Luxury | Super-Luxury | Time on site |
|---|---|---|---|---|---|
| Garage or basement conversion, 300 to 800 sq ft, studio or 1 bedroom | $150,000+ | $200,000+ | $275,000+ | $375,000+ | 3 to 5 months |
| Attached ADU, 400 to 1,000 sq ft, 1 or 2 bedrooms | $250,000+ | $325,000+ | $425,000+ | $550,000+ | 5 to 8 months |
| Detached DADU, 300 to 600 sq ft, studio or 1 bedroom | $325,000+ | $400,000+ | $500,000+ | $650,000+ | 5 to 8 months |
| Detached DADU, 600 to 1,000 sq ft, 2 bedrooms | $425,000+ | $500,000+ | $625,000+ | $800,000+ | 6 to 10 months |
Source: Cost of an ADU or DADU in the Greater Seattle Area, our ADU pricing guide, updated October 2026. Starting prices reflect typical completed projects and are not a quote.
Why does an ADU conversion cost less than a new DADU?
A conversion reuses a foundation, walls and a roof that already stand. Our Basic conversion at $150,000+ pays for what turns that space into a home: insulation, drywall and code egress windows, a kitchen with standard cabinets and quartz, one bath, a separate locking entrance, fire and sound separation from the main house, LVP flooring and a ductless heat pump.
A garage was never built to be lived in. In Seattle a detached garage converted to a DADU must fully meet the Seattle Energy Code, so the insulation, windows and heat usually change completely. Our Garage Conversion Across the Greater Seattle Area page covers that work.
A new DADU builds everything from the ground up: foundation, framing, roof, siding, windows and every system. Utility runs across the yard are priced per site, on their own lines. That is why a 300 to 600 sq ft cottage starts at $325,000+. That is more than twice the Basic conversion price for a similar size.
What does an ADU cost per square foot and by size?
A recent 1,000 sq ft DADU we contracted came to about $476 per sq ft for construction, per our ADU pricing guide. A small unit costs more per foot, because the kitchen, the bathroom and the utility connections cost about the same at any size.
Size has a ceiling in Seattle. Seattle Municipal Code 23.42.022 (SMC 23.42.022, amended 2025) caps most ADUs at 1,000 square feet with up to two bedrooms, or 1,200 square feet with three or more (23.42.022.G.1), with a 1,500 square foot exception for some lots in LR zones (23.42.022.G.1.c). An attached ADU in part of a house that existed on July 23, 2023 may go over 1,000 square feet (23.42.022.H.4). Underground stories do not count toward that limit (23.42.022.G.2.b), and a lot may have two ADUs (23.42.022.C). Our 600 to 1,000 sq ft DADU tier runs to 1,200 sq ft in Bellevue, per our pricing guide.
Within the cap, more square feet, a second bedroom or a second bath add framing, plumbing and time. A two story unit adds a stair and structure. A one story unit adds foundation and roof area.
What costs are not in the starting price of an ADU?
Our starting prices cover the build. These items are priced in writing only when your lot needs them:
- Design, structural engineering, survey and permit fees, priced in the design phase.
- City plan review and utility connection fees.
- The King County sewer capacity charge.
- A new electrical service, water meter or side sewer.
- Long utility trenches, a sewer pump, rock or retaining walls.
- Tree removal and arborist or geotechnical reports.
- Fire sprinklers where the city requires them.
- Appliance purchase and installation (we prepare the connections).
King County bills an attached or detached ADU at 0.59 of a home, about $46 a month for 15 years, for units connecting in 2026, with a discounted lump sum available (King County Wastewater Treatment Division, capacity charge, 2026).
One cost Seattle removes: "No off-street motor vehicle parking is required for an accessory dwelling unit" (SMC 23.42.022.I, amended 2025).
How do ADU layout and interior design choices change the cost?
An ADU is small, so its layout and interior design decide how it lives and what it costs. A compact plan with the kitchen and bathroom sharing a wet wall uses less pipe, less framing and less finish than a spread out one. Daylight, storage and access all have a price, and each shows up at a set finish level in our pricing guide.
Other choices cost little when made early. Place windows so the unit and the main house do not look into each other. Plan a level entry and wider doors before anyone needs them.
| Design choice | Where it appears in our prices | Starts at |
|---|---|---|
| Durable floors and efficient heat | Basic: LVP flooring and ductless heat pump | $150,000+ conversion |
| Laundry inside the unit | Mid-Range: in-unit laundry (attached) or laundry closet (DADU) | $325,000+ attached |
| More daylight | Mid-Range DADU: covered entry and larger windows | $400,000+ |
| Level entry and wider doors | Luxury conversion, on request | $275,000+ |
| Vaulted ceiling and clerestory windows | Luxury DADU, 300 to 600 sq ft | $500,000+ |
| Covered porch or deck | Luxury DADU, 300 to 600 sq ft | $500,000+ |
| Loft or two story layout | Luxury DADU, 600 to 1,000 sq ft, with a custom stair | $625,000+ |
| Built-in storage and smart controls | Super-Luxury conversion: custom millwork and built-ins, smart lighting | $375,000+ |
Source: the tier lists on our ADU pricing guide, updated October 2026.
Which permits does an ADU need, and do they add cost?
Yes, every ADU needs permits, and their fees sit outside our starting prices. In Seattle, SDCI issues the building permit, with separate electrical, plumbing and side sewer permits. In Bellevue, permits are filed through MyBuildingPermit.com. We draw the site plan, floor plans, energy calculations and structural plans, file them and answer any correction letter the city sends. Clean drawings keep corrections short, and design and permits usually take 4 to 9 months before construction starts.
An ADU is not the same choice as building a new house or adding onto yours. Our guide How Much Does It Cost to Build a House in Seattle? compares a DADU with a rebuild, a gut renovation and an addition.
How can you keep the cost of an ADU down?
Five steps keep the cost down:
- Check what the lot allows before paying for custom drawings, because the lot decides more than the design does.
- Look at a city pre-approved DADU plan. We can build from one.
- Convert existing space where you can: a basement or garage conversion starts at $150,000+.
- Keep the kitchen and the bath on one wet wall.
- Settle every selection before construction, because changes after the walls close cost the most.
Think through who will live in the unit before the layout is fixed. A parent who should not live alone may need a level entry and a walk-in shower. An adult child may want a second bedroom.
How do we price an ADU for your lot?
We start with a lot check: we measure the lot and the house, check zoning, sewer and power, and tell you which type of ADU the site can take. Then we design the unit with you, show it in 3D and price it in writing, line by line, with the site costs on their own lines. The full service is on ADU Builders Across the Greater Seattle Area. We build across King and Snohomish counties, including Home Remodeling Contractors in Seattle, WA.
Frequently Asked Questions
How much does an ADU above a garage cost?
A studio over an existing garage is priced as a conversion, from $150,000+. A unit over a new garage is priced as an attached ADU, from $250,000+. A larger backyard unit over a garage is priced as a 600 to 1,000 sq ft DADU, from $425,000+. All three prices are for the Basic finish level in our ADU pricing guide.
What warranty comes with an ADU?
Every ADU we build carries a 2-year workmanship warranty, extendable to 5 years. Materials carry their manufacturer's warranty.
Can an ADU be financed?
Financing options are available for ADUs and DADUs. See Home Renovation Financing Across the Greater Seattle Area for the options.
