An ADU costs what a small house costs to build, and the number swings widely with five choices: whether the unit is converted from existing space or built new, how big it is, how far the sewer, water and power have to run, what the site is like, and the finishes inside. ADUs we build start at $150,000+ for a garage or basement conversion and $250,000+ for an attached ADU. Detached DADUs start at $325,000+ for a small cottage and $425,000+ for a 600 to 1,000 sq ft unit. See current ADU prices. Below is what moves the cost, and the permit path in Seattle, from the city's own pages.
Seattle recognizes two kinds of ADU, and the kind sets most of the cost.
The Seattle Department of Construction and Inspections (SDCI) defines an accessory dwelling unit as "a separate living space within a house or on the same property." It names two kinds: "A legally permitted unit in the home is called an attached accessory dwelling unit (AADU)," and "A legally permitted unit on the property (but not within the home) is called a detached accessory dwelling unit (DADU)." Either way the unit needs its own kitchen, bathroom and living space.
- Converting a basement or a lower floor into an attached ADU reuses the foundation, the walls and the roof. The cost goes into the kitchen, the bathroom, a separate entrance and the work that separates the unit from the house above.
- Converting a detached garage into a DADU reuses a structure, but a garage was never built to be lived in, so the floor, walls, windows, insulation and heat usually change completely. See Garage Conversion Across the Greater Seattle Area.
- Building a new DADU means a foundation, framing, roof and every system from scratch, plus the path and the utility runs across the yard. It is usually the largest budget of the three.
Size is capped, and every square foot is priced.
SDCI announced that "Size limits for ADUs have been raised to 1000 square feet across all zones where they are allowed," and that the city allows "two ADUs per lot in addition to a primary unit on all lots" (Building Connections, "ADUs and Middle Housing," July 7, 2025). Within that cap, a compact plan saves money: a kitchen and a bathroom that share a wet wall need less pipe and framing than rooms at opposite ends of the unit. A second story adds stairs and structure, and a single story adds foundation and roof area, so neither is automatically cheaper.
Sewer, water and power are often the biggest unknown.
A new home on the lot needs its own plumbing and electrical service, and the cost depends on distance and capacity:
- Power. SDCI notes that "You may also need to apply for electrical service changes or new services from Seattle City Light." If your panel cannot carry a second home, the service gets upgraded.
- Plumbing and sewer. In Seattle, Public Health, Seattle and King County issues plumbing permits "Whenever you are going to install, relocate, or change a plumbing or gas piping system." A detached unit far from the main sewer line means a longer trench.
- The county sewer charge. SDCI says it "will report all ADU permits to King County for sewer treatment capacity charges," and you sign a form acknowledging those charges with the application.
The permit path is part of the cost.
Every ADU in Seattle needs a permit, and the type of permit follows the type of unit. From SDCI: "Adding within an existing house. You need a construction addition / alteration permit." "Building a detached unit. You need a construction addition / alteration permit." "Legalizing an existing unit. You need a construction permit to establish use." The drawings, the energy calculations and the structural plans the city reviews are real design work, and corrections from the reviewer add time. One cost Seattle removes: SDCI lists parking among the standards that do not apply to ADUs.
Outside Seattle, each city sets its own ADU rules. In Bellevue, permits come from Development Services; our page ADU and DADU Builders in Bellevue, WA covers the Bellevue rules.
The site and the finishes move the rest of the price.
- Access. A backyard reached only through a narrow side yard means materials move by hand and small equipment does the digging.
- Ground and trees. A slope, poor soil or large roots in the footprint add excavation and foundation work.
- Finishes. Cabinets, counters, flooring, tile and appliances are chosen the same way as in a kitchen or bathroom remodel, and they move the total the same way.
Two steps keep an ADU budget under control.
First, check what your lot allows before paying for custom drawings, because the lot decides more than the design does. Second, settle the full scope before the build starts: changes after the walls close cost the most. An ADU is a home for someone close to you, such as a parent who should not live alone or an adult child, so think through who will live there and what they need before the layout is fixed.
Here is how we price an ADU.
We start with a lot check: we measure the lot and the house and look at zoning, sewer and power. Then we design the unit with you, show it in 3D, price it in writing line by line and file the permit. See ADU Builders Across the Greater Seattle Area for the service and ADU vs DADU: what is the difference? to compare the two. Every ADU needs a kitchen and a bathroom, the two rooms our crews build most often; you can see finished rooms in the Kitchen Remodeling Portfolio Across the Greater Seattle Area and the Bathroom Remodeling Portfolio Across the Greater Seattle Area.
ADU cost questions
How big can an ADU be in Seattle?
Seattle raised the size limit for ADUs to 1,000 square feet across all zones where they are allowed, and a lot may have two ADUs in addition to the main house (SDCI Building Connections, "ADUs and Middle Housing"). Zoning, critical areas and shoreline rules can still limit a particular lot, so we check yours first.
Does a Seattle ADU need its own parking space?
No. SDCI lists parking among the standards that do not apply to accessory dwelling units, so a parking space is not part of the cost.
Can I get a price before paying for full drawings?
Yes. We start with a lot check: we measure the lot and the house, look at zoning, sewer and power, and tell you which type of ADU the site can take. The written, itemized price follows the design, before any work starts.
Thinking about an ADU? Book a consultation below. By appointment. Limited start dates.
