ADU type
New Detached ADU
A detached ADU is a standalone home built in your backyard or side yard, separate from the main house. It is the most flexible ADU type and usually the most private for both households.
What a detached ADU is
A detached ADU is a fully independent dwelling on the same lot as your primary home, with its own entrance, kitchen, and bathroom. Because it stands on its own, it offers the most design freedom of any ADU type, and it keeps the new unit physically separate from the main house.
Typical uses
- Long-term rental income from a separate backyard unit.
- Multigenerational living, such as housing for aging parents or adult children.
- A guest house, home office, or studio that can later become a rental.
- Added long-term property value and flexibility.
What the builder handles
A licensed local builder handles design, engineering, permitting with your jurisdiction, and construction. You contract with that builder directly.
What to consider before you start
- Your jurisdiction sets the allowed size, height, and setbacks, so confirm whether your lot can support an ADU before you plan.
- Lot size, slope, trees, and access affect what is buildable and the cost.
- Utility connections (water, sewer or septic, power) are a major cost driver for a detached unit.
- The Washington 2023 ADU law, HB 1337, changed several local rules. See our HB 1337 guide.
Keep researching
What it costs, and what happens after you send it
- What does an ADU cost in the Spokane area?
- For a detached new-build adu, published estimates put the all-in figure at $110,000 to $260,000. Across types: detached new-build $110,000 to $260,000, garage conversion $50,000 to $120,000, prefab or modular installed $80,000 to $180,000. Add roughly $3,000 to $5,000 in permit, plan review and impact fees, which is an estimate from one source rather than a published fee schedule, so confirm it with your permitting jurisdiction.
- How solid are those numbers?
- Treat them as planning ranges, not prices. They come from two third-party estimator sites, ADU Zoning and ADU Wizard, both read in full on 2026-08-17, neither of them a government publication. The two land in nearly the same place on total project cost, which is why totals are shown here. They differ by roughly a factor of two on cost per square foot, so a per-square-foot rate is not a reliable way to budget an ADU here and none is quoted.
- How fast will someone get back to me?
- Your request goes to an independent local ADU builder, usually within about an hour during the day. The builder then contacts you to look at the property and price the project on their own schedule.
- What types are covered?
- Detached ADUs, attached ADUs, garage conversions and basement junior ADUs. See every type.
Only a builder standing on your lot can price your actual project.
Get a free quote from a Spokane ADU builder
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