ADU type
Attached ADU and Home Additions
An attached ADU shares at least one wall with your primary home, or is built as an addition designed to function as a separate unit. It can be a cost-effective path when your lot or budget favors building onto the existing structure.
How an attached ADU differs from a detached unit
An attached ADU connects to the main house, so it can share structure, a wall, or utility runs. That can lower some costs compared with a standalone build, but it ties the project more closely to the existing home and its systems. A home addition built to serve as an independent unit, with its own entrance, kitchen, and bathroom, falls into the same category.
Typical uses
- Adding a rental or in-law unit without giving up backyard space.
- Extending an existing home and converting part of it into a separate dwelling.
- Lots where a separate detached structure is not practical.
What the builder handles
A licensed local builder who serves your jurisdiction handles design, engineering, permitting, and construction, including how the addition ties into your existing home. You contract with that builder directly.
What to consider
- How the new unit connects to the existing structure, roofline, and foundation.
- Shared versus separate utilities and metering.
- Size and setback rules for your jurisdiction, which differ from lot to lot.
- Whether your goals are better served by a detached unit or a conversion instead.
Not sure an attached unit fits your property? Work through whether you can build an ADU on your lot first.
Keep researching
What it costs, and what happens after you send it
- What does an ADU cost in the Spokane area?
- 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
Tell us about your lot and project. A builder who serves your jurisdiction prices it, free and with no obligation.
Updated