ADU type
Basement ADU and Junior ADU Conversion
Interior conversions turn existing space inside your home into a separate unit. A basement ADU uses below-grade space, and a junior ADU (JADU) is a smaller unit created within the walls of the existing house. Both can be a lower-cost entry point.
Basement ADU
A basement ADU converts below-grade space into an independent dwelling with its own entrance, kitchen, and bathroom. Because the shell already exists, the main work is egress, ceiling height, moisture control, insulation, and adding the kitchen and bathroom. A separate, code compliant exit is usually the most important feasibility question.
Junior ADU (JADU)
A junior ADU is a smaller unit created within the existing footprint of the house, typically with a more limited kitchen. JADUs are generally the smallest and least expensive ADU type, though the exact rules, including any size cap, depend on your jurisdiction.
What to consider
- Egress: a basement unit needs a compliant second exit, which can require a new window well or door.
- Ceiling height and moisture, which are common limiting factors below grade.
- Sound separation and a clear division between the unit and the main home.
- Whether your jurisdiction recognizes junior ADUs and what size and kitchen rules apply.
Egress and jurisdiction rules decide most interior conversions, so confirm whether your lot can support an ADU early.
What the builder handles
A licensed local builder who serves your jurisdiction assesses the space, designs the conversion, handles permitting, and performs the work. You contract with that builder directly.
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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.
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