ADUs in Puget Sound: Costs, Rules and How to Hire
A detached accessory dwelling unit in the Seattle area costs $300,000 to $500,000 all-in, a garage conversion $120,000 to $220,000, and an attached or basement unit $150,000 to $300,000. Washington law requires cities to allow at least two ADUs on most residential lots, so the question for most homeowners is no longer whether an ADU is allowed but what fits on their lot and what it costs.
What ADUs cost here
| ADU type | All-in range | Typical timeline |
|---|---|---|
| Detached ADU (DADU), 400 – 1,000 sq ft | $300,000 – $500,000 | 12 – 18 months from design to move-in |
| Garage conversion | $120,000 – $220,000 | 8 – 12 months |
| Attached or basement ADU | $150,000 – $300,000 | 9 – 14 months |
All-in means design, engineering, permits, utility connections, construction and mid-range finishes. Utilities are the line most often underestimated: trenching water, sewer and power to a backyard unit can run $25,000 to $60,000.
What Washington's ADU law allows
State law requires cities in urban areas to allow at least two ADUs per lot in any combination of attached and detached, removes owner-occupancy requirements, limits parking requirements near frequent transit, sets a floor on allowed size, and caps impact fees for ADUs below those for houses. Cities still regulate height, setbacks, lot coverage, trees and critical areas. Each city's code adds its own details.
Three kinds of unit
Detached ADU. A separate small house in the yard. Most flexible and most rentable, and most expensive because it needs its own foundation, roof and utilities.
Garage conversion. The cheapest route when the garage is structurally sound. The work is the floor, insulation, the garage-door wall, a kitchen and bath, and meeting egress rules.
Attached or basement ADU. A unit inside or attached to the house with its own entrance. It shares the structure, which saves money, and needs fire separation between units and adequate ceiling height and egress.
What limits a lot
Critical-area buffers, lot coverage and impervious limits, large trees, sewer capacity, steep slopes and height limits on sloped lots are what most often shrink or stop an ADU. A feasibility check before design is worth the cost. The ADU planning guide walks through it.
ADU permits
An ADU needs a building permit with architectural and structural drawings, an energy worksheet and utility plans, plus trade permits and often side-sewer, tree and stormwater reviews. Several cities publish pre-approved ADU plans that shorten review when they fit a lot.
How to vet an ADU builder
- Confirm L&I registration, bond and insurance.
- Ask how many ADUs they have completed in your city and visit one.
- Ask who does feasibility, design, engineering and permitting.
- Ask how they price utility trenching and connection fees.
- Ask for a schedule from contract to certificate of occupancy.
- Ask what is excluded: landscaping, appliances, fences, driveway changes.
ADU quote red flags
- A per-square-foot price that excludes site work and utilities.
- No feasibility step before design fees.
- No mention of side-sewer or water-meter requirements.
- A builder unfamiliar with your city's ADU code.
ADU guides by city
Questions about ADUs
Can I rent my ADU?
Yes, long-term. Short-term rental rules vary by city.
Does an ADU need its own utility meters?
Usually not; many share the main house's services. Separate meters are possible at added cost.
Will an ADU raise my property taxes?
Yes, in proportion to the added value, once the permit is complete.
Can I sell an ADU separately?
State law allows cities to permit ADUs to be sold as condominiums, but local rules and condominium law apply. Ask a real-estate attorney before planning on it.
How long do ADU permits take?
Several weeks to several months depending on the city and whether a pre-approved plan is used.