Planning an ADU: From Feasibility to Move-In
An accessory dwelling unit can house a parent, an adult child or a tenant, and Washington's ADU law means most residential lots in Seattle and the Eastside can have up to two. Getting from idea to move-in usually takes twelve to eighteen months for a backyard cottage and eight to twelve for a conversion. The projects that go smoothly are the ones that check the lot carefully before spending on design.
Step 1: Decide what the unit is for
- Family member: accessibility, proximity to the main house, privacy for both households.
- Rental income: a layout that rents well, separate utilities or sub-meters, durable finishes, parking or transit access.
- Flexibility: a home office now, a rental later, family use after that.
The purpose drives size, layout and finish level.
Step 2: Choose a type
| Type | Typical all-in cost | Pros | Cons |
|---|---|---|---|
| Detached cottage | $300,000 – $500,000 | Most privacy, most rentable | Highest cost, utilities and site work |
| Garage conversion | $120,000 – $220,000 | Lowest cost if the garage is sound | Loses parking and storage |
| Attached or basement unit | $150,000 – $300,000 | Shares structure | Needs headroom, egress and fire separation |
Step 3: Check the lot
This is the step people skip and regret.
- Zoning and ADU rules. Your city's code sets size, height, setbacks and placement within state law.
- Critical areas. Wetlands, streams, steep slopes and their buffers can remove buildable area.
- Lot coverage and impervious limits. On smaller lots, the house and driveway may use most of the allowance.
- Trees. Significant or protected trees can decide where a cottage fits.
- Utilities. Where sewer, water and power connect, and how far and how deep the trench to a backyard unit must go.
- Access. Alley access, side yard width and how construction equipment reaches the back.
- Covenants. HOA covenants are private agreements; if yours seem to restrict ADUs, get legal advice.
A feasibility review by a designer or builder, often for a modest fee, answers these before design.
Step 4: Budget beyond the building
The building itself is the visible cost. These lines surprise people:
- Utility trenching and connections: often $25,000 to $60,000 for a detached unit.
- Side-sewer work where the existing line is old or undersized.
- Impact and connection fees, which state law caps for ADUs but does not eliminate.
- Site work: grading, drainage, paths, fencing, landscaping repair.
- Appliances and furnishings for a rental.
Small units do not save as much as people expect, because a kitchen, a bathroom, a foundation and a roof cost nearly the same at 600 square feet as at 900.
Step 5: Design
Some cities offer pre-approved ADU plans that shorten review when they fit the lot. Custom designs take longer but fit the site and the purpose. Design choices that pay off: a separate entrance with privacy from the main house, plenty of light, storage, in-unit laundry, and a layout that works for the intended occupant.
Step 6: Permits
An ADU needs a building permit with architectural and structural drawings, an energy code worksheet and utility plans, plus electrical, plumbing and mechanical permits, and often tree, stormwater and side-sewer reviews. Review times run from several weeks to several months.
Step 7: Construction
A detached unit takes four to seven months on site: site work and utilities, foundation, framing, roof, rough-ins, insulation, drywall, finishes. Conversions take two to four months. Expect the utility work to be the most disruptive part for the main house.
Step 8: Move-in or rent
A certificate of occupancy ends the permit. If renting, check whether your city requires rental registration or inspection, set up a lease that meets Washington landlord-tenant law, and confirm insurance for the rental use.
Common mistakes
- Paying for design before checking buffers, trees and utilities.
- Building too small to rent well.
- Underestimating utilities and site work.
- Assuming a pre-approved plan fits without adjustment.
- Ignoring covenants until late in the process.