Home Additions & ADUs — King County
Home Additions & ADUsin Seattle.
Seattle has the friendliest ADU rules in the state and the slowest permit desk. We build the unit, and we build the schedule around the review.
( Seattle )
Home Additions & ADUs for Seattle homes
Seattle allows up to two accessory units on most single-family lots, with no owner-occupancy requirement and no off-street parking in most zones, which is why ADUs are now one of the highest-return projects in the city. The tall basements under Ballard, Wallingford, Green Lake and Ravenna craftsmans convert especially well: seven-foot-plus ceilings, a separate entry down the side of the house, and existing plumbing stacks close to where the new kitchen and bath go. Mid-century houses in View Ridge and Wedgwood with daylight basements on sloped lots are the other natural candidates.
Detached backyard cottages are a bigger project and a bigger return, with the Seattle rules permitting more height and floor area than most neighboring cities. What governs the cost is the site: lot coverage, the required setbacks, whether the sewer side-sewer line has to be rerouted, and how materials get to the back yard of a lot with a four-foot side path. SDCI review for an ADU commonly takes months, so we submit before the design is fully finished where the code allows it, answer correction rounds within days, and sequence the build so the permit wait never becomes idle site time.
Permits for Seattle projects run through Seattle Department of Construction & Inspections (SDCI), and we handle the filing and inspections as part of the job. For the full picture on how we approach this work, see our home additions & adus page — and for everything we build locally, our Seattle contractor page.
Typical Seattle investment
Broad ranges for orientation — your written estimate is free and itemized.
| Tier | Range | What it covers |
|---|---|---|
| Garage conversion | $40k–$150k | Living space at the low end; full ADU with kitchen, bath and entrance at the high end. |
| Ground-level addition | $250–$450/sq ft | Foundation, framing, envelope, systems and finishes, tied into the existing house. |
| Second story / detached ADU | $350–$550/sq ft | Structural work below, or a standalone unit with its own foundation and utilities. |
Seattle questions
- How many ADUs can I build on a Seattle lot?
- Up to two on most single-family lots: one attached (usually a basement unit) and one detached, or two of the same type in some zones. There is no owner-occupancy requirement and no off-street parking requirement in most of the city. We confirm what your zone allows at the feasibility visit.
- What does an ADU cost in Seattle?
- Basement ADUs in Seattle typically run $120,000 to $250,000 in 2026 depending on ceiling height, the entry and how much plumbing moves. Detached backyard cottages commonly run $300,000 to $450,000 including design, engineering, SDCI permits and side-sewer work, which is often the hidden line on a Seattle lot.
- How long does an ADU permit take in Seattle?
- SDCI review for an ADU is measured in months, not weeks, and detached units usually take longer than basement conversions. We file as early as the design allows, respond to corrections fast and schedule construction around the approval so the wait costs calendar time, not money.
- Can I rent out a Seattle ADU?
- Yes, long-term rental is permitted and is the most common reason owners build one. Short-term rental rules are separate and change; check Seattle's current short-term rental licensing before you plan on it. Either way the unit needs its own address, a legal kitchen and bath, and fire separation from the main house.
Let’s talk about
your place.
Free in-home estimates across Everett, Seattle & the Eastside. You talk to Eduard directly, not a call center.