Can I build a DADU on my lot in Seattle?

Homeowners do it constantly — hire an architect, pay for plans, then discover a steep-slope overlay, a setback, or an HOA covenant that shrinks or kills the project. As one Seattle homeowner on Reddit put it after the fact:

“I'll spend $5K on plans and then find out I can't build.” — a Seattle homeowner on Reddit

This page is the screen you run first. None of it is a substitute for a formal review, but it tells you whether your lot is a clear yes, a probable yes with conditions, or a likely no. Work through the seven checks below in order. Any one of them can stop a project, so don't skip ahead.

What HB 1337 already settled (so you can stop worrying about it)

A lot of the fear online is about rules that no longer exist. Washington's HB 1337 (effective in Seattle June 30, 2025) preempted local zoning and made these statewide:

• Up to 2 ADUs per lot (a detached DADU plus an attached AADU, in most cases).
• No owner-occupancy requirement— you don't have to live on the property.
• No off-street parking required near frequent transit.
• Detached DADU up to 1,000 sq ft in Seattle.
• Condoization legalized — you can sell the ADU separately (RCW 36.70A.681).
• Non-conforming existing structures can be convertedeven if they don't meet modern setbacks.

So the question is no longer “am I allowed an ADU?” — almost certainly yes. The question is “does my specific lot physically and legally fit one?”That's what the seven checks resolve.

The 7-point feasibility checklist

1. Zoning

Confirm your parcel is in a residential zone that permits a DADU. In Seattle this is the Neighborhood Residential zones; in unincorporated King County it depends on whether you're inside an Urban Growth Area (up to 2 ADUs) or a Rural Area (limited to 1 ADU, and detached only if you meet a minimum lot size — e.g., RA-2.5 requires ~1.875 acres). The Eastside cities each have their own code but all now allow 2 ADUs per lot post-HB 1337.

2. Lot coverage

Your lot has a maximum percentage that can be covered by structures. Add your house, garage, decks, and the proposed DADU footprint — if the total exceeds the cap, the cottage has to shrink or move. Seattle's 2025 interim changes loosened lot-coverage limits for some configurations, but the cap still governs how big a footprint you can put down.

3. Setbacks

A DADU has to sit a required distance from each property line and from your main house. Setbacks are what most often shrink a backyard cottage from the size you pictured to the size that fits. Some cities loosened these post-HB 1337 (Seattle reduced certain setbacks; Kirkland reduced alley setbacks). Sketch your buildable envelope: lot minus setbacks minus existing structures = the box your DADU has to fit inside.

4. ECA / steep slope

This is the Seattle-specific dealbreaker. Environmentally Critical Areas— steep slopes, landslide-prone soils, wetlands, riparian corridors — restrict or block development. Steep slopes (roughly 40%+) trigger geotechnical reports, shrink buildable area, and add real cost. Seattle's 2025 interim zoning cut allowed unit counts on lots with more than 50% steep slope. ECA constraints are concentrated in West Seattle, Magnolia, Queen Anne, and parts of the Eastside.

If your backyard slopes noticeably, assume an ECA flag until a map says otherwise. A Bellevue note: lots east of 148th Ave sit on till/outwash soils that frequently trigger mandatory geotech.

5. Shoreline (within 200 ft of water)

If any part of your lot is within 200 feet of a shoreline— Lake Washington, Puget Sound, a major creek — you're under the Shoreline Management Act. In Seattle that adds roughly 4–8 weeks and $5,000–$15,000 in consulting to the permit. It rarely kills a project, but it changes the budget and timeline, so know early.

6. Septic (if you're not on city sewer)

In unincorporated King County and pockets of the Eastside, you may be on an on-site septic system (OSS), and an ADU adds load. King County sizes septic at 150 gallons/day per bedroom; an independent OSS needs a minimum 1,000-gallon tank for a 1–2 bedroom unit. A shared system gets re-evaluated as multifamily, which can force drainfield expansion. The hidden risk: many older septic systems are at or past their design life— if yours needs replacing, that's a five-figure surprise. And if you're within 200 ft of public sewer in a UGA, major work may force you to abandon septic and connect.

7. HOA / CC&Rs (the one that surprises people)

HB 1337 did not override your HOA. This is the single most common misconception. State law preempts city zoning — it does notvoid private covenants (CC&Rs). A homeowners association or recorded covenant can still legally block your DADU, and the city won't enforce it either way; only your neighbors or HOA will. Covenant fights are live in Somerset, Bridle Trails, and Sammamish subdivisions.

Pull your title report and read the covenants before you design anything. If you bought recently, this is in your closing documents.

How to actually check your lot

You don't have to guess on all of this. Tools exist:

• Seattle's ADUniverse(aduniverse-seattlecitygis.hub.arcgis.com) — the city's own mapping tool. Enter your address and it confirms baseline ADU eligibility, zoning, and flags overlays. Start here for any Seattle parcel.
• King County iMap / Parcel Viewer — for zoning, ECA layers, and parcel dimensions countywide, including unincorporated areas on the DPER (not SDCI) permit track.
• Your title report— the only place your CC&Rs live. No public map shows private covenants.
• A pre-application review with the jurisdiction — the formal check, worth doing before committing to design on any lot with a slope, water, or septic flag.

Reading your result