Seattle ADU
City guide · permits · cost · zoning
Read guide →REGION_01 / SEATTLE CORE
Seattle Core
ADU builder.
The City of Seattle has the most permissive ADU code in Washington — two ADUs per lot, no owner-occupancy, no parking minimums. We file dozens of SDCI permits a year and know exactly where in the city review cycles slow down (ECA overlays, tree protection, side-sewer capacity).
- Counties
- King County (City of Seattle)
- Cities
- 9
- Best for
- Detached ADU (DADU)
- HQ
- Seattle, WA
Cities in Seattle Core
SPEC_01 / COVERAGEBallard
Active coverage
Queen Anne
Active coverage
Capitol Hill
Active coverage
Magnolia
Active coverage
West Seattle
Active coverage
Wallingford
Active coverage
Fremont
Active coverage
Greenwood
Active coverage
SPEC_02 / PERMITS
Permit complexity
SDCI typically issues a clean DADU permit in 12–16 weeks; pre-approved plan sets run faster (6–10 weeks). ECA-encumbered lots (steep slope, liquefaction, peat) and Tier-1/Tier-2 trees add 4–8 weeks for geotech and arborist review.
SPEC_03 / COST
Cost ranges
Detached ADUs in Seattle run $310–$430/sqft turnkey for mid-range finishes; garage conversions land $185–$260/sqft. Side-sewer capacity upgrades on older Ballard / Wallingford blocks are the most common scope add.
Full pricing breakdown →SPEC_04 / ROI
Rental ROI
Long-term rents on a 600–900 sqft DADU range $2,400–$3,400/mo across the Seattle Core. Removed owner-occupancy means whole-house + ADU rental is allowed (RRIO registration required).
Run the ROI calculator →BEST FIT IN THIS REGION
Detached ADU (DADU)AADU basement conversionGarage conversionTwo-ADUs-per-lot stacks
SPEC_05 / FAQ
Seattle Core ADU questions
- Do I need to live on the property to build an ADU in Seattle?
- No. Seattle removed the owner-occupancy requirement in 2019. You can build a DADU or AADU and rent both the primary home and the ADU once you register with the Rental Registration & Inspection Ordinance (RRIO).
- Can I build both a DADU and an AADU on the same Seattle lot?
- Yes — most NR2 and NR3 lots allow one detached and one attached ADU in addition to the primary residence. The DADU is capped at 1,000 sqft of gross floor area; the AADU is bound by the underlying envelope.
- How long do Seattle SDCI ADU permits take?
- Budget 12–16 weeks from intake to issuance on a clean submittal. Pre-approved plans run 6–10 weeks. ECA overlays or Tier-1/Tier-2 trees can add 4–8 weeks for geotech and arborist review.
- Are there off-street parking requirements for a Seattle ADU?
- No. Seattle eliminated parking minimums for ADUs citywide. We still recommend a hardstand on parking-saturated blocks — it's a small cost that protects resale.
OTHER PUGET SOUND REGIONS
FAQ
Frequently asked
Does Golden State serve Seattle Core?
Yes — Seattle Core is inside our active service area. We dispatch crews weekly to projects across Seattle Core and maintain established relationships with each AHJ's plan reviewers, side-sewer departments, and inspection teams. The advantage of working with a regional builder is reviewer familiarity: when a Seattle Core reviewer recognizes the contractor and the standard details, first-cycle approvals are dramatically more common.
What is unique about building ADUs in Seattle Core?
Each Seattle Core jurisdiction interprets WA statewide preemption (RCW 36.70A.681 / HB 1337) slightly differently — height to peak vs. height to midpoint, setbacks measured to siding vs. eaves, drainage thresholds, side-sewer ownership rules. Seattle Core cities also vary on fee structures and SDCs. Our Seattle Core feasibility report names the exact code section, fee schedule version, and reviewer interpretation that will apply to your lot.
How does Seattle Core compare on ADU cost and timeline?
Construction labor and material cost is within ~6% across Seattle Core jurisdictions because the same subcontractor pool services the region. The real cost variance is in fees (SDCs, capacity charges, school impact) and timeline (median 8–16 weeks permit, depending on AHJ workload). We publish per-city permit medians monthly so you can compare Seattle Core cities directly before choosing a lot.
Do I need a different contractor for each Seattle Core city?
No — one license, one warranty, one point of contact across every Seattle Core jurisdiction. Golden State holds an active Washington general contractor license (GOLDNAB882L2) good statewide, plus relationships with L&I-licensed plumbing, electrical, and mechanical specialty contractors who work the full region. Using one regional builder for multiple Seattle Core sites also helps when you want consistent rental specs across a small portfolio.