An Accessory Dwelling Unit (ADU) is a self-contained residential unit on the same lot as a primary home. Washington State's HB 1337 — combined with Seattle's landmark 2019 ADU reform (CB 119544 / Ordinance 125791) — has created the most favorable ADU-building environment in the region's history. Most residential lots across Seattle and King County can now support up to two ADUs with no owner-occupancy requirement.
This guide covers everything King County homeowners need to know: what types of ADUs exist, where you can build, what the rules are, how much it costs, and how to get started.
Types of ADUs in King County
Detached ADU (DADU)
A separate structure on your lot — backyard cottage, guest house, or standalone rental. Maximum privacy and rental value. Up to 1,000 sq ft per SMC 23.44.041.
Attached ADU
A unit built within or connected to your existing home — basement conversions, side additions, above-garage units. Lower infrastructure costs since you share structural elements.
Garage Conversion
Convert an existing garage into a permitted dwelling unit. The most affordable path. Existing structure conversions allowed even if the building doesn't meet current setback standards.
Basement ADU
Turn existing below-grade space into a rental-ready unit. In Seattle, basement conversions are classified as AADUs under SMC 23.44.025 with no additional lot coverage impact in most SF5000 zones.
Key Rules You Need to Know
Two ADUs Per Lot
Most residential lots in urban growth areas can support up to two ADUs in any configuration — two attached, two detached, or one of each. ADUs are exempt from density limitations.
Source: RCW 36.70A.680 / SMC 23.44.041/23.44.025
No Owner Occupancy
You are no longer required to live on the property. You can rent both your main home and your ADU(s) independently. This was eliminated statewide under HB 1337.
Source: RCW 36.70A.681(1)(b)
1,000 Sq Ft Minimum Floor
Local governments cannot set maximum ADU size below 1,000 sq ft. Cities can allow larger, but cannot restrict below this threshold.
Source: RCW 36.70A.681(1)(c)
Reduced Parking
No parking required if within ½ mile of a major transit stop. Link Light Rail and RapidRide service areas in Seattle and King County qualify.
Source: RCW 36.70A.681(1)(e)
Existing Structure Conversion
You can convert existing legal nonconforming structures (garages, barns) into ADUs even if they don't meet current setback or lot coverage standards.
Source: RCW 36.70A.681(1)(d)
Condo Ownership
ADUs may be sold as condominiums independent of the principal unit. The land stays with the primary homeowner, but the ADU structure can have separate ownership.
Source: RCW 36.70A.681(1)(g)
Shoreline Management Act Consideration
Properties within 200 feet of Lake Washington, Lake Union, Green Lake, or Puget Sound may require Shoreline Management Act (SMA) review. Unlike some environmental restrictions, the SMA does not prohibit ADU construction — but it adds 4–8 weeks and $5,000–$15,000 to the permitting process. Our feasibility study identifies SMA jurisdiction automatically.
Get a free feasibility studyADU Guides by City
ADU rules vary by jurisdiction. Click your city for a detailed breakdown of local regulations, permit processes, and what to expect.
City of Seattle
Seattle DCI permitting, SMC 23.44.041, 2019 ADU reform (CB 119544), and neighborhood-level ADU details.
Read guideUnincorporated King County
DPER permitting, KCC Title 21A, rural lot requirements, UGA regulations, and 2025 updates.
Read guideBellevue
Bellevue ADU rules, lot sizes, and permitting through Bellevue Development Services.
Read guideRenton
Renton ADU zoning, permit process, and why Renton is one of King County's best ADU values.
Read guideWhat Does an ADU Cost?
| ADU Type | Cost Range |
|---|---|
| Garage Conversion | $200K – $325K |
| Basement ADU | $225K – $400K |
| Attached ADU | $300K – $500K |
| Detached ADU (DADU) | $450K – $800K+ |
Additional costs include permit and impact fees ($20K\u2013$50K), utility connections ($10K\u2013$30K), and site prep ($5K\u2013$20K). See full ADU cost breakdown for King County.
Ready to Get Started?
Get a free feasibility study to confirm your lot qualifies, or explore our ADU permitting service — your permit investment is applied as a build deposit.
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Last updated: March 2026. This guide is for educational purposes only and does not constitute legal advice.
