King County DADU Rules: Urban Lots, Rural Lots and Permits
First establish that King County, rather than an incorporated city, reviews the parcel. The county’s current code distinguishes urban lots, rural towns and other rural or resource land, so one countywide lot-size rule is not enough.
Sources checked November 5, 2026. Confirm the requirements for your parcel with its permitting authority.
Separate urban and rural eligibility
King County Code 21A.08.030 B.7, in the code updated September 9, 2026, allows two ADUs on urban lots in the listed attached and detached configurations. Its rural and natural-resource provisions generally allow one ADU per lot, with additional conditions for a detached unit.
For that rural detached-unit test, the code requires no more than one primary dwelling and distinguishes a rural-town lot of at least 3,200 square feet from property outside a rural town that must meet the applicable zone’s minimum lot area. Other use restrictions still matter. Ask staff to apply the full code to your parcel.
Official guidance: King County Code Title 21A, including 21A.08.030 B.7.
Use current guidance when summaries disagree
The county’s March 2026 ADU handout also distinguishes urban and rural areas. Some older county FAQ language describes different rules. For a new project, use the current code and ask Permitting to resolve any conflict rather than choosing the more favorable summary.
Save the code section, handout date and staff response with your project records. This is especially useful around Fall City and rural Eastside properties, where a mailing address alone does not establish the land-use designation or governing requirements.
Official guidance: King County ADU information, March 2026.
Resolve the property questions before choosing a plan
Make a property checklist covering jurisdiction, zone, rural-town or urban status, existing dwellings and available site records. Add the intended bedroom count, utility information and any known easements or environmental constraints.
Then compare possible locations for the detached home. Consider access during construction, the relationship to the main house and how services might reach the proposed building. Have the appropriate professionals investigate open questions before using a preliminary footprint as the basis for a firm construction price.
Prepare for the application
King County accepts online applications through MyBuildingPermit. Its preparation guidance calls for property research and a complete electronic package, including applicable septic materials where required. Check the current project-specific document list before submitting.
TimberworX can help you start the conversation about a backyard home with a free initial consultation. Detailed feasibility research, design and budgeting are paid preconstruction services. Bring the parcel information and existing records so the team can identify what needs confirmation next.
Official guidance: King County permit application steps.
A few things to know
Does the same King County rule apply inside Redmond or Kirkland?
No. Incorporated cities administer their local zoning and permits. Determine the jurisdiction before using an unincorporated-county guide.
Can I assume a small rural lot qualifies because urban ADUs are allowed?
No. The county has separate rural provisions and a rural-town distinction. Confirm the applicable designation and full eligibility requirements for the lot.
Keep planning
Can a DADU use septic or existing utilities?
Check septic capacity, sewer availability, water service and utility routes before committing to a DADU design in King County.
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