King County ADU / DADU Regulations — Highlights
- National Design
- 2 days ago
- 2 min read
In unincorporated King County, accessory dwelling units (ADUs) are allowed on lots that contain an existing dwelling, as defined in King County Code Title 21A (KCC 21A.08.030 B.7).
Maximum Size:
The heated floor area of an ADU may not exceed 1,000 square feet, plus up to 1,000 square feet of unheated area (such as garage, storage, or unfinished basement).
If the ADU is entirely located within an existing basement or attic, the size limit may not apply.
In some urban zones, the heated area may be increased up to 1,500 square feet, provided the unheated area is reduced accordingly.
Number of ADUs per lot:
Currently, one ADU per single-family dwelling is allowed in unincorporated King County.
However, due to Washington State House Bill 1337 (2023), counties are required to update their codes to allow up to two ADUs per lot; King County is in the process of aligning its regulations accordingly.
Minimum Lot Size, Zoning, and Height:
ADU or DADU must comply with all underlying zoning and dimensional standards for the property.
Detached ADUs may be limited or prohibited in certain rural or resource zones, depending on the minimum lot area (for example, in R-5 rural zone if the lot is under 5 acres).
The height limit for an ADU generally cannot exceed the height allowed for the principal dwelling.
Notice on Title Requirement:
A Notice on Title must be recorded with the King County Recorder’s Office, stating that the property includes an approved ADU.
This notice remains attached to the property deed and informs future owners.
Other Requirements:
ADUs must comply with all applicable residential building, mechanical, electrical, plumbing, and energy codes.
Adequate water and wastewater service must be demonstrated (either public utilities or approved private systems).
ADUs must also comply with critical area regulations, fire access standards, and any design review required for the zoning district.
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