King County, WA Accessory Structures: ADU Impact Fees (2026)
Key Facts
- Exemption
- ADUs excluded from school impact fees
- Citation
- K.C.C. § 21A.43.070.A.9
- Administered by
- Dept. of Local Services, Permitting Division
- Applies to
- Unincorporated King County only
- ADU defined at
- K.C.C. 21A.06.350 / 21A.08.030.B.7.a
Summary
Unincorporated King County exempts accessory dwelling units from school impact fees under County Code § 21A.43.070.A.9, so building an ADU triggers no per-unit school fee that a new single-family home would owe.
A. The following are excluded from the application of the impact fees: ... 9. Accessory dwelling units as defined in K.C.C. 21A.06.350 and K.C.C. 21A.08.030.B.7.a.
Full Breakdown
King County Code § 21A.43.070.A lists nine categories excluded from school impact fees charged on new dwelling units in unincorporated areas of districts that have adopted a fee. Item 9 excludes accessory dwelling units as defined in K.C.C. 21A.06.350 and K.C.C. 21A.08.030.B.7.a. That means a homeowner adding a detached or attached ADU under Title 21A's ADU standards owes no separate school impact fee, unlike a new principal dwelling unit, which is billed per the school district's adopted fee schedule administered by the Department of Local Services, Permitting Division, under K.C.C. 21A.43.050.
Violations & Fines
Building an ADU without paying a school impact fee is not a violation, the exemption is automatic under § 21A.43.070.A.9; permits are still required through King County's normal ADU review process.
Frequently Asked Questions
Does King County charge a school impact fee for an ADU?
Does this exemption apply inside Seattle or other King County cities?
Sources & Official References
Other rules in King County
Compare King County to another location·View the Washington accessory structures overview
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