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Stormwater drainage facility maintenance

If you own property in unincorporated King County, you are required to maintain any privately owned stormwater drainage facilities on that property. Drainage refers to flow control and treatment facilities, and any conveyances to, between, and from the facilities. Maintenance is required to ensure that these remain effective at protecting the environment and properties as intended.

How to inspect your stormwater facility/facilities

For questions about inspecting your facilities, email us at or call us at 206-477-4811.

Stormwater drainage facility maintenance checklist

Owners of commercial and multifamily residential properties with privately maintained drainage and stormwater facilities receiving a stormwater fee reduction use this drainage facility checklist:

Commercial and Multifamily Drainage Facility Maintenance Checklist (504 KB)

Drainage Maintenance Standards

Appendix A of the King County Surface Water Design Manual (SWDM) gives information on the county's maintenance standards for drainage facilities.

2021 SWDM Appendix A (532 KB)

The Appendix entries are brief and formal. For more detail on maintenance, go to Washington State Department of Ecology's instructions in their:

Stormwater Management for Western Washington (SWMMWW) (139 MB)

Note that the King County SWDM makes a distinction between engineered facilities that require maintenance per Appendix A, and non-engineered best management practices (BMPs); whereas Ecology's SWMMWW naming convention refers to both facilities and best management practices as BMPs.

For example, Ecology's 2019 SWMMWW calls detention ponds "BMP D.1"; but despite the 'BMP' designation, this is an engineered facility, comparable to 2021 SWDM Section 5.1.1 detention ponds.

Additional information: