2021 Surface Water Design Manual

A technical manual detailing stormwater management requirements for parcel development in King County, WA. Also applicable in any other municipalities that have adopted this manual. At present the most current version is the 2021 Surface Water Design Manual (SWDM).

Important notice about projects permitted under earlier versions of the SWDM

Per requirements in King County's Phase 1 Municipal Stormwater Permit (effective date August 1, 2019), projects permitted under the 2016 SWDM that have not started construction by July 1, 2026, and projects permitted under earlier versions of the SWDM that have not started construction by July 1, 2021, must be revised to comply with the requirements of the 2021 SWDM, or subsequent versions as directed by state regulations. Revisions will need to be approved by the Permitting Division of the King County Department of Local Services (DLS-PD), or its successor agency prior to construction.

*"Started construction" means, the site work associated with, and directly related to the approved project is at a stage where rough grading is complete or utilities are installed. For rough grading to be considered complete, elevations are within 1 foot of final design elevations.

The Department of Natural Resources and Parks has adopted a public rule (PUT 8-3-6) revising the King County Surface Water Design Manual (SWDM). In accordance with King County Code, Chapter 2.98, Rules of County Agencies and Executive Policy INF 7-6-1-EP "Drafting and Implementing Rules", this public rule was adopted June 23, 2021. The 2021 SWDM is effective as of July 23, 2021.

Notice regarding addition of bioretention designs with high performance bioretention mixes

Note that King County has proposed an amendment to the 2021 SWDM that adds bioretention designs using high performance bioretention mixes (HPBSM) for water quality treatment facilities and as a flow control best management practice (BMP). A draft of the proposed public rule, redlined sections of the SWDM, and a summary of proposed changes are available at kingcounty.gov/swdm2021rev2024. The comment period is closed.

Printed copies of the 2021 SWDM are not being provided. Interested parties can download the electronic version of the manual and then print and bind their own manual. Contact David Batts at david.batts@kingcounty.gov for further information on suggested printing specifications.

The entire 2021 SWDM including Chapters 1-6, Appendices A through D, Definitions, and Reference sections is available as file downloads from links below. The manual is searchable and the bookmarks within the navigation pane allow for quickly moving through document. Note that although some of the materials in the Reference section of the 2021 SWDM may have been adopted by administrative rule or ordinance, the administrative rule that adopted the SWDM has not formally adopted any of the materials in the Reference section.