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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.

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 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 for download in Adobe Acrobat PDF format. The electronic version of the manual is searchable and the bookmarks function within the navigation pane allows 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 this section.

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.

2021 Surface Water Design Manual documents

Combined SWDM including all chapters, appendices, definitions, and reference sections (19.3 MB)
The combined SWDM PDF is markup capable; the individual Chapters and other sections are not at this point in time.

SWDM errata edits

Errata and edits log (2021/10/05) (110 KB)

SWDM chapters

Front matter - title page, acknowledgments, table of contents and introduction (178 KB)

Chapter 1-Drainage Review and Requirements (1.2 MB)

Chapter 2- Drainage Plan Submittal (351 KB)

Chapter 3-Hydrologic Analysis and Design (1.5 MB)

Chapter 4-Conveyance System Analysis and Design (2.2 MB)

Chapter 5-Flow Control Design (1.5 MB)

Chapter 6-Water Quality Design (3 MB)

Definitions (457 KB)


Appendix A-Maintenance Requirements for Flow Control, Conveyance, and Water Quality Facilities (532 KB)

Appendix B-Master Drainage Plan Objective, Criteria, Components, and Review Process (81 KB)

Appendix C-Simplified Drainage Review Requirements (2.4 MB)

Appendix D-Construction Stormwater Pollution Prevention Standards (1.7 MB)

Design Manual drawings:

Standard drawings from the 2021 King County Design Manual can be downloaded individually in AutoCAD DWG format. Additionally, the AutoCAD DWG drawings can be downloaded as a single file or chapters can be downloaded individually.

Select a group of drawings to download and extract:

Standard drawings in AutoCAD DWG format


Combined References, Forms and Worksheets (3.6 MB)

2021 SWDM cover pages

Full Manual binder cover (2.8 MB)

Full Manual binder spine (0.98 MB) 4-up version (2.2 MB)

Appendix C and D covers (2.3 MB)

Surface Water Design Manual application maps

Find the most up-to-date information posted on iMap

SWDM maps in PDF format:

Flow Control Application Map (1.4 MB)

Water Quality Application Map (1.4 MB)

Landslide Hazard Drainage Areas Map (1.5 MB)

SWDM signed public rule and 2016-2021 summary of changes

SWDM Public Rule PUT-8-3-6 (297 KB)

SWDM Summary of Changes (95.3 KB)

Blanket adjustments and clarifications

None at the present time.

If you have questions about these changes, contact Mark Wilgus at

Information on key provisions of King County Drainage Code Title 9

Key provisions of King County Code 9.04 and 9.12 (138 KB)

For questions about the stormwater services website, please contact Mark Wilgus, Engineer, King County Stormwater Services Section.