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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 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 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.

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)

Corrected page 6-40: Flow spreader options C and D graphics (64 KB)

SWDM chapters

Front matter- title page, acknowledgements, 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 Surface Water 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)

NEW as of 9/1/2022

Declarations of Covenant for use in unincorporated King County for residential single family properties (164 KB) (zip file)

Errata and updates

  • Reference 8-H, Site Improvement Bond Quantities Worksheet:
    • The web links on full SWDM pdf file pages 979 and 1030 are incorrect.
    • The web link on the separate References pdf file page 56 is incorrect.
    • The web link on full SWDM pdf file page 149 is correct as it is here.
  • Modeling bioretention
    •  At time of publication MGS Flood was not approved for modeling bioretention explicitly. Version 4.56 or later is now approved by Washington State Department of Ecology for this purpose. 

2021 SWDM cover pages 

Full Manual binder cover (2.8 MB)

Full Manual binder spine (0.98 MB) and 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 in 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 (95.2 KB)

Blanket adjustments and clarifications

None present 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)