2026 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 this 2026 Surface Water Design Manual (SWDM) is the most current version.

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, 2024), projects permitted:

On or after July 1, 2026,
Prior to July 1, 2016, that have not started construction by July 1, 2021,
Prior to July 1, 2021, that have not started construction by July 1, 2026, or
Prior to July 1, 2026, that have not started construction by July 1, 2031

must be revised to comply with the requirements of the 2026 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 8-3-8-PR) for the 2026 SWDM, in accordance with King County Code, Chapter 2.98, Rules of County Agencies and Executive Policy INF 7-6-2-EP "Drafting and Implementing Rules". The 2026 SWDM is effective as of July 1, 2026.

Printed copies of the SWDM are not being provided. Interested parties can download the electronic version of the manual and then print and bind their own manual.

The entire 2026 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.

When viewing the SWDM manual documents through some web browsers, the entirety of the nested bookmarks may be unavailable. To address this, you can download the file and view it in a PDF reader.

Note that although some of the materials in the Reference section of the 2026 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.

2026 SWDM documents

Full 2026 SWDM (31.9 MB) - combined SWDM including all chapters, appendices, definitions, and reference sections.

Individual 2026 SWDM Appendices, References, and drawings

Appendices

Appendix A - Maintenance Requirements for Flow Control, Conveyance, and Water Quality Facilities (558 KB)

Appendix B - Master Drainage Plan Objective, Criteria, Components, and Review Process (184 KB)

Appendix C - Simplified Drainage Review Requirements (2.7 MB)

Appendix D - Construction Stormwater Pollution Prevention Standards (2 MB)

Design Manual drawings:

Standard drawings from the SWDM can be downloaded individually in AutoCAD DWG format. Additionally, the AutoCAD DWG drawings can be downloaded all at once in a single zip file; or chapters can be downloaded as separate zip files.

Select a group of drawings to download and extract:

Standard drawings in AutoCAD DWG format