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Stormwater drainage resources for professionals
Requirements and standards for designing surface and stormwater management systems in King County to mitigate the impacts of new development and redevelopment on natural and existing man-made drainage systems.
Important notice about projects permitted under earlier versions of the Surface Water Design Manual (SWDM)
The 2021 SWDM (amended 2024) is the most current version.
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.
Other resources
Additional permit-required stormwater management resources for development and re-development.