Grading, Drainage, and Clearing Requirements (Land Disturbing Activity)

Why is regulation of stormwater runoff important?

Stormwater runoff is rain that runs off hard surfaces, rooftops, streets, parking lots, and landscaped areas. As water runs off these surfaces, it can pick up pollutants such as oils, dirt, and fertilizers. Polluted stormwater runoff is a leading contributor to water pollution in the Puget Sound region.

Runoff can also cause and contribute to flooding. When runoff is uncontrolled, it could result in the destruction of property, harm to fish and wildlife, damage to aquatic habitats, loss in aesthetic values, and cause threats to public health due to contaminated food, drinking water supplies, and recreational waterways.

Snohomish County is committed to complying with federal and state requirements to provide effective and efficient stormwater management in our community. As part of our work, we maintain stormwater regulations that guide new development and redevelopment to minimize pollution and harmful effects of stormwater.


Where can I find the County’s Stormwater regulations?

Please visit the Stormwater and Drainage Regulations webpage for more information on the permitting process and links to associated checklists and assistance bulletins.

The County’s current stormwater and drainage requirements are found in:

Please visit our LID webpage for resources and information.

These rules and regulations have recently been updated effective July 1, 2021. The updated requirements, along with documents that summarize the changes are available here.

Stormwater, drainage, and land disturbing activity regulations are updated from time to time. Generally, projects vest to a particular version of the regulations depending on when project applications are submitted. However, a recent Washington State Supreme Court decision and the County’s Phase I Municipal Stormwater Permit (“Phase I Permit”), issued to the County by the Washington State Department of Ecology, now require a different result.


On December 29, 2016, the Washington State Supreme Court issued a decision in Snohomish County v. Pollution Control Hearings Board related to the County’s Phase I Permit. The Court held the County’s updated stormwater regulations are not “land use control ordinances” subject to state vesting statutes. This is a significant change in the law for developments across the state.

Snohomish County is a Phase I jurisdiction under the federal Clean Water Act National Pollutant Discharge and Elimination System (NPDES) program. The County must comply with its Phase I Permit. One aspect of the County’s compliance with its Phase I Permit is the adoption of updated stormwater drainage regulations, which the County accomplished effective January 22, 2016, and which the County must further update by July 1, 2021. County documents that will be updated by July 2021 include: Chapters 30.63A and 30.63B SCC, Snohomish County Drainage Manual (Volumes I through V), and Snohomish County Design and Development Standards (Chapters 1, 5 and 11). Please visit for more information on the proposed updates to the stormwater regulations.

What Does This Mean for Development Projects?

The County’s Phase I Permit requires the County to impose updated drainage regulations to projects that have not started construction as follows:

  • Approved permits and permit applications subject to pre-January 22, 2016, stormwater drainage regulations that have not started construction by July 1, 2021, must be revised by the project proponent to comply with updated stormwater drainage regulations that will be made effective by the county on July 1, 2021. For projects with a phasing plan, compliance with the started construction requirement will be assessed separately for each phase.

  • Approved permits and permit applications subject to pre-July 1, 2021, stormwater drainage regulations in effect between January 22, 2016, and June 30, 2021, that have not started construction by July 1, 2026, must be revised by the project proponent to comply with updated stormwater drainage regulations that will be made effective by the County on July 1, 2021. For projects with a phasing plan, compliance with the started construction requirement will be assessed separately for each phase.

The term “started construction” as it pertains to the County’s Phase I Municipal Stormwater Permit, is defined in Snohomish County Code (SCC) 30.70.310 to mean either:

  • (a) 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; or

  • (b) for public works projects performed under the authority of the director of public works or the county engineer which are subject to public bid laws, the project has been advertised per public bids laws, legislatively approved for construction, awarded to contractor, site work has begun, and the contractor has a schedule for completion.

Click here to view a document summarizing the revised Snohomish County definition of “started construction” for the Phase I (NPDES) Stormwater Management Permit.

PDS is preparing to implement the new stormwater requirements effective July 1, 2021. PDS recognizes the Supreme Court decision and the requirements Ecology imposed on the County under its Phase I Permit may impact significantly some development projects, and is evaluating ways to facilitate the permitting process to minimize that impact while fully implementing the County’s obligations under its Phase I Permit. PDS performed a detailed review of its records, followed by hundreds of site inspections, to identify potentially impacted projects. PDS sent notices to alert applicants or permit-holders that may be impacted by the July 1, 2021, started construction deadline.

Planning and Development Services has issued a Director's Interpretation of Administration and Enforcement of National Pollution Discharge Elimination System (NPDES) Phase I Municipal Stormwater Permit. This interpretation provides guidance and transparency about how PDS will administer and enforce the requirements of recent regulations that apply to projects subject to a revised definition of "started construction," as defined by the recent change in county code (SCC 30.70.310), which was adopted to comply with federal regulations in April of 2021. Planning and Development Services is committed to ensuring all projects are compliant with county code, and will be conducting inspections on July 1, 2021 of all projects which are subject to land disturbing activity permit approvals based on stormwater regulations in effect prior to January 22, 2016. For more information, please see PDS Director's Interpretation.

Contact Us

  • If you have questions about a specific application already submitted to PDS, please contact your project manager/contact. To locate your project manager/contact, enter your project file number on our Postcard page.
  • If you are preparing to submit an application for a permit, see information on the main permitting page or visit
  • For general information about the County’s stormwater regulations, please email Ken Crossman or call 425-262-2227.