Stormwater Code

See also: Side Sewer Code, Side Sewer Permit

What Is It?

The Stormwater Code and Manual are in the process of being updated and are scheduled to be effective July of 2021. See our Updating Stormwater Regulations page for more information.

The 2016 Stormwater Code and Manual became effective on January 1, 2016. 

Our stormwater regulations protect people, property, and the environment from damage caused by stormwater runoff. Our stormwater codes also satisfy the City’s obligation to comply with our Municipal Stormwater Discharge National Pollutant Discharge Elimination System (NPDES) Permit, issued by the Washington State Department of Ecology.

Our Stormwater Code addresses:

• Drainage control submittal and review requirements
• Where the stormwater from your site needs to go
• On-Site Stormwater Management (previously referred to as Green stormwater infrastructure) and why it's important
• Erosion control requirements for your project during construction and grading activities
• Flow control and stormwater treatment requirements
• How we enforce the Stormwater Code

Do I Need Drainage Review?

You need to get a drainage review for your construction or grading permit if:

• You are disturbing more than 750 square feet of land
• You are adding or replacing more than 750 square feet of hard surface, such as pavement
• You are adding or replacing more than 750 square feet of a building (as measured by the roof outline)
• A Grading Permit is required for your project 
• Your project is within a Category I peat settlement-prone area per SMC 25.09.020 

Read the Code

The 2016 Stormwater code is Title 22, Subtitle VIII of the Seattle Municipal Code (SMC). You can find the stormwater code on the Seattle Municipal Code website.  

• 2016 Stormwater Code
• Seattle Municipal Code 22.800 - 22.808
• 2016 Stormwater Code Ordinance, Council Bill 118465

Read the Manual

The City of Seattle Stormwater Manual consists of five volumes and a set of appendices.

• Stormwater Manual - Volumes 1-5 and Appendices (Directors' Rules SDCI 17-2017/SPU DWW 200)
• Clarification Sheet for the 2016 Stormwater Manual
• Clarification Attachment 1: On-Site Performance Standard BMP Design (Addition to Appendix F - Hydrologic Analysis and Design)
• Clarification Attachment 2: BMP 10 Fueling at Dedicated Stations (Clarifications for Volume 4, Section 3.2.2)

Below is the Stormwater Manual, broken into sections.

• Volume 1 – Project Minimum Requirements
• Volume 2 – Construction Stormwater Control
• Volume 3 – Project Stormwater Control
• Volume 4 – Source Control
• Volume 5 – Enforcement
• Appendix A – Definitions
• Appendix B – Chemical Treatment
• Appendix C – On-Site Stormwater Management BMP Infeasibility Criteria
• Appendix D – Subsurface Characterization and Infiltration Testing for Infiltration Facilities
• Appendix E – Additional Design Requirements and Plant List
• Appendix F – Hydrologic Analysis and Design
• Appendix G – Stormwater Control Operations and Maintenance Requirements
• Appendix H – Financial Infeasibility Documentation for Vegetated Roofs and Rainwater Harvesting
• Appendix I – Integrated Pest Management

Forms and Documents

Required Documents

Projects with more than 750 square feet of land disturbing activity must submit the following documents.

• Small Demolition Construction Stormwater Control Soil Management Plan. This pre-approved plan may only be used for small demolition permits which disturb less than 5,000 square feet of land. The plan must be submitted without edits. Drainage review isn't required. To create a site specific plan or if you plan to disturb more than 5,000 square feet of land, use the Standard CSC and Soil Amendment Plan listed below.
• Standard Construction Stormwater Control and Soil Amendment Plan (PDF) | CAD Version | CTB File
• Standard Drainage and Wastewater Control (DWC) Plan (PDF) | CAD Version | CTB File
• On-site Stormwater Management – List Approach Calculator (If you find errors or experience any difficulty with this draft calculator, please email sidesewerinfo@seattle.gov).
• Simple Infiltration Checklist
• Pilot Infiltration Test (PIT) Checklist
• Pre-Sized Flow Control Calculator - For projects with required flow control of less than 10,000 square feet of hard surface
• Memorandum of Drainage Control
• Side Sewer Easement Agreement
• Side Sewer Joint Use and Maintenance Agreement
• Certification / Attestation of Mailing Notification Form
• King County Sewage Treatment Capacity Charge - Residential Sewer Use Certification
• King County Sewage Treatment Capacity Charge - Non-Residential Sewer Use Certification
• King County Sewage Treatment Capacity Charge - Alternative Housing Sewer Use Certification

Note: If your project has 5,000 square feet, or greater, of new plus replaced hard surface area, your application must be prepared by a professional engineer. See the “For Professional Engineers” later on this webpage.

On-site Stormwater Management Documentation

In the 2016 Stormwater Code and Manual, “On-site Stormwater Management” (OSM) requirements replace the previous Green Stormwater Infrastructure (GSI) requirements. On-site Stormwater Management feasibility, Best Management Practice selection, and Best Management Practice sizing must be documented in the On-site Stormwater Management – List Approach Calculator.

Infiltration Investigation Requirements

If your project requires On-site Stormwater Management or must meet Flow Control or Water Quality Standards, you must first investigate the feasibility of infiltration facilities (such as a dry well) to meet these standards. The exception to this is if your project is located in area that is mapped as "Infiltration Investigation Not Required." The “Infiltration Investigation Not Required” map is available to you as a Base Map Layer on our GIS website. If you are required to do infiltration testing, you must submit the infiltration checklists with your plans. If infiltration is not required, you must indicate the reason on the Site and Drainage Control Summary.

Nutrient Critical Receiving Waters

At the time this Manual was developed, there were no nutrient-critical receiving water segments determined to be impaired due to phosphorus contributed by stormwater. In the future, the City may designate a waterbody as a nutrient-critical receiving water as defined by the SMC, Section 22.801.150.

Reference Materials

Here's a list of documents to help you understand our stormwater code:

• Director's Rule: DWW-430.1, Identified Public Combined Sewer Basins Where Flow Control is Not Required (Note: if the immediate downstream drainage system is considered to be "Capacity Constrained," Flow Control may still be required.)
• Tip 520, Rainwater Harvesting for Beneficial Use
• Tip 531, Post Construction Soil Management
• Approved Street Tree List
• Permanent Groundwater Dewatering
• Vegetated Roof Media
• SPU Allowable Permeable Pavement Wearing Course Materials
• Director's Rule 4-2011/SPU 2011-004, Requirements for Design and Construction of Side Sewers (Drainage and Wastewater Discharges) (see this document for allowable materials and construction methods for drainage and detention pipes and structures).