Stormwater Code

See also: Grading Code, Grading Permit, Side Sewer Code, Side Sewer Permit

What Is It?

The Seattle Stormwater Code and Manual regulate stormwater runoff in Seattle. The 2021 Seattle Stormwater Code and Manual became effective on July 1, 2021.

See the bottom of this page for the 2009 and 2016 Code and Manual and associated forms and documents.

Read the Code

The 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.

• 2021 Stormwater Code
• Summary of Recent Code Changes - Changes from the 2016 Code to the 2021 Code

Read the Manual

• Clarifications to the 2021 Stormwater Manual - Errata and clarifications to the published Stormwater Manual. Note: the manual below is the original, published manual and does not include these clarifications. 
• 2021 Full Stormwater Manual - Volumes 1-5 and Appendices Directors' Rules SDCI 10-2021/SPU DWW 200
• Summary of Recent Manual Changes - Changes from the 2016 Code to the 2021 Code
• 2021 Full Stormwater Manual with Tracked Changes - Revisions and changes from the 2016 Manual are underlined and in blue text

The 2021 City of Seattle Stormwater Manual consists of five volumes and a set of appendices. 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 -Additional Submittal Requirements
• 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 - Landscape Management Plans and Integrated Pest Management

Seattle Public Utilities' New Public Drainage System Requirements

• SPU Public Drainage System Requirements Director's Rule

This new SPU Public Drainage System Requirements Director's Rule (SPU DWW-210) includes Storm Mainline Extension, Approved Point of Discharge, and Ensuring Sufficient Capacity Requirements as well as the Public Drainage System Design Requirements that were in SPU CAM 1180. This rule replaces SDCI's Small Project Point of Discharge protocol and prohibits any new drainage discharges to the "formerly combined" public sanitary sewers and requires mainline extensions for certain projects in the combined sewer and ditch and culvert areas.

Forms and Documents

Submittal documents: If drainage review is required, some or all the following documents must be submitted with the building or grading permit application.

• 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 and that do not require Drainage Review or a Grading Permit. The plan must be submitted without edits. 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 (CSC/SOIL) Plan (PDF) | CAD Version | CTB File
  Required for all projects that require drainage review.
• Standard Drainage and Wastewater Control (DWC) Plan PDF) | CAD Version | CTB File
  Required for all projects that require drainage review.
• SDCI Typical Drainage Details (PDF) | Extra DWC Detail Sheet Template (PDF) | CAD Version | CTB File
  Select the applicable detail to put on the DWC Plan and, if needed, the extra DWC Detail Sheet.
• SDCI Standard Drainage Notes (PDF Version) | CAD Version
  Add the applicable Standard Notes to the Drainage Plans if you're not using the Standard CSC/SOIL Plan and DWC Plan Templates
• On-site Stormwater Management – List Approach Calculator 
  Required for all projects that require drainage review. Note: always start new projects with this link. Do not use a calculator that you have previously saved.
• Pre-Sized Flow Control Calculator
  May be used in lieu of continuous runoff modeling for projects that require Flow Control and have less than 10,000 square feet of hard surface.
• Memorandum of Drainage Control
  Required for all projects that require drainage control facilities for On-site Stormwater Management, Flow Control, Water Quality, On-site Infiltration, or Dispersion.

Infiltration Checklists: If your project requires On-site Stormwater Management (OSM), Flow Control, or Water Quality you must first investigate the feasibility of infiltration facilities (such as a dry well) to meet these standards. Submit the applicable infiltration checklist below with supporting documentation.

• Simple Infiltration Checklist
• Pilot Infiltration Test (PIT) Checklist

King County Sewage Treatment Capacity Charge Forms: Submit the applicable form(s) below for projects that add or replace dwelling units or buildings or that convert buildings to a new use. See sewage treatment capacity charge - King County for more information on capacity charges. Note: these documents are required even if drainage review is not required.

• 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
  

Additional Forms: You may also be required to submit one or more of these forms.

• Side Sewer Easement Agreement
• Side Sewer Joint Use and Maintenance Agreement
• Certification / Attestation of Mailing Notification Form

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" section below.

Do I Need Drainage Review?

You need to get a drainage review for your SDCI construction or grading permit if one or more of the following apply:

• You are disturbing more than 5,000 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
• You are applying for a subdivision, short plat, unit lot subdivision, or lot boundary adjustment
• You are applying for a master use permit (MUP) for development on a property that has non-disturbance areas associated with environmentally critical areas (ECA) such as steep slopes, landslide prone areas, riparian corridors, wetlands buffers, etc.
• Your project is within a category I peat settlement-prone area per SMC 25.09.020
• Other less common criteria as described in SMC 22.807.020.A / Stormwater Manual Volume 1, Chapter 8

We use the Drainage Review Checklist as an aid when reviewing permit applications. While it is not comprehensive and does not include all requirements, applicants can use it as a reference to ensure that their drainage plans address the most common requirements to expedite our review.

Training

We have several opportunities for you to learn about the 2021 Stormwater Code.

Recorded Video Trainings

• Drainage Review 101 and 2021 Code Updates
  This training covers when drainage review is required, what to submit, and what drainage control will be required. It also highlights many of the updates to the 2021 Stormwater Code/Manual.
• Infiltration Feasibility and Testing
• 2021 Updates to Drainage BMP Requirements

Reference Materials

• Managing Stormwater on Your Site - Implementation Guide (this fact sheet describes why we require on-site stormwater management, answers some common questions, and provides recommendations for implementing on-site stormwater management into development projects)
• Director's Rule: DWW-430.1, Identified Public Combined Sewer Basins Where Flow Control is Not Required (Note: if groundwater will be discharged or the immediate downstream drainage system is considered to be "Capacity Constrained," Flow Control may still be required.)
• Tip 531, Post Construction Soil Management
• Approved Street Tree List
• Vegetated Roof
• 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)

For Professional Engineers

Important Updates to Stormwater Modeling Requirements for 2021 Stormwater Manual

The following are a few of the stormwater modeling requirements that have changed with the 2021 Seattle Stormwater Manual. This is not a comprehensive list, but highlights some of the more impactful changes.

• The Peak Flow Control Standard has changed and will result in facilities that are larger. See Volume 1, Section 5.3.5. Note: the Pre-sized Flow Control Factors for projects with less than 10,000 square feet of hard surface did not increase significantly.
• The Pervious Land Segment (PERLND) and Impervious Land Segment (IMPLND) Runoff Parameters are required to be adjusted based on the project site characteristics. See the HSPS Parameter Modification section in Appendix F, Section F-4 and Table F.11.
• If using MGS Flood, Version 4.56 (or later) with the new “Ecology Bioretention” element must be used. See items numbers 3, 7, 8, and 14-16 in the Clarifications to the 2021 Stormwater Manual

Drainage Reports

A drainage report prepared by a licensed civil engineer is required for projects with 5,000 square feet or more of new plus replaced hard surface or 1 acre or more of land disturbing activity. See Volume 1, Chapter 8 for comprehensive drainage review submittal requirements and Appendix B of the Seattle Stormwater Manual for drainage report requirements.

Pre-Sized Versus Stormwater Modeling for Flow Control and Water Quality

Projects less than 10,000 square feet of new and replaced hard surface may use the Pre-Sized Flow Control Calculator or the pre-sized equations in Volume 3 of the Stormwater Manual. The pre-sized equations are in the in the BMP Sizing section for most of the BMPs in Volume 3, Chapter 5.

If there is more than 10,000 square feet of new and replaced impervious surface, then continuous runoff modeling with an approved stormwater model using the Seattle 158-year Precip / Evap series is required.

Guidance for the Seattle 158-Year Precipitation / Evaporation Time Series