Stormwater Code

Note: The 2026 Seattle Stormwater Code and Manual will become effective on July 1, 2026. Projects with a completed application on July 1, 2026, or later must comply with the 2026 code. You may choose to comply with the 2021 code for 2026 code for projects with a completed SDCI permit application submitted prior to July 1, 2026. An application is complete when intake has been approved and all intake fees have been paid.

See also: Grading Code, Grading Permit

What Is It?

The Seattle Stormwater Code and Manual regulate stormwater runoff in Seattle. See the bottom of this page for the 2016 and 2021 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). It will be available on the Seattle Municipal Code website on July 1, 2026.

• 2026 Stormwater Code

Read the Manual

• 2026 Full Stormwater Manual - Volumes 1-5 and Appendices Directors' Rules SDCI 10-2021/SPU DWW 200. 
• 2026 Full Stormwater Manual with Tracked Changes - Revisions and changes from the 2021 Manual are underlined and in blue text.

The 2026 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 - Drainage Control Report and Additional Submittal Requirements
• Appendix C - On-Site List Infeasibility Criteria
• Appendix D - Subsurface Investigation and Infiltration Testing for Infiltration Facilities with BMPs
• Appendix E - Additional BMP Design Requirements
• 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 Plans
• Appendix J- Plant and Tree Lists for Stormwater BMPs

Seattle Public Utilities' Public Drainage System Requirements

• SPU Public Drainage System Requirements Director's Rule DWW -  210

This 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.

Submittal Documents 

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

Note: The plans and standard notes and details documents below are provided as references for building professionals using Autodesk or similar programs. They are not for communicating information to the general public and are not screen reader compliant. If you need assistance accessing these files, contact sci_ada@seattle.gov.

Plans

• Small Construction Stormwater Control Soil Management (CSC/SOIL) Standard Plan (PDF) | CAD Version
  This pre-approved standard plan may only be used for construction projects with less than 5,000 square feet of new and replaced hard surface or land disturbance. This standard plan is made to be submitted as is, without edits. Note: For projects with 5,000 square feet or more of new and replaced hard surface or demolition permits with 5,000 square feet or more of land disturbance, a custom construction stormwater control and soil management plan sheet is required. You may use the Standard Notes and Standard Details, provided below, on your stamped plans.
• Stormwater Plan (Stormwater Plan PDF) | CAD Version 
  This plan may be used for all projects that require stormwater review. See the "Do I Need Stormwater Review" section below.

Note: If your project has 5,000 square feet, or greater, of new plus replaced hard surface area or 10,000 square feet, or greater, of land disturbing activity, your application must be prepared by a professional engineer. See the "For Professional Engineers" section below.

Standard Notes and Details

Select the applicable notes and details to put on the Stormwater Plan (template), your custom stormwater plan, or your custom CSC/SOIL plan.  

• SDCI Typical Stormwater Details (PDF) | Extra Stormwater Plan Detail Sheet Template (PDF) | CAD Version 
• SDCI Standard Stormwater Notes (PDF) | CAD Version

Calculators

• 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.

Infiltration Checklists

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

Do I Need Stormwater Review?

You need stormwater 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.

SDCI uses the Stormwater 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 stormwater 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
• 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 2026 Stormwater Manual

The following are a few of the stormwater modeling requirements that have changed with the 2026 Seattle Stormwater Manual. This is not a comprehensive list, but highlights some of the more impactful changes.

• The Seattle 158-Year Precipitation / Evaporation Time Series was renamed to the Seattle Extended Precipitation Time Series (SEPTS-99) in the 2026 Stormwater Manual.  Currently, there is no change to the values in the time series.  Continue to use the SPU 158 Year 5min time series in WWHM and MGS Flood.  See additional guidance below. 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. (This was updated in 2021.)
• 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. (This was updated in 2021.)
• If using MGS Flood, Version 4.56 (or later) with the new “Ecology Bioretention” element must be used. 

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 10,000 square feet or more of land disturbing activity. See Volume 1, Chapter 8 for comprehensive stormwater 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 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 Extended Precipitation Time Series (SEPTS-99)