The Seattle Stormwater Manual consolidates regulations governing stormwater management, aligning with Chapter 22.800 and Directors Rules. It guides design, permitting, and compliance for new projects, ensuring sustain drainage across the city, while supporting health and environmental protection.
Purpose and Scope of the Manual
The Seattle Stormwater Manual is the city’s central, authoritative guide for all stormwater design, construction, and compliance activities. It integrates the 2026 Stormwater Code, Chapter 22.800, and Directors Rules 10‑2021, providing a single, cohesive reference that translates legal mandates into practical, actionable requirements. The Manual’s purpose is to equip developers, engineers, architects, and permitting officials with clear, step‑by‑step instructions that ensure every project meets regulatory standards while protecting water quality, public health, and the urban environment. Its scope covers every phase of a project—site analysis, design, construction, inspection, and post‑construction monitoring—addressing impervious surface control, infiltration, detention, and best‑management practices. The document also delineates the roles and responsibilities of the Seattle Department of Construction and Inspections, Seattle Public Utilities, and other stakeholders, fostering coordinated oversight and streamlined review. By standardizing requirements and facilitating compliance, the Manual helps the city achieve its long‑term goals of reducing runoff, preventing flooding, and safeguarding the Puget Sound and downstream ecosystems. The Manual is updated annually through a structured revision cycle that incorporates public comment periods, stakeholder workshops, and lessons learned from field performance, ensuring practitioners always have access to the most current best practices and regulatory expectations. It also includes guidance on performance monitoring, reporting, and public engagement, encouraging transparency and community involvement at every stage of stormwater management.
Legislative Framework
Seattle’s stormwater regulations rest on Chapter 22.800 and Directors Rule 10‑2021, both enforced by the Department of Construction and Inspections and Public Utilities. These laws mandate design, permitting, and monitoring to protect water quality. Compliance ensures sustainablewater stewardship.
Stormwater Code (Chapter 22.800) and Directors Rules
The Seattle Stormwater Manual is anchored by Chapter 22.800 of the Seattle Municipal Code, which establishes the legal framework for stormwater management across the city. This chapter sets forth the requirements for controlling runoff from new and existing developments, detailing design standards, permitting procedures, and monitoring obligations. The Directors Rule 10‑2021 supplements the code by providing specific interpretive guidance on how the statutory provisions should be applied in practice. Rule 10‑2021 clarifies the responsibilities of developers, architects, and engineers, and outlines the documentation needed for drainage review. It also defines the roles of the Seattle Department of Construction and Inspections (SDCI) and Seattle Public Utilities (SPU) in administering the code, ensuring that all stormwater projects meet the city’s environmental objectives. Together, Chapter 22.800 and Directors Rule 10‑2021 create a comprehensive, enforceable system that balances development needs with the protection of water quality and public health. The manual’s detailed sections on design criteria, permitting steps, and compliance monitoring are directly derived from these legislative sources, making it the authoritative reference for anyone involved in stormwater projects within Seattle’s jurisdiction. The integration of statutory law and administrative rules ensures that all stakeholders— from private developers to municipal agencies—operate under a unified set of standards, thereby promoting consistency, transparency, and accountability in stormwater management practices across the city. This legislative foundation is critical for maintaining Seattle’s commitment to sustainable water stewardship and for safeguarding the community against flooding and water pollution risks.
Recent Code & Manual Updates
Stakeholders received a 30‑day comment period, and manual incorporates infrastructure.!
These changes align strategy with resilience goals.!!
Public feedback was solicited through workshops and portals, ensuring community voices shaped the provisions.!!
2026 Stormwater Code and Manual Revision Timeline
The 2026 revision of Seattle’s Stormwater Code and Manual follows a structured, multi‑phase timeline designed to balance regulatory rigor with stakeholder engagement. The process began in early 2025 with the release of a draft “Stormwater Code and Manual Update 2026” on the SDCI website, inviting public comment through an online portal and a series of community workshops. Draft documents were published on July 8, 2025, and the public review period extended until August 15, 2025, allowing developers, engineers, and residents to submit feedback, propose clarifications, and suggest additional performance criteria. In September 2025, the City incorporated a draft, refining language on infiltration, detention, and green‑infrastructure requirements. A second draft circulated in early 2026, incorporating the majority of public comments and aligning the Code with the City’s Climate Resilience Strategy. The final documents were approved by the City Council on June 15, 2026, and posted to the official Stormwater Code page. Effective July 1, 2026, the new Code and Manual replace all prior versions, including Directors Rules 10‑2021, and introduce updated design standards, permitting thresholds, and enforcement mechanisms. The transition framework provides a 12‑month grace period for existing projects to comply, with technical assistance offered through the Department of Construction and Inspections and Seattle Public Utilities. This update aligns with Seattle’s resilience goals now.
Effective Dates & Transition
The new Seattle Stormwater Code and Manual take effect July 1, 2026, replacing all prior rules. A 12‑month transition period allows existing projects to meet updated requirements, with guidance from the Department of Construction and Inspections and Public Utilities. Compliance is mandatory for all.
July 1, 2026 Effective Date and Transition Requirements
On July 1, 2026 the City of Seattle enacts its revised Stormwater Code and Manual, superseding all prior versions. The transition period extends through June 30, 2027, granting developers, owners, and contractors a full year to align existing and ongoing projects with the new standards. Projects that began before the effective date must submit a Transition Compliance Report by December 31, 2026, detailing current stormwater controls and any planned upgrades. The Department of Construction and Inspections will review submissions, issue compliance certificates, and, if necessary, mandate corrective actions. Projects that commence after the effective date must obtain a new drainage review permit, demonstrating compliance with updated design criteria, performance standards, and documentation requirements. The Manual provides a step‑by‑step transition checklist, including required calculations, design drawings, and field inspection protocols. Public utilities will coordinate with the Department to ensure that utility‑related stormwater infrastructure meets the new code. Non‑compliance will trigger enforcement actions, including fines, project hold‑ups, or mandatory remediation. The transition framework is designed to protect water quality, reduce flooding risk, and promote sustainable development across the city. Stakeholders are encouraged to consult the online transition guide, attend scheduled webinars, and submit queries to the Stormwater Office to ensure timely compliance. All stakeholders should review the updated guidance documents, participate in the public comment, and prepare their compliance plans before the July 1, 2026 deadline. Failure to comply may penalties and delays!
Administrative Oversight & Agencies
Seattle’s Stormwater Manual is governed by the Department of Construction and Inspections (DCI) and Seattle Public Utilities (SPU). DCI enforces code compliance, while SPU manages water‑quality monitoring and permits, ensuring coordinated oversight.
The Seattle Department of Construction and Inspections (DCI) serves as the primary regulatory authority for stormwater compliance, overseeing permit issuance, design review, and enforcement of Chapter 22.800. DCI’s Stormwater Manual provides detailed guidance on design standards, construction practices, and post‑construction monitoring. It collaborates closely with the Seattle Public Utilities (SPU), which manages the city’s water‑quality monitoring network, collects and analyzes stormwater effluent data, and issues water‑quality permits. SPU’s role includes ensuring that discharges meet federal and state water quality standards, conducting compliance inspections, and maintaining the public database of monitoring results. Together, DCI and SPU coordinate permitting processes, share technical expertise, and jointly enforce compliance through inspections, penalties, and remediation orders. Their partnership also facilitates public outreach, training, and the integration of new regulatory updates into the Stormwater Manual. This collaborative framework ensures that stormwater management aligns with environmental protection goals and supports sustainable urban development across Seattle.
The manual outlines coordination with federal and state agencies, such as the EPA and Washington State Department of Ecology, to align stormwater practices with broader water quality objectivesThe training technical assistance are provided to ensure compliance across all project types.
Design & Construction Standards
Seattle’s Stormwater Manual mandates low‑impact development, green infrastructure, and best‑management practices for new construction; It specifies design criteria, stormwater control devices, and monitoring protocols to meet Chapter 22.800 and protect water quality. It meets federal standards
Stormwater Control Requirements for New Construction Projects
New construction in Seattle must comply with the Stormwater Manual’s detailed control requirements, which align with Chapter 22.800 and Directors Rule 10‑2021. Projects are required to incorporate low‑impact development (LID) strategies, such as rain gardens, bioretention cells, and permeable pavement, to reduce runoff volume and velocity. Green roofs are mandated for all multi‑story buildings exceeding 10,000 sq ft, with a minimum 2‑inch drainage layer and vegetative cover that meets local species criteria. Stormwater detention and retention basins must be designed to capture at least 50 % of the first‑flush stormwater and to maintain a 24‑hour detention period for residential developments, while commercial projects must retain 75 % of the first‑flush and provide a 48‑hour detention period. Infiltration trenches and subsurface infiltration systems are required for projects on permeable soils, with a minimum infiltration rate of 0.5 in/hr and a design storm of 0.5 in. All stormwater control devices must be inspected annually, and maintenance records must be submitted to the Department of Construction and Inspections (DCI) and Seattle Public Utilities (SPU) within 30 days of installation. The Manual also specifies that stormwater quality must meet the City’s water quality standards, requiring sediment and pollutant control measures such as sediment basins, oil‑water separators, and fine‑mesh filtration for projects adjacent to water bodies. Construction activities must follow the “Construction Stormwater Control” guidelines, including erosion and sediment control plans, daily site inspections, and a final site clearance before occupancy. The Manual’s design criteria are based on the 2025 storm‑event data, ensuring that new developments do not increase peak discharge rates beyond the 100‑year floodplain limits. All design documents, including hydraulic calculations, stormwater modeling reports, and LID schematics, must be submitted for review under the DCI’s Drainage Review 101 process. The Manual’s requirements are enforced through a combination of permit conditions, on‑site inspections, and post‑construction monitoring, with penalties for non‑compliance ranging from fines to project shutdowns. By adhering to these standards, Seattle ensures that new construction projects protect water quality, reduce flooding risk, and promote sustainable urban water management.
Permitting & Review Process
The permitting process begins with a preliminary review, followed by a detailed drainage review. Applicants submit plans, calculations, and LID designs. DCI evaluates compliance with the Manual, issues permits, and schedules inspections Compliance mandatory!
Drainage Review 101 and Required Documentation
Before submitting a stormwater permit, applicants must complete the Drainage Review 101 training, which explains the Seattle Stormwater Manual’s requirements, design criteria, and permitting workflow. The training covers the 2026 Code updates, LID best practices, and the role of the Department of Construction and Inspections (DCI) and Seattle Public Utilities (SPU). After training, applicants must prepare a comprehensive set of documents: a site plan showing existing and proposed impervious areas, a stormwater management plan (SWMP) that includes design calculations, detention or infiltration volumes, and a LID inventory, a drainage review application form with project details, a compliance statement, and any required environmental impact statements. All drawings must be in PDF or DWG format, with layers labeled per the Manual’s naming conventions. The application must also include a cost estimate, a schedule of inspections, and a copy of the signed LID compliance certification. DCI will review the submission for completeness, verify that all calculations meet the 2026 Code thresholds, and assess whether the proposed controls meet the required performance standards. If deficiencies found, the applicant receives a deficiency notice and must correct the issues before a permit is issued. The entire process is designed to ensure that stormwater runoff is managed safely, sustainably, and in compliance with Seattle’s regulatory framework. Documentation must be submitted electronically.
Compliance, Enforcement & Public Engagement
Seattle’s manual mandates regular monitoring, annual reporting, and public dashboards. Violations trigger fines, corrective orders, and possible civil action. Community meetings and online portals foster stakeholder input and transparency.2026
Monitoring, Reporting, and Public Participation Mechanisms
Monitoring, reporting, and public participation mechanisms are central to the Seattle Stormwater Manual’s compliance framework. All regulated projects must submit quarterly monitoring reports that capture runoff volumes, pollutant concentrations, and control device performance. These reports are uploaded to the city’s Stormwater Dashboard, a publicly accessible web portal that displays real‑time data and trend analyses. The dashboard also hosts downloadable datasets and a GIS layer of monitoring sites, enabling researchers, planners, and residents to evaluate watershed health. In addition to electronic reporting, the manual requires annual compliance reviews conducted by the Seattle Department of Construction and Inspections (SDCI) in partnership with Seattle Public Utilities (SPU). During these reviews, inspectors verify that control measures meet design specifications and that monitoring equipment is calibrated. If deficiencies are identified, corrective action plans are issued and tracked through the SDCI online case management system. Public participation is facilitated through community meetings at the SDCI office and via an online comment platform that allows stakeholders to submit public draft feedback request site visits.! The manual also mandates that project owners provide public notices in the Seattle Daily Journal of Commerce and the Land Use Information Bulletin, ensuring that local residents are informed of stormwater activities and can engage in the decision‑making process. By integrating data transparency, routine oversight, and open dialogue, the manual promotes accountability and encourages continuous improvement in stormwater management across the city.