AG 1050: Regional Voluntary Bioretention in the Right-of-Way Projects

Regional Voluntary Bioretention in the Right-of-Way Projects Overview

The City of Seattle supports regional, voluntary bioretention projects in the public right-of-way that manage public stormwater runoff. “Regional” facilities manage pollution-generating hard surface from a contributing area beyond the immediate surrounding block. These projects might be installed by a redeveloping parcel property as an amenity for tenants and the public, or led by a collection of community organizations, neighborhood groups, or private funders that want to help improve water quality in our lakes, streams, and Puget Sound.

Bioretention systems are a type of green stormwater infrastructure (GSI) and a preferred best management practice for mitigating the negative impacts of stormwater runoff. These systems collect and filter rainwater, allowing it to soak into the soil and reducing the amount of polluted runoff from adjacent streets, sidewalks, and private parcels from entering Seattle’s waterways. They can be integrated into the streetscape to provide habitat, beautify streetscapes, and serve as a buffer between the sidewalk and street to create a safe and welcoming place for people to walk. The following street type designations in Streets Illustrated are most likely to support large voluntary bioretention systems:

  • Downtown Neighborhood Street
  • Urban Village Main
  • Urban Village Neighborhood
  • Neighborhood Corridor
  • Industrial Access
  • Neighborhood Yield
  • Neighborhood Curbless

This Applicant Guide addresses the requirements for the design, construction, and maintenance of regional voluntary bioretention projects in City right-of-way. Because these facilities are voluntary, they do not typically fulfill all Seattle Stormwater Code or Seattle Green Factor requirements for adjacent parcel projects. For guidance on smaller, less complex bioretention projects designed to infiltrate runoff from a single block or less, reference CAM 2309 Voluntary Curbside Bioretention.

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Section 1: Project Design

Projects should reference the City's standard plans and specifications and the City of Seattle Stormwater Manual for design requirements. Additionally, projects should assume the following:  

  • Infiltrating bioretention is preferred whenever feasible. 
  • For stormwater design elements, the applicant must adhere to City Design Standards & Guidelines (DSG). For other design standards, such as Standard Specifications for Road, Bridge and Municipal Construction, a deviation may require additional permit submittals that could extend the permitting timeline.
  • Projects with unnecessary or non-standard structures typically trigger additional permitting, require extensive maintenance, and may increase the risk of the facility failing. Any non-standard assets, even those detailed in the Stormwater Manual like walls and weirs, will remain the responsibility of the neighboring property owner to maintain.
  • Underground Injection Control (UIC) wells or facilities and pit drains are not allowed. 
  • Any new street trees and plants within and adjacent to bioretention cells should be selected, designed, and planted according to the City of Seattle Stormwater Manual, Appendix E, Additional Design Requirements and Plant Lists and Streets Illustrated Section 3.7 – Street Tree Design Standards and in response to any feedback received from Seattle Department of Transportation (SDOT) Urban Forestry as part of design guidance meetings and review during the permit process. 
  • Project shall include a bypass system provision for maintaining flow to the downstream City-owned drainage system (Municipal Separated Storm Sewer System [MS4] or combined sewer system [CSS]) in the event the system fails or must be taken off-line. For example, provisions could include valves, adjustable flow splitters, conveyance pipes, or other components as needed to bypass or divert flows safely downstream if the facility fails or the City deems it necessary to fill in the facility area.
  • A licensed professional engineer is required to submit a completed asset onboarding spreadsheet with the 90% design submittal. Be sure to request the most current version of the asset onboarding spreadsheet, which may include long-term operation and maintenance expenses and labor hours required to achieve planned maintenance.

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Section 2: Project Permitting

All regional bioretention in the right-of-way projects must go through the SDOT Street Improvement Permit (SIP) process, detailed in Applicant Guide (AG) 2000. A licensed professional civil engineer should prepare the SIP plans and typically facilitates the permitting process with SDOT. Seattle Public Utilities (SPU) will also review the design and, if acceptable, approve the plans during the SIP permitting process. Applicants must also comply with SPU's GSI Construction Inspection Requirements for bioretention facilities in the right-of-way to ensure the inspection process is followed.

In general, applicants should note the following early in the permitting process:   

  1. For projects that are part of an associated parcel development project, submit a preliminary building and land use permit application at the Accela Permitting Portal. Frontage improvements associated with the parcel development will eventually require SIP plans.
  2. When filling out the preliminary assessment questions, indicate in the notifications section that you are interested in a free consultation with SPU to discuss voluntary bioretention in the right-of-way as part of your project. If you cannot locate this section, email to schedule an appointment.
  3. If, after consultation, SPU confirms that the voluntary regional bioretention aspect of the project is viable, SPU will help coordinate design review with the project’s SDOT Project Manager (PM) once the SIP application is submitted.

Note: Projects seeking to build voluntary regional bioretention in the right-of-way as a standalone project (meaning there is no associated development project) should email to schedule an initial consultation.

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Section 3: Project Ownership & Maintenance

The City expects these assets to perform for decades, so accessibility, maintainability, and durability are essential. Most permitted projects will feature subsurface components, including, but is not limited to, pipes, flow restrictors, catch basins, inlets, underdrains, manholes, and upstream pre-settlement structures. For projects with subsurface components that meet the following conditions, the City commits to maintaining those components over the functional life of the asset. 

  • Project discharges to a City-owned system (MS4 or combined sewer system) 
  • Project extends or is plumbed into the public drainage system (e.g. due to poor infiltration rates in the right-of-way)
  • Project manages at least a half-acre (21,780 square feet) or more of contributing area from pollution-generating hard surfaces 
  • Project is deemed safely accessible and maintainable by City operations staff 

The City will not maintain additional furnishings or architectural features. Any non-standard assets must be reviewed and approved by SPU’s GSI Asset Management staff as part of the SIP process. Maintenance of any approved non-standard assets will remain the responsibility of the property owner.

Maintenance Responsibilities by Asset Component

The following table lists maintenance responsibilities for a sample project. It is for illustration purposes only. A detailed final version of the table and the completed asset onboarding sheet will be included in the final agreement (see Agreement Terms section).

Asset Component Seattle Public Utilities ResponsibilityAdjacent Property Owner/Manager or Project Steward Responsibility
    Softscape Assetsa
Vegetation  x
Mulch  x
Bioretention Soil Mix (BSM)b x
Aggregate x
    Hardscape Assets
Pipes x
Laterals x
Underdrains  x
Catch basins  x
Inlets x
Junction boxes x
Flow restrictors x
Sedimentation chambers x
Conveyance facilities needed to maintain flow to downstream City-owned drainage system  x


a. The adjacent property manager or project steward will be responsible for maintaining landscaped elements, including vegetation, mulch and bioretention soil according to guidelines in the Seattle Stormwater Manual, Appendix G, Stormwater Control Operations and Maintenance Requirements.

b. Replacing the bioretention soil and vegetation is typically the responsibility of the property owner or project steward. Periodically, the City will need to replace the bioretention soil as part of regular maintenance, at which point vegetation may have to be removed. The property owner or project steward will incur the cost to replace bioretention soil and vegetation.

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Section 4: Agreement

The City requires a formal agreement to document the project ownership, asset types, and maintenance responsibilities. The agreement, including an asset onboarding sheet, is to be completed in full and accompany the 90% design SIP submittal. A sample agreement can be referenced here.  

Voluntary Bioretention with Adjacent Redevelopment 

The applicant will coordinate with SPU and SDOT staff throughout design on a declaration of covenant and terms for ongoing facility maintenance. The covenant will be recorded on the deed of the adjacent parcel and all current and future parcel owners become the project sponsor in perpetuity.

  • As stated in the Maintenance Responsibilities table, projects discharging to City MS4 and CSO basins will split maintenance between the property owner (softscape) and City (hardscape).
  • For facilities that manage right-of-way runoff from non-City jurisdictions (e.g. WSDOT, King County, large institutions), the adjacent property owner will work with the sponsoring jurisdiction directly on additional necessary permitting and/or agreements that cover all aspects of the facility including ownership and maintenance (softscape and hardscape). 

Voluntary Bioretention without Adjacent Redevelopment 

The applicant will coordinate with SPU and SDOT staff throughout design on the declaration of covenant and terms for ongoing facility maintenance. The organization (or group of organizations) will identify a long-term steward for the facility to be named as grantor in the covenant.

  • As stated in the Maintenance Responsibilities table, projects discharging to City MS4 and CSO basins will split maintenance between the facility steward (softscape) and City (hardscape). 
  • For facilities that manage right-of-way runoff from non-City jurisdictions, the project steward will work with the sponsoring jurisdiction directly on additional permitting and/or agreement that covers all aspects of the facility including ownership and maintenance (softscape and hardscape). 

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Section 5: Partnering Opportunities

Depending on the project, the City may be willing to enter a financial partnership to maximize stormwater management potential. In general, projects that meet the following conditions are most likely to be eligible for partnering:

  • Project discharges to a City-owned MS4 or combined sewer basin 
  • Project manages runoff from a priority drainage basin where SPU has known system issues (e.g. flooding or sewer backups) 
  • Project manages greater than a half-acre of pollution-generating hard surface area from the right-of-way 
  • Project is designed to meet a flow control or water quality performance standard as defined in the City of Seattle Stormwater Manual

Note in your preliminary building and land use permit application if you believe your project is capable of meeting these conditions and would like to explore a financial partnership. To schedule a consultation with SPU, email

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Greg Spotts, Director
Address: 700 5th Ave, Suite 3800, Seattle, WA, 98104
Mailing Address: PO Box 34996, Seattle, WA, 98124-4996
Phone: (206) 684-7623

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