City of Seattle, Washington Stormwater Code

The City of Seattle, Washington’s stormwater regulations are implemented in order to improve stormwater management for new development in Seattle, including on-site stormwater management. Seattle’s Stormwater Code imposes retention requirements on residential properties.  These requirements vary according to several factors, including the type of sewer system or water body to which the site discharges and the size of the land disturbance or impervious surface on that site.  For example, if a parcel discharges into small lake basins and its total new-plus-replaced impervious surface is 2000 square feet or more, it must manage stormwater from a 25-year rainfall event (a storm that has a 4% chance of occurring in any given year). Additionally, construction sites are required to maintain natural drainage patterns, protect downstream properties from erosion, and implement green infrastructure, to the maximum extent feasible.

As described by the City, stormwater runoff can cause flooding, landslides, and erosion that can damage homes, businesses, and property. Stormwater runoff carries oil and grease, fertilizers, pesticides and other toxic chemicals to  creeks, lakes, bays, rivers, and other waterways.

By not only requiring retention based on the water- quality considerations (discharge destination), but also by requiring green infrastructure onsite to manage the stormwater, Seattle is avoiding high levels of impervious surface coupled with stormwater basins in favor of green infrastructure.


Publication Date: 2009

Related Organizations:

  • City of Seattle, Washington

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  • Regulation

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