Drainage & Wastewater

Property owners and developers must properly convey the following to an approved discharge location:

  • Stormwater
  • Groundwater
  • Wastewater

In the City of Seattle, all proposed development needs a review if it will disturb more than 750-square feet of land or hard surface (such as pavement). This review determines the impact on existing drainage and wastewater infrastructure.

You may need to make infrastructure improvements when existing infrastructure is unavailable or inappropriate.


Drainage is stormwater and groundwater. It must be managed to avoid damage to people, property, or the environment.

The Stormwater Code and Manual has requirements for onsite stormwater management when certain development thresholds are exceeded.


Wastewater is sewage. Examples include water from toilets, bathtubs, commercial applications, or other sources of contaminated water.

Not all contaminated water can be disposed of as wastewater in Seattle’s or King County’s sewer systems. As outlined in SDCI’s Side Sewer Code site, water must be pre-treated by a specialized facility or managed in another way if it’s contaminated with:

  • Petroleum products
  • Certain chemicals
  • Bio-hazardous materials