Stormwater Facility Credit Frequently Asked Questions (FAQ)

What is the Stormwater Facility Credit Program?

This SPU program provides a discount on drainage bills for private stormwater systems that reduce stormwater flow and/or provide water quality treatment, which help lessen the impact to the City’s stormwater systems. Systems must be built to City Code standards and be properly maintained and functioning to be considered for credit.

  • Permeable paving is a paving system which allows rainwater to percolate into the underlying soil.
  • Permeable paving is designed to slow the flow of stormwater.
  • Permeable paving can be used in walkways or driveways instead of soil, concrete or cement surfaces.
  • Porous cement can be used for sidewalks or driveways and allows water to drain to the underlying soil below.
  • Porous cement is an open-graded pavement system with small air pockets encased within the pavement.
  • Detention ponds are surface water basins that temporarily store rainfall to help prevent flooding.
  • They are typically designed to fill up during heavy rain events then drain in between storms.
  • Media filter systems are filtration systems that use sand or crushed granite to filter out pollutants from rainfall on impervious surfaces.
  • They can be designed as cartridges in a maintenance hole.
  • They can also be designed as cartridges in a vault underground.
  • Oil water separators are designed to separate oil and water. This allows the oil to stay in the system while the clean water discharges. There are two types of oil water separators; the baffle system, called American Petroleum Institute (API), and the coalescing plate system (CP) as shown here.
  • A detention system is a large pipe or vault that holds rainwater on the property and then allows water to flow slowly through a “flow control structure.”
  • The flow control structure has a small hole at the bottom of the pipe that meters the amount of water that can drain out of the detention system.
  • You should never enter a detention system without having had the proper training.
  • Bioretention systems are shallow depressions in the ground with designed soil mix and plants adapted to the local climate and soil conditions.
  • Bioretention systems can be designed to both detain and treat stormwater.
  • They can be used in common landscaped areas at business or residential properties.
  • They are intended to be used in small areas, with no one system larger than 800 square feet of bottom area.
  • They can be designed to take rainwater from rooftops, driveways or parking areas.
  • Green roofs are living vegetation systems installed on top of buildings to slow the flow of stormwater via soil storage, evaporation and transpiration. Green roofs consist of several layers of material to achieve the desired vegetative cover and drainage requirement.

What is a stormwater system?

Stormwater systems are structures such as vaults, rain gardens, permeable pavements and infiltration systems that provide water quality treatment and/or slow down stormwater flow from impervious surfaces like rooftops, driveways or walkways. Retrofitted systems can also apply as long as they meet code requirements.

How do I know if I have a stormwater system on my property?

Here are four ways you can determine if you have a stormwater system on your property. Some of these systems may have access points that are in landscaped areas or parking lots where you may need to expose the access point.

  • Locate plans used for original development and/or redevelopment of your property. Review the drainage plans for pipes and stormwater systems such as detention vaults or oil water separators.
  • Evaluate your property while it is raining. Starting from the roof downspouts and catch basins, follow the path of the water to see and hear where it goes. You might need a crowbar to open grates and a flashlight to inspect the downstream pipes for the direction of flow. Follow flow to see if there are any structures in that direction.
  • Look at your property’s sewer side sewer card. These site maps will outline the drainage structures on your property and may show any vaults or treatment systems that were constructed during the construction phase. Look up your address at Seattle’s Department of Planning and Development Side Sewer Cards & Maps.

If my application is approved, when can I expect to see the credit on my drainage bill?

All applications received by November 1 are eligible for credit on the drainage bill for the next calendar year. The drainage bill is shown on the annual property tax bill from the King County Assessor.

How big will my credit be?

Credit amounts are both site and system specific. The maximum allowable parcel credit is 50 percent. In 2008, the average awarded credit was 9 percent.

Do I have to reapply for the credit program every year?

No, the credit will continue to remain on your account as long as your stormwater system is operating and maintained properly. SPU’s drainage fee is billed on the King County property tax statement. Learn more about SPU’s drainage rates.

You can view your current drainage fee by entering your property tax account number into the King County Property Tax Information System. The drainage fee is shown as the “Surface Water” or Drainage line item on the statement and will be net of any approved credits. Your property tax account number is found on the property tax statement you receive in the mail from King County. You may also find your property tax account number (or parcel number) by using the “Parcel Viewer” search feature found at the above King County Web site.

What are the benefits of building a low impact development system?

Low-impact development (LID) and Green Stormwater Infrastructures (GSI) systems, such as rain gardens and swales, generally provide a higher level of stormwater management and thus receive a larger credit than traditional stormwater systems such as detention vaults. Low impact development is a way to decrease the amount of impervious surface on your property during the development and/or construction phases. By decreasing the amount of impervious surface, you reduce the amount of rainwater runoff from the site onto the City street and right-of-ways.

How often will my system be inspected?

SPU may inspect systems once a year.

What if the inspector tells me I need to clean my system?

If your stormwater system has not been properly maintained or is not working properly, you will have one month to repair, clean and maintain the system before a second inspection. Failure of a second inspection will terminate your credit. Non-functioning systems may be subject to follow-up enforcement actions as required by code. 

I own a condo. How do I get a credit for stormwater systems that manage my building’s run-off?

If your condo property has a stormwater system, each unit in the condo may be eligible for stormwater facility credit. Any approved facility credit will be shared equally among owners of individual units. To be considered for a credit, your condo homeowner association will need to submit an application form. If the application is approved, you will receive the credit on the surface water fee that is billed on your individual unit’s property tax statement. The condo association will be responsible for ensuring that the necessary maintenance is completed in order to continue receiving the credit.

I own a townhouse with no homeowners’ association. How do I get this stormwater facility credit?

If your townhouse development has no homeowners’ association, any individual owner may submit an application. Any approved credit will be divided among the townhouses in the development. SPU requires all stormwater systems be properly maintained in order to qualify for and continue to receive a facility credit. In the absence of a homeowners’ association, townhouse owners must work out the details regarding system maintenance among themselves. Often, guidelines regarding maintenance of common systems are addressed in covenants specific to the townhouse development.