How SPU Determines Your Credit

SPU has developed a formula that is used to calculate the percentage credit for each eligible parcel. In this "credit calculator", SPU has assigned a uniform percentage credit for each type of approved stormwater system based on a weighting of the stormwater performance goals and the appropriate drainage discharge point (see Drainage Basin Map pdf) for that parcel. The credit calculator then considers information specific to the parcel (entered into the calculator by SPU), such as the percentage of the parcel’s impervious surface managed by the approved facility and the parcel's drainage rate category assignment.

The output of the credit calculator is a credit which may not exceed 50 percent, per Section 21.33.040. The credit is then applied as a discount off the parcel’s annual drainage bill.

Credit calculators

Learn how to apply for a credit >


Please contact the program manager at or (206) 615-0550.