Our goal: The King County Wastewater Treatment Division (WTD) is considering updating the basis of its capacity charge to better reflect the types of housing our region is building today. WTD levies a 15-year capacity charge for all new connections to the sewer system. This helps pay for the wastewater system—pipes, treatment plants and other facilities—that serves a growing region. Our mission is to protect public health and enhance the environment by collecting and treating wastewater while recycling valuable resources for the Puget Sound region.

Since the capacity charge program began in 1990, King County has experienced significant growth and change (the current rate design was approved in 2001). We are seeing a wider range of residential building types: multi-unit, microhousing, and accessory dwelling units like basement apartments and backyard cottages. It’s time to re-evaluate the approach for allocating the capacity charge among today’s building types. We want to develop a basis for the capacity charge that reflects the relative amount of wastewater each type of unit sends to the sewer and is administratively workable. Visit our frequently asked questions for more information on the capacity charge.

Current status: The capacity charge is based on building type (for residential) and the number of plumbing fixtures (for commercial). (We are not proposing any changes for commercial buildings.)  

For residential buildings, a single detached home is considered one residential customer. The capacity charges for other types of residences—for example multi-unit, accessory dwelling units, or microhousing—are based on a percentage of one residential customer equivalent (RCE). Most of the percentages of an RCE assigned to a different rate classification were established in 2001. The King County Council established an interim rate classification for accessory dwelling units in June 2019.

Proposal: The proposal would use data from the U.S. Census Bureau and other housing surveys on the average persons per household (PPH) to set the percentage of one RCE for each type of residence. Using PPH would reflect the relative amount of wastewater each type of unit sends to the sewer. Using survey data means the capacity charge basis can be updated regularly to reflect new housing trends as new data become available.

Two options: We are considering two options that primarily differ in how single detached dwelling units would be charged. We could continue to have one category for all single detached  homes or create three categories for small, medium, and large single detached homes. Small homes would pay less for the capacity charge and large homes would pay more. The data show that three categories would better reflect the average PPH and, therefore, the amount of wastewater a home is likely to send to the sewer. However, it adds some administrative complexity.

The following table presents estimates of how much the capacity charge (paid monthly) for residential buildings would cost by each option, including the current rate structure:   

The table below presents the percentages of one RCE that would be used to determine the capacity charge for each type of building. The current RCE percentages closely resemble the percentages that are based on survey data. So, in fact, there would be little change. However, using survey data means the capacity charge basis can be updated regularly to reflect new housing trends as new data become available.

Thank you for your feedback. If you'd like to join the capacity charge studies mailing list, contact Eunice Lee at elee@kingcounty.gov. For more information, visit King County's capacity charge studies page.