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Rate structure reviews

King County periodically reviews its capacity charge rate structure to make sure the capacity charge program accurately reflects current demographics and housing trends.

A rate structure study implemented in 2017 served as the basis for updates to the capacity charge rate design that were recently approved by the King County Council. Effective January 1, 2021, the County will now use average persons per household as the new basis of the capacity charge for residential structures.

The purpose of updating the rate structure is to respond to changing housing trends by making the capacity charge better reflect the amount of wastewater a home is likely to send to the sewer. Using average persons per household allows King County to update the basis for the capacity charge as demographics change.

Since the capacity charge program began in 1990, King County has experienced significant growth and change. Emerging trends include micro-housing, small efficiency dwelling units, accessory dwelling units, detached accessory dwelling units, group housing, adult care homes, and the installation of low-flow plumbing fixtures.

The rate design study was designed to evaluate the best approach for allocating the capacity charge in light of the wastewater demand for today’s building types.

What does the update mean?

This change to the capacity charge residential rate structure will tie the amount that people pay to the average number of people per household by housing type, which is based on data from the U.S. Census Bureau.

For single-family homes, the updates mean that small homes will pay less for the capacity charge and large homes will pay more beginning in 2021. The Residential Customer Equivalent (RCE) assigned to single family homes would be based on size:

  • Small (less than 1,500 sq. ft.) = 0.81 RCE
  • Medium (1,500 to 2,999 sq. ft.)  = 1.0 RCE
  • Large (3,000 sq. ft. or greater) = 1.16 RCE


Using persons-per-household data will also result in small changes to RCEs assigned to multi-family units. These changes will also confirm a previous interim change for accessory dwelling units, like backyard cottages and basement apartments, and set a 0.59 RCE for those units. The approved changes are designed to be revenue neutral so that it does not increase the overall amount that King County collects through the capacity charge.

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