King County warns sewer rates likely to rise as wastewater capital forecast jumps; council seeks oversight and options

The Budget and Fiscal Management Committee received an extended staff briefing May 28 on the proposed 2026 sewer wholesale rate and a new 10‑year capital forecast for King County’s Regional Wastewater Treatment Division (WTD). Council staff and WTD reported that a sharp rise in projected capital spending, driven largely by regulatory projects tied to a modified combined sewer overflow (CSO) consent decree, is the principal driver of higher sewer‑rate forecasts.

Key figures and schedule: Jenny Giambattista, council staff, said the 10‑year capital forecast is about $11,400,000,000 — roughly $3,100,000,000 more than the prior 10‑year forecast. WTD staff reported that regulatory projects (largely CSO work) now represent a larger share of the portfolio; WTD’s estimate for CSO projects is approximately $5,300,000,000. The “Mouth of the Duwamish” program — a set of projects to control overflows from five CSO outfalls into the Duwamish River, including a wet‑weather treatment facility, storage tanks and new conveyance — contributed most of the change, with staff saying updated cost estimates added roughly $2,000,000,000 to the forecast.

Forecast uncertainty and cost accuracy: Staff cautioned that early cost estimates for large projects have wide accuracy ranges; WTD told the committee the cost estimate for the Mouth of the Duwamish project at this stage has an estimated accuracy of about minus 50% to…