ADUs Often Pay Full-Size Utility Fees

Utility hook-up fees may be creating an added barrier to accessory dwelling unit development in Washington.

Published in May 2026, the Washington Center for Housing Studies report Tapped Out: How Water Utility Hook-Up Fees Hit ADUs compares water and sewer connection charges for ADUs and standard single-family homes across 13 sample municipalities and utility providers.

Most jurisdictions in the study did not maintain separate ADU rates and instead charged ADUs the same fees as full-size single-family homes. Only three jurisdictions had an ADU-specific rate for at least one of the two utilities examined. Where separate rates existed, ADUs were charged between 50% and 86% of the corresponding single-family rate.

The study concludes that many fee structures may not reflect the smaller size and proportional utility impact of ADUs. It recommends that jurisdictions reassess their methods and establish connection fees based on an ADU’s relative impact on the utility system.

Read the full report for the methodology, jurisdiction comparisons and policy recommendations.