King County, Washington septic permits and requirements

Permitting authority

Public Health — Seattle & King County, On-site Sewage/Septic System Program

Environmental Health Services: 206-477-8050. King County also provides a septic technical-question line at 206-477-8177.

Washington local-health-jurisdiction framework

Washington State Department of Health places property-level onsite sewage permitting and management with local health jurisdictions. Local agencies review and approve OSS siting, design, installation, repair, and operation under Chapter 246-272A WAC and may adopt local requirements that are more protective than the statewide minimums.

New-system design and installation permit

King County requires both an approved site application/design and an installation permit for construction of a new onsite sewage system. The county states that construction must not begin until the installation permit is issued and posted at the site. A septic designer submits the site application and a county-certified installer performs the installation.

Repairs and replacements require county review

King County states that a repair proposal and permit are required when changes or system replacements are made. The repair workflow generally uses a certified master installer, while larger replacement work can require a certified designer or professional engineer. County Public Health inspects regulated repair and replacement work and requires the applicable as-built documentation.

Building and land-use projects depend on septic approval

For projects involving septic-served property, including new installations, repairs or replacements, remodels, expansions, subdivisions, boundary-line adjustments, and rezones, King County Permitting can require the approved septic permit and site plan. The county states that an application lacking the required Public Health approval is not complete.

Current local code

King County Board of Health revised its onsite sewage system code effective April 1, 2025. Property owners and professionals should use the current Title 13 requirements rather than relying on older county OSS rules.

Official sources

SepticScope summarizes public-agency information for planning purposes. The permitting agency's current instructions control if they differ from this guide.