King County, Washington: septic system inspection at sale
Checked on November 9, 2026 against the official rule. Rule set by Seattle & King County Public Health.
General information, not legal advice. This is our summary of the official rule as we read it. The ordinance or code and the health department decide: confirm with them before you list, sign or close.
The rule
Washington also has a statewide rule: see the Washington page. Where both apply, follow both.
| Question | What the official text says |
|---|---|
| What is required | Inspection by certified on-site system maintainer before title transfer; maintainer reports to Public Health; OSSM notice recorded; O&M program fee Source: cdn.kingcounty.gov |
| Triggered by | sale / title transfer Source: cdn.kingcounty.gov |
| Deadline | before closing; Public Health reviews within 10 business days Source: cdn.kingcounty.gov |
| Who arranges it | seller schedules inspection; buyer pays O&M program fee Source: cdn.kingcounty.gov |
| Who pays | seller Source: cdn.kingcounty.gov |
| Exemptions |
Source: cdn.kingcounty.gov |
| Fee | Buyer pays O&M program fee at transfer (amount not on page) Source: cdn.kingcounty.gov |
| Who may inspect | King County certified maintainers. official page (kingcounty.gov) Source: cdn.kingcounty.gov |
| Official forms | cdn.kingcounty.gov Source: cdn.kingcounty.gov |
This is our summary of the official rule as we read it. The ordinance or code and the health department decide: confirm with them before you list, sign or close. Rule set by: Seattle & King County Public Health.
Checked on November 9, 2026: read on the official site.
Private wells in King County
In 2015 about 128,846 of King County’s 2,117,125 residents (6%) got their drinking water from a private well, by the USGS estimate. That is the 39th highest share of the 39 Washington counties in the USGS table, and the highest number of private-well users.
As we read it, this rule covers the septic system, not the well.
Source: USGS county water use 2015 (Dieter et al., 2018). Ranks count the Washington counties in the USGS table (1 = highest).
Certified labs nearest to King County for a well water test
Labs on the Washington certified drinking-water list that the list marks as taking samples from homeowners or outside clients, certified for coliform bacteria, nearest first. 5 of them are in King County.
| Lab | Town | Distance | Certified on the list for |
|---|---|---|---|
| AmTest Laboratories | Renton, WA · in this county | 6 miles | coliform bacteria, nitrate, arsenic |
| Seattle-King Co Dept of Public Health Lab | Seattle, WA · in this county | 7 miles | coliform bacteria |
| MCL Food Testing | Seattle, WA · in this county | 9 miles | coliform bacteria |
| Alliance Technical Group, LLC-Seattle | Seattle, WA · in this county | 10 miles | coliform bacteria |
| Eurofins EPK Built Environment Testing, LLC | Seattle, WA · in this county | 12 miles | coliform bacteria |
Source: WA Ecology - Environmental Laboratories Accredited for Drinking Water Analyses (with scopes) (Information current as of 11/13/2026; read November 15, 2026). Straight-line distance from the centre of King County (the average of its Census ZIP-area points); “approx.” = lab placed at its ZIP centre. Call the lab first: lists do not say which household tests each lab takes or what it charges.
Nearest other sale-time rules
Buying or selling near a county line? These are the closest other local rules in our census to King County.
| Rule | State | Distance | Covers | Checked |
|---|---|---|---|---|
| Pierce County | Washington | about 29 miles | Septic system | November 9, 2026 |
| Island County | Washington | about 44 miles | Septic system | November 9, 2026 |
| Thurston County | Washington | about 52 miles | Septic system | November 9, 2026 |
| Skagit County | Washington | about 63 miles | Septic system | November 9, 2026 |
| Clallam County | Washington | about 96 miles | Septic system | November 9, 2026 |
Straight-line distance between county centres (the average of each county’s Census ZIP-area points). Townships, villages and health districts are placed at the centre of their county.
All rules: by state · Washington.