If a home you're considering runs on a septic system or a private well instead of city sewer and water, King County generally won't let title transfer until the septic system has been inspected by a certified on-site system maintainer — and your lender may separately require the well's water tested. Here's exactly what's required, what's just smart due diligence, and how both fit into a Washington purchase contract.
Why this comes up more on the Eastside than you'd expect
Most of the dense, incorporated Eastside — Bellevue, Kirkland, and most of Redmond — sits on city sewer and water. But move toward the edges and it changes fast: larger-lot pockets of Woodinville, parts of Sammamish and Issaquah, and much of unincorporated King County toward the Snoqualmie Valley still rely on an on-site sewage system (a septic tank and drainfield) and a private well rather than municipal utilities. It's not rare, it's not a red flag by itself, and it often comes with the acreage and privacy buyers are looking for in the first place — but it does add a layer of due diligence a city-sewer, city-water home doesn't need.
King County's septic property-transfer inspection rule
Under the King County Board of Health Code, a property served by an on-site sewage system generally cannot transfer title until the system has had a monitoring and performance inspection completed by a licensed on-site system maintainer (OSM). In practice:
- The seller arranges it. The seller schedules the inspection with a certified OSM before closing.
- The report goes straight to the county. After the inspection, the OSM submits a report to Public Health – Seattle & King County, which reviews it within about 10 business days. You (or your agent) can look up a completed report through the county's online records system by searching the property's application history.
- Some systems can skip it. The inspection can be waived in specific situations — for example, a maintenance or performance report already filed within the last six months, new construction that's never been occupied, or a system that's been abandoned because the home now connects to public sewer.
- A separate notice gets recorded. Before closing, the seller must also record a Notice of On-site Sewage System Operation and Maintenance Requirements (OSSM) at the King County Recorder's Office — unless one is already recorded against the title from an earlier sale — and give the buyer a copy.
- The buyer owes a fee. At the time of transfer, the buyer is responsible for submitting the county's Operation and Maintenance Program fee to Public Health; check the Public Health Permit Center for the current amount, since county fee schedules change.
King County has said it's in the process of updating these local rules to align with a new statewide standard (below), so always confirm the current requirement with Public Health – Seattle & King County's sales-and-transfers page rather than relying on last year's process.
A statewide rule is coming — but it isn't in effect yet
Washington's Department of Health has adopted a rule change under WAC 246-272A-0270 that creates a statewide property-transfer inspection requirement for on-site sewage systems, tied to the deed transfer. That requirement takes effect February 1, 2027. Until then, whether an inspection is required — and exactly how it's handled — still depends on your specific county's local health code; King County already requires one, as described above, while other counties are still drafting their own local ordinances to implement the new state standard. If you're buying outside King County, ask your agent or the local health district what currently applies.
How this fits into your purchase contract
A septic-served property doesn't use a different inspection contingency, but it does add addenda to the standard NWMLS Form 35 inspection contingency used in most Washington offers. Expect a Septic Addendum covering the county inspection and report, and — if there's a well — a separate Well Addendum giving you the right to review the seller's well documents and attend the water test. Your disapproval window to respond to what the inspection or test turns up typically runs a set number of days (commonly five, if the contract doesn't specify otherwise) from the day you receive the report, mirroring the general inspection contingency you'd use on any home. One quirk worth knowing: in King County, the time-of-sale septic inspection is a county requirement — it still has to happen even if you've otherwise waived your inspection contingency under Form 35. Exact form numbers and default timelines are revised periodically, so confirm the current versions with your agent before you rely on them.
Private wells: no inspection mandate, but don't skip testing
Unlike septic, there's currently no state or King County inspection requirement tied to the sale of a home with an existing private well. That doesn't mean it's optional in practice — many lenders require a water-quality test or a full well inspection (flow rate, pressure, physical condition of the well and pump) before they'll fund the loan, and it's worth doing even if yours doesn't. The Washington State Department of Health's baseline recommendation for any well is to test, at minimum, for:
- Coliform bacteria — the top health priority; the standard is zero present, and DOH recommends testing yearly.
- Nitrate — the other annual-testing priority; the state's maximum is 10 mg/L, and a result of 5 mg/L or higher is worth re-checking within six months.
- Arsenic — the state's maximum is 0.010 mg/L; because levels can shift seasonally, DOH suggests testing twice across a year (summer and winter) once you own the well.
Order testing from a certified lab, and if the home has a water treatment or filtration system, test both before and after it to confirm the system is actually doing its job. Budget for it as part of your overall closing costs and due-diligence spending — it's inexpensive insurance relative to what a contaminated or low-producing well could cost you after closing.
Your due-diligence checklist for a septic-and-well home
- Confirm the county's septic requirement applies and ask who's arranging the OSM inspection and when the report is due.
- Request the system's records — as-built drawing, tank size, drainfield location, and maintenance history — from the seller or the county's online records.
- Order a well water test covering at minimum coliform, nitrate, and arsenic, and ask about a flow/yield test if your lender or contract calls for one.
- Check the OSSM notice recorded (or to be recorded) against the title, and read it before your disapproval period closes.
- Add a sewer-scope mindset to septic, too — just as an older home's side sewer can hide expensive problems, an aging or undersized drainfield can be costly to replace. Ask the OSM directly about the system's remaining useful life.
- Budget for ongoing costs. Septic systems typically need pumping every few years and periodic maintenance; wells need recurring water testing. Factor this into your long-term cost of ownership, not just your closing costs.
Where a flat fee agent helps
A septic-and-well purchase has more moving pieces than a standard city-utility sale — extra addenda, a county inspection and filing, a recorded notice, and a water test to track down alongside your usual timelines. A traditional buyer's agent charging roughly 2.5% commission on a $1.1M Eastside property costs about $27,500. A flat $2,999 buyer's agent handles the same coordination — making sure the septic inspection, OSSM notice, and well test all land inside your contingency windows — while leaving tens of thousands of dollars for the down payment or whatever the inspection turns up. See flat fee vs. a traditional buyer's agent and who pays the buyer's agent in Washington. New to the process end to end? Start with the first-time buyer guide for King County & the Eastside.
General information, not legal, engineering, or environmental health advice. Septic and well requirements, fees, NWMLS form numbers, and default timelines vary by county and change over time — confirm the current rules with Public Health – Seattle & King County (or your local health jurisdiction), the Washington State Department of Health, and your agent before relying on any of the details above. Shi Hao Liu is a licensed WA real estate broker (License #26003789) with Kelly Right Real Estate, not a septic or water-quality professional. Verify any WA broker at the WA Department of Licensing.