Radon Testing When Buying a House in Snohomish or King County (2026)

Short answer: Radon is a natural radioactive gas from soil that you can’t see or smell, and testing is the only way to know the level in a specific house. EPA puts both Snohomish and King counties in its lowest radon zone, but it still recommends testing every home below the third floor, including when you buy. If you want a radon test, build it into your inspection period: a real-estate radon test takes at least 48 hours plus time for results. EPA recommends a fix at 4 pCi/L or more.

This is general information, not medical, legal or engineering advice. For a specific house, talk to a certified radon measurement professional, and check your purchase agreement with your agent.

What the county maps say, and don’t

EPA’s map of radon zones dates to 1993 and sorts counties by predicted average indoor radon. In EPA’s county list, King and Snohomish are both Zone 3, which EPA describes as low potential, with a predicted average below 2 pCi/L. The map’s own notes say elevated levels have been found in all three zones and that all homes should be tested. EPA’s zone page (updated August 2026) says the map shouldn’t be used to decide whether to test a particular home.

The Washington Department of Health keeps a Radon Dashboard through the Washington Tracking Network and says radon can build up inside a building to tens or even hundreds of times the outdoor level. So treat the county zone as background, not an answer for the house you’re buying.

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Data from: Washington DOH radon page; EPA Map of Radon Zones page, county zone list and map notes; EPA Home Buyer’s and Seller’s Guide to Radon (March 2024 revision).

Fitting a radon test into your inspection period

EPA’s Home Buyer’s and Seller’s Guide to Radon recommends three short-term options for real estate:

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Based on: EPA Home Buyer’s and Seller’s Guide to Radon, short-term testing options for real estate transactions.

A few rules from the same guide that affect a purchase:

• Where. Test in the lowest level of the house that could be used regularly, finished or not. If you plan to use the basement, test there, even if the seller doesn’t.
• Closed house. For a test under four days, windows and outside doors should be closed for at least 12 hours before and during the test, except for normal coming and going.
• Placement. At least 20 inches above the floor, away from drafts, high heat, humidity and outside walls.
• Close calls. Short-term results near 4 are less certain. EPA’s example: if two short-term tests average 4.1 pCi/L, there’s about a 50 percent chance the year-round average is a bit above or below 4.

Timing is the catch. DOH has a limited supply of free test kits, but it says to allow 8 to 10 weeks for delivery, which is too slow for most inspection periods. A store-bought kit or a radon measurement professional is the realistic option during a purchase. EPA’s guide suggests hiring a qualified tester, and DOH links to the national certifying organizations, including the National Radon Proficiency Program, to find certified professionals. Ask your inspector whether they offer radon testing, since a standard inspection may not include it; see what a standard home inspection covers.

Put it in the contract

EPA’s guide lists what a buyer and seller should agree on in writing: where the test happens, who does it, what kind of test, when, how results and costs are shared, and when any mitigation happens and who pays. In a Washington sale that usually lives in your inspection contingencies and any addendum your agent writes. Missing a contingency deadline can affect your earnest money, so keep the radon timeline inside it.

Also read the seller’s disclosure. Form 17 asks whether there are environmental concerns on the property and lists radon gas among them. A Yes or a past mitigation system is a reason to ask for test results and system details. RCW 64.06.020 currently has two versions on the Legislature’s site: one marked effective until January 1, 2027, and one effective January 1, 2027. The questions quoted here appear in both. If your sale runs across the new year, check which form is being used.

If the seller already tested

EPA’s guide tells buyers to ask the seller for radon results and, if there’s a mitigation system, any information about it. Before you rely on an old test, ask who did it, where the device sat, and whether anything has changed since. The guide says a new test makes sense if the last one is older than about two years, the house has been renovated, or you plan to use a lower level than the one tested.

If the level is high

EPA says high radon can be reduced, even at very high levels. The common method is sub-slab depressurization: a vent pipe and fan pull soil gas from under the house and send it outside. EPA doesn’t recommend sealing cracks alone. The guide says a mitigation system costs about the same as other common home repairs, and there are ongoing costs to run the fan. Get an estimate from one or more qualified mitigators.

Two more points from the guide. There’s a potential conflict of interest when the same firm tests and installs the system, so ask about it. And test again after any system is installed. If you plan to finish a basement later, EPA suggests testing before and after the project; adding a system during renovation is usually cheaper than after.

Buying new construction

EPA’s page on radon-resistant new construction says builders can use common materials and techniques to make new homes resist radon entry, and that it’s cheaper to do during construction than to fix later. Ask the builder whether those features were included. EPA’s page still says all homes should be tested.

Houses on a well

Radon can also enter a home through well water, according to EPA’s guide. If the house has a private well, ask your well tester about it; our guide to buying a house with a private well covers the rest of the well checks.

FAQ

Is radon a problem in Snohomish and King County?

EPA’s county radon zone list puts both King and Snohomish counties in Zone 3, the lowest of its three zones, where the predicted average indoor level is under 2 pCi/L. But EPA’s map notes say homes with elevated levels have been found in all three zones, and EPA says the map shouldn’t be used to decide whether to test. The only way to know about a specific house is to test it.

What radon level is too high?

EPA recommends fixing a home when radon is 4 pCi/L or higher, and says to consider fixing it between 2 and 4 pCi/L. For comparison, the Washington Department of Health says outdoor air averages about 0.4 pCi/L.

How long does a radon test take during a home purchase?

EPA’s buyer and seller guide says every radon test should run at least 48 hours, and some devices need longer. For real estate, it suggests two short-term tests at the same time, two in a row, or a continuous monitor for at least 48 hours. Add time for the lab or the tester’s report, and make sure it all fits inside your inspection period.

Can I use the seller’s radon test?

You can, but EPA suggests checking who did it, where in the house it was placed, and whether it was done under closed-house conditions. EPA’s guide says a buyer may want a new test if the old one isn’t recent (for example, within two years), if the house has been renovated since, or if you plan to use a lower level than the one tested.

What if the radon test comes back high?

Under EPA’s guide, the usual fix is a mitigation system that pulls soil gas from under the slab and vents it outside with a pipe and fan. EPA says the cost is about the same as other common home repairs. Whether the seller fixes it, gives a credit, or you walk away depends on your purchase agreement and inspection contingency. After any fix, test again.

Talk it through with a local agent

Peter Kim and Odigo Real Estate Club can help you plan inspections, including radon, so they fit your contract deadlines, with 25+ years combined experience, 593+ closings, and 122 five-star Zillow reviews. See our home buying services.

Odigo Real Estate Club · REAL Brokerage, WA #112064 · 16108 Ash Way, Suite 201, Lynnwood, WA 98087 · (425) 409-3823 · Mon–Sat 9 AM–6 PM, closed Sunday

Sources

• U.S. Environmental Protection Agency, Home Buyer’s and Seller’s Guide to Radon (EPA 402/K-24/001) (March 2024 revision: test below the third floor, lowest usable level, 48-hour minimum, real estate test options, closed-house conditions, contract items, seller tests, mitigation, conflict of interest, well water), https://www.epa.gov/sites/default/files/2015-05/documents/hmbuygud.pdf , accessed Nov 2, 2026.
• U.S. Environmental Protection Agency, EPA Map of Radon Zones (map not for deciding whether to test; fix at 4 pCi/L, consider 2 to 4; updated Aug 2026), https://www.epa.gov/radon/epa-map-radon-zones , accessed Nov 2, 2026.
• U.S. Environmental Protection Agency, Radon zones by county (spreadsheet) (King and Snohomish counties, Washington: Zone 3), https://www.epa.gov/system/files/documents/2024-05/radon_zones-spreadsheet.xls , accessed Nov 2, 2026.
• U.S. Environmental Protection Agency, Map of Radon Zones with notes (Zone 3 predicted average under 2 pCi/L; elevated homes found in all zones), https://www.epa.gov/system/files/documents/2024-05/radon-zones-map_text_link.pdf , accessed Nov 2, 2026.
• U.S. Environmental Protection Agency, Find a radon test kit or measurement and mitigation professional (test kits and professionals), https://www.epa.gov/radon/find-radon-test-kit-or-measurement-and-mitigation-professional , accessed Nov 2, 2026.
• U.S. Environmental Protection Agency, Radon-resistant new construction for home buyers (builders can add radon-resistant features; cheaper during construction; all homes should be tested), https://www.epa.gov/radon/radon-resistant-new-construction-home-buyers , accessed Nov 2, 2026.
• Washington State Department of Health, Radon (0.4 pCi/L outdoor level, 2.7 pCi/L used elsewhere, free kits with 8 to 10 week delivery, certified professionals, contact), https://doh.wa.gov/community-and-environment/contaminants/radon , accessed Nov 2, 2026.
• Washington State Department of Health, Radon in Washington (DOH 323-002) (EPA action level 4 pCi/L; test kits), https://doh.wa.gov/sites/default/files/2022-02/323002-Radon-in-WA.pdf , accessed Nov 2, 2026.
• Washington State Department of Health, Washington Tracking Network: Radon (Radon Dashboard; indoor levels up to tens or hundreds of times outdoor air), https://doh.wa.gov/data-and-statistical-reports/washington-tracking-network-wtn/radon , accessed Nov 2, 2026.
• Washington State Legislature, RCW 64.06.020 (seller disclosure statement form; version effective until Jan 1, 2027 and version effective Jan 1, 2027), https://app.leg.wa.gov/RCW/default.aspx?cite=64.06.020 , accessed Nov 2, 2026.

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