Radon-Resistant New Construction in Spokane, WA
Spokane County new homes carry a real code requirement most homeowners have never heard of. Here's what Washington's WAC 51-51 Appendix F requires, and how North Idaho's rules differ just across the state line.
Does Washington Require Radon-Resistant Construction
Yes, and this is the single most differentiated fact on this site: Spokane County new homes are legally required to include radon-resistant construction, not just encouraged to. Washington's residential building code, WAC 51-51 Section R332.1, applies Appendix F's radon control provisions to buildings constructed in "high radon potential counties (zone 1)," and Spokane is one of seven counties named. Unvented crawl spaces aren't even permitted in these counties, which tells you how seriously the code treats soil gas entry. This isn't a voluntary best practice a builder can skip to save money; it's a code requirement inspected like any other part of the build.
What's Actually Required
New homes need a minimum 4-inch gas-permeable aggregate layer under slabs (or an equivalent sand-plus-drainage-matting alternative), a soil-gas retarder membrane rated at 10-mil polyethylene minimum, sealed foundation penetrations, and one continuous, sealed, minimum 3-inch-diameter vent pipe terminating at least 12 inches above the roof and more than 10 feet from any window or chimney, visibly labeled "RADON VENT."
That 10-mil membrane figure is thicker than the 6-mil number some generic national code summaries cite; Spokane County's own locally controlling bulletin is the figure that actually applies here. A dedicated 110V junction box near the vent stack is also required at this stage, sized for a future inline fan even though the fan itself isn't installed yet, which is the subject of the next question below.
Is a Fan Required at Construction
No, not yet, and that distinction matters if you're budgeting for a new build. Washington's code requires the passive system components, the membrane, the vent pipe, the sealed penetrations, plus electrical provision for a future inline fan: a dedicated 110V junction box near the vent stack. It does not require the fan itself to be installed and running at construction; the system is designed to be activated later if needed. After the home is built, a post-construction test tells you whether the passive system alone keeps levels below 4 pCi/L. If not, a fan gets added to that junction box to make the system active, without redoing any of the underlying construction.
How North Idaho Differs
Here's the honest regulatory gap: Idaho has no statewide radon-resistant construction mandate. Kootenai County, including Coeur d'Alene and Post Falls, shares the exact same EPA Zone 1 designation as Spokane County and the same underlying geology, but no source shows Kootenai County or either city has locally adopted an equivalent code requirement. That means a new home built today in Zone 1 Kootenai County is not legally required to include the passive radon system Washington mandates just across the state line. See our service area page for coverage details, or our Zone 1 explainer for the full geology and regulatory story.
Confirm your home's radon system
Call to ask about post-construction testing, or send a request.
New Construction Questions, Answered Straight
Does Washington require radon-resistant features in new homes?
Yes, in Zone 1 counties. Washington's residential building code, WAC 51-51 Appendix F, makes radon-resistant construction mandatory for new homes in Spokane and six other high-radon-potential counties, including a sub-slab gas-permeable layer, a sealed soil-gas membrane, and a labeled vent pipe routed to above the roofline.
What does radon-resistant new construction actually include?
Per Spokane County's own building guidance, it includes a minimum 4-inch gas-permeable layer under slabs, a soil-gas retarder membrane, sealed foundation penetrations, and one continuous 3-inch vent pipe running from beneath the slab to at least 12 inches above the roof. A junction box is also required for a future fan, even if one isn't installed yet.
Is a fan required in a new home's radon system?
Not at construction. Washington's code requires the passive system components and electrical provision for a future inline fan, but not the fan itself. If a post-construction test comes back at or above 4 pCi/L, the fan is then installed to activate the system and bring levels down, without redoing the underlying construction.
Does North Idaho require the same radon construction standards as Spokane?
No. Idaho has no statewide radon-resistant construction mandate, and no source shows Kootenai County, Coeur d'Alene, or Post Falls has adopted one locally. That means a new home in equally high-radon Kootenai County is not legally required to include the passive radon system Washington mandates just across the state line.
See why Spokane is EPA Zone 1
Real historical data and the science behind the designation.