Radon-Resistant New Construction in Spokane and North Idaho

Unincorporated Spokane County: Building & Planning, 1026 W Broadway Ave, 1st Floor, (509) 477-3675. Inside a city, that city’s building office handles the radon parts.

What Washington requires in Spokane County

Section R332 of the Washington residential code applies Appendix F to buildings constructed in the high radon potential counties of Table AF101(1): Clark, Ferry, Okanogan, Pend Oreille, Skamania, Spokane and Stevens. The state text says the techniques are meant “to resist radon entry and prepare the building for post-construction radon mitigation, if necessary.”

Spokane County spells out the details in its handout BP-12: at least 4 inches of clean aggregate under each slab, a 10-mil polyethylene retarder with 12-inch laps, and a gas-tight pipe at least 3 inches across. The pipe ends at least 12 inches above the roof and 10 feet from any opening, with a 110-volt junction box near its top.^{1, 2, 3, 6}

The test-and-activate rule since March 2024

Washington’s 2021 residential code took effect on March 15, 2024. Its radon appendix adds section AF104: where radon-resistant construction is required, the finished house is tested before the final report goes to the code official.

The test runs at least 48 hours with the HVAC operating, never in a closet, hallway, bathroom or kitchen. The builder, a registered design professional or an approved third party may run it with two averaged test kits or a continuous radon monitor.^{4, 5}

Idaho: no state rule, so check the city

Idaho’s state code adopts Parts I, II, III and IX of the 2018 IRC and names none of its appendices, so the radon appendix does not apply. Idaho’s health department says these methods “are not required by law in Idaho unless a city or county has specifically adopted them into their local building codes.”

Coeur d’Alene’s city code adopts the 2018 IRC with Appendices J, K and Q only, and Hayden’s with J and K. Kootenai County’s building code ordinance, in force since January 1, 2025, adds the patio cover and tiny house appendices, not the radon one. Post Falls lists the 2018 IRC without naming an appendix; for Rathdrum we found no published list.

The answer matters for new housing. In Post Falls and Rathdrum the median home was built in 2005.^{14, 15, 16, 17, 18, 19, 20, 22, 24}