Why Timing Decides Your Fort Collins Radon Test Result

Every residential purchase contract in Colorado now carries a paragraph in bold type, the kind of legal language buyers usually skim past on the way to the signature line. It warns that radon is a Class A carcinogen and the leading cause of lung cancer in nonsmokers. Since Colorado's SB23-206 took effect in August 2023, state law has required it on every sale.

Most people read that paragraph, feel a flicker of concern, order the test, and move on. Few ask the question that actually determines what the test will show: what time of year is it? In Larimer County, that question matters more than almost anything else about the house itself.

The Paragraph Every Fort Collins Contract Now Contains

Colorado's SB23-206 changed the mechanics of every residential sale in the state. Sellers are now required to disclose any known radon test results, any history of testing or mitigation, and whether a mitigation system currently exists on the property. If none of that history exists, the contract still has to carry the state's standard warning language and a copy of the health department's radon brochure.

For a Fort Collins buyer, this means the radon conversation isn't optional anymore and it isn't something an agent has to remember to bring up. It's baked into the paperwork before either side sits down to negotiate anything else. The law didn't create a new health risk. It just made explicit what careful buyers in this market were already doing on their own.

The Geology Underneath Every Basement

Fort Collins sits where the Great Plains meet the Front Range foothills, on ground shaped by uranium-rich granite. As that uranium decays underground, it releases radon gas, which moves up through soil and finds its way into homes through foundation cracks, sump pits, and crawl spaces. Larimer County's health department puts the county's average indoor radon level at 7.8 picocuries per liter, nearly six times the national average of 1.3 pCi/L, and well above the EPA's action threshold of 4.0 pCi/L.

That average isn't evenly distributed. County guidance is direct on this point: your neighbor's test result tells you nothing about your own house, because soil composition and foundation design vary even on the same block. The takeaway for a Fort Collins buyer isn't that one specific address is dangerous. It's that an elevated reading here is closer to the expected outcome than the exception, which changes how you should treat the number when it comes back.

The Mechanism Most Buyers Never Learn About

Here's the part that rarely comes up in a walkthrough or an open house conversation. Radon levels in a Colorado home aren't static. They rise and fall with the seasons, and the swing is large enough to change a negotiation.

During winter, homes get sealed tight against the cold. Furnaces run constantly, warm air rises and escapes through the upper floors, and that rising air creates negative pressure at the foundation that pulls soil gas up into the basement. Larimer County's own extension office recommends retesting in colder months for exactly this reason, specifically flagging that a test taken during warmer months, when houses are open and ventilated, may not reflect what the same house will read once winter sets in.

A buyer who closes on a July purchase with a clean radon test isn't necessarily buying a low-radon house. They may be buying a house that simply wasn't tested under the conditions most likely to reveal a problem. That gap is the actual friction point in this market, more than the disclosure law itself.

What Happens After the Test Comes Back Elevated

An elevated result doesn't usually blow up a deal in Fort Collins, because it's common enough that most buyers and sellers have a script for it. The standard fix is a sub-slab depressurization system, which draws radon from beneath the foundation and vents it outside through a pipe and fan. Larimer County's health department estimates most installations run between $1,000 and $3,000, and even homes built with passive radon-resistant features can still test above the action level, so a newer build isn't automatically exempt from the conversation.

In practice, this becomes a negotiating line item rather than a dealbreaker. Boulder County's sample contract language ties results to the 4.0 pCi/L threshold, and if the short-term test, which typically runs two to seven days, comes back above that number, the parties negotiate who pays for mitigation rather than walking away. Sellers who test proactively and disclose a clean result, or one that's already been mitigated, tend to move through this part of the process with less back-and-forth than sellers who are disclosing for the first time mid-contract.

A Few Questions Worth Asking Before You Sign

1. Has this specific house been tested before, and if so, in what season?
2. If a mitigation system is already installed, when was it last inspected for proper airflow?
3. Is the test being scheduled as part of the general home inspection window, or separately?
4. If the result comes back near the 4.0 pCi/L line, has the contract specified who covers mitigation costs?

None of these questions require a specialist. They just require knowing that the number on the report is a snapshot of one week's conditions, not a permanent fact about the house.

A Few Common Questions

Does a passing summer test mean the house is safe year-round? Not necessarily. Larimer County's own guidance recommends retesting in winter if a home was purchased during warmer months, precisely because sealed, heated conditions tend to produce higher readings than an open, ventilated summer test.

Can a buyer test again after moving in? Yes, and it's a reasonable step if the purchase closed during a season unlikely to reflect the home's typical winter conditions. A short-term test kit or a call to a certified measurement professional can confirm whether the pre-closing result still holds once the heating season arrives.

Radon in Fort Collins isn't a red flag on any single listing. It's a fact of the ground this whole city is built on, and the buyers and sellers who handle it best are the ones who understand that the test date matters almost as much as the test result. If you're weighing a purchase or preparing to list in Fort Collins and want a clearer read on how this shows up in a real contract, Catherine Montgomery is ready to walk through it with you. Start Your Home Search today.

This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.