A few months ago, a Longmont seller was days from closing when a radon test came back high. The buyer's agent called in a mitigation contractor, and the crew found an unfinished basement was the obvious spot for a sub-slab depressurization system, except the home's enclosed back porch blocked the standard exhaust route along the rear wall. They rerouted the PVC piping to a cove at the front of the house instead, got the fan installed, and kept the deal moving. It's the kind of fix a local mitigation company like North Point Radon handles routinely when a Boulder County agent calls with a closing date already on the calendar.
That story used to be a footnote. In Longmont's market right now, it's closer to a preview of what a lot of sellers are about to experience for the first time.
The law already changed. The market just caught up to it.
Colorado's radon disclosure law isn't new. Governor Jared Polis signed SB23-206 on June 5, 2023, and it took effect that August. Every residential sales contract in the state now has to carry a bold-faced warning about radon risk, along with whatever the seller actually knows: past test results, mitigation work performed, and whether a system is currently installed. Sellers also have to hand buyers a current brochure from the Colorado Department of Public Health and Environment. None of this requires a seller to test before listing. It only requires honesty about what they already know, and the Colorado Division of Real Estate has been clear that skipping the disclosure exposes a seller to a buyer's claim for damages.
For the first few years after it took effect, that law mostly lived in the fine print. Sellers had backup offers lined up, buyers waived contingencies to compete, and a radon number rarely became a real negotiation. That's the part of the picture that's shifted.
Why the geology makes this a Longmont problem, not a generic one
Longmont sits in the Environmental Protection Agency's Zone 1, the highest risk tier, where average indoor radon levels are predicted to exceed the 4.0 picocurie-per-liter action threshold before you even test. SWAT Environmental, a national mitigation company that works Boulder County, puts the local numbers at roughly half of tested homes in the county coming back above that action level. That's not a rounding-error risk. It's closer to a coin flip on any given house, older or new, because the source is the uranium-rich soil under the Front Range, not the age of the building.
The city takes it seriously enough to run its own outreach. Longmont's Mayor Susie Hidalgo-Fahring proclaimed January 2026 as National Radon Action Month in Longmont, and the city still offers a free mail-in test kit through its Longmont Sustainable Home program, plus radon detectors residents can borrow from the library. If City Hall is running a public health campaign around this, it's a reasonable bet that most buyers touring a home this year already know to ask about it.
What's actually different in 2026
Here's the part that changes the math for sellers. Regional MLS data covering March through June 2026 shows Longmont single-family homes typically taking around 40 days to find a buyer, and by the end of June, inventory had climbed to about 300 active listings, a level the local MLS hadn't seen in 11 years. Over that same April-through-June window, the median sale price for Longmont single-family homes landed roughly 1% below the original list price. For comparison, buyers in the city of Boulder and unincorporated Boulder County were securing closer to 2.3% off list over the same stretch, a gap that traces back to Longmont's relatively lower price point drawing a deeper pool of buyers even as the broader county cools faster.
None of that describes a market in free fall. Pending sales were still up modestly heading into July, both from the prior month and from a year earlier, so buyers haven't disappeared. But the math has shifted from where it sat during the tightest years. A seller with three competing offers in 2021 could absorb a radon finding by pointing to the next buyer in line. A seller today, with a single offer on the table and a home that's already sat on the market for close to six weeks, has a lot less room to say no when that same buyer comes back asking for a mitigation credit.
That's the actual shift. The radon disclosure law gave buyers a piece of paper. The current market is what's giving that piece of paper leverage.
What a positive test sets in motion
Once a test comes back above 4.0 pCi/L, the path is fairly predictable, even if the specifics vary by contract:
- The buyer's agent typically raises it as part of the inspection objection process, alongside any other findings.
- The seller and buyer negotiate who pays for mitigation, whether that's a credit at closing or the seller hiring a contractor directly.
- A licensed radon professional installs a system, most commonly a sub-slab depressurization setup that vents soil gas above the roofline.
- A follow-up test confirms the fix worked before or shortly after closing.
Local pricing for a mitigation system generally runs from several hundred dollars up into the low thousands, according to Longmont Radon Mitigation, depending on the home's layout and whether the crew is activating an existing passive system or building one from scratch. New construction in Colorado often ships with a passive system already roughed in, which can make the fix faster and cheaper than in an older home starting from zero.
The part that catches sellers off guard isn't the science. It's the timing. A positive test showing up during an active under-contract inspection period, with a closing date already set, is a very different negotiation than testing on your own schedule before you ever list.
What this means if you're selling
Testing before you list, even though the law doesn't require it, puts the timeline back in your hands. If the result comes back clean, you've got a positive fact to disclose and one less unknown for a buyer to negotiate around. If it comes back high, you can get a mitigation system installed on your own schedule instead of a buyer's, and you walk into showings with the fix already documented rather than promised.
If you already have a mitigation system from a previous test, that's a good starting point, but Longmont Radon Mitigation notes that systems still warrant periodic rechecking, especially after any renovation work that could have disturbed the seal or the piping. A system installed five years ago isn't automatically proof of a safe reading today.
What this means if you're buying
Ask for whatever radon history the seller has before you're deep into an inspection window. It's part of what SB23-206 already entitles you to. If the seller hasn't tested, budget the time for a short-term test as part of your due diligence rather than an afterthought squeezed in at the end. And know that in the current market, if the number comes back elevated, you have more standing to ask for a credit or a completed mitigation system than a buyer would have had two or three years ago. That's not a threat to make lightly. It's simply where the leverage sits right now.
A few common questions
Does Colorado actually require me to test before I sell? No. SB23-206 requires disclosure of whatever radon information you already have, along with a standard warning statement in the contract. It doesn't mandate testing itself.
What happens if I don't disclose radon information I already have? The law gives the buyer a claim for damages against a seller who fails to disclose known radon test results or mitigation history. It's a real legal exposure, not just a courtesy.
My home has a mitigation system already. Am I in the clear? Mostly, but not automatically. A working system installed years ago is a good sign, but retesting after any renovation, and periodically in general, is the only way to confirm it's still doing its job.
Radon isn't a reason to panic about a Longmont home, and it's not a reason to walk away from one either. It's a specific, fixable line item that behaves very differently depending on when in the process you deal with it and what kind of market you're negotiating in. Right now, that market favors the buyer who asks the question early over the seller who hopes it doesn't come up.
If you're weighing when to list, what to test for before you do, or how to handle a result that already came back high, Milestone Real Estate has walked Longmont sellers and buyers through exactly this. Let's talk about your property.
