Every residential sales contract in Colorado now carries a bold-faced radon warning, a legal fixture since Senate Bill 23-206 took effect in August 2023. Most buyers in Westminster read that language and assume the radon question has already been asked and answered by the time they see the seller's disclosure form. It hasn't. The law requires sellers to report what they already know. It never requires them to find out.
That gap matters more in Westminster than in most Denver suburbs, because Westminster is one of the few cities in the metro that sits across two counties with two different radon track records, and because its housing stock leans heavily on the exact floor plan, finished basements, where radon concentrates.
What the disclosure form actually asks a seller to say
The Colorado Real Estate Commission's Seller's Property Disclosure asks whether a radon test has been conducted, what the most recent results showed, and whether a mitigation system has ever been installed. If none of that has happened, the honest answer on every line is "unknown" or "none," and that is a fully compliant disclosure. Nothing about the law obligates a seller to order a test before listing.
The Division of Real Estate's own broker guidance on SB23-206 lays out what has to appear in writing: a radon warning statement, any records of past testing, a description of results and remediation, and a copy of the state health department's radon brochure. Disclosure of history. Not a mandate to create one.
If a seller in Legacy Ridge, The Ranch, Bradburn, or Cotton Creek has owned the home for a decade and never tested, the paperwork will say so honestly, and a buyer who treats that paperwork as reassurance is mistaking silence for a clean bill of health.
Two counties, one city, two different baselines
Westminster is split between Jefferson County and Adams County, and both fall under the EPA's Zone 1 designation, the agency's highest radon-potential category. But the counties don't test identically. Looking at pre-mitigation radon tests submitted to the Colorado Department of Public Health and Environment between 2005 and 2025, Jefferson County shows 50.3% of 30,229 tests at or above the EPA's 4 pCi/L action level, with a median reading of 4.0 pCi/L. Adams County shows 46.5% of 12,383 tests at or above that same threshold, with a median of 3.5 pCi/L.
| County | Tests above 4 pCi/L | Sample size | Median reading |
|---|---|---|---|
| Jefferson | 50.3% | 30,229 | 4.0 pCi/L |
| Adams | 46.5% | 12,383 | 3.5 pCi/L |
| Statewide | 46.4% | 237,408 | — |
Neither number is good news, and neither is a reason to skip testing on the other side of the city line. Radon behaves like a property of the individual lot, not the jurisdiction. Boulder County's own radon guidance for real estate transactions makes this point directly: soil conditions vary from one building to another even within the same neighborhood, so the decision to test should never be based on a neighbor's result or a zone map. What the county split does tell a buyer or seller is that Westminster's own baseline is not uniform, and a Jefferson-side home starts the conversation with slightly worse odds on paper than an Adams-side one, before a single test gets ordered.
The 10 to 14 day window that decides everything
Colorado's standard Contract to Buy and Sell gives buyers a set inspection period, typically 10 to 14 days, to complete due diligence and raise objections. A radon test has to fit inside that window if the result is going to carry any negotiating weight.
The accepted method for a real estate transaction is a continuous radon monitor placed in the lowest livable level of the home for a minimum of 48 hours, with many inspectors recommending 96 hours for a more stable reading. That means the test needs to start within the first day or two of the inspection period, not the last few days of it. A buyer who waits until day nine to order a radon test may get a result back after the objection deadline has already passed, at which point there is nothing left to negotiate.
Sellers who test ahead of listing avoid this crunch entirely. A documented pre-listing result, high or low, removes radon as a late surprise and lets the seller control the timeline instead of reacting to it during a buyer's inspection window.
What a high reading is actually worth in Westminster
Colorado law does not obligate a seller to pay for mitigation. What actually happens is a negotiation, usually one of three shapes: the seller installs a system before closing, the seller credits the buyer's closing costs for the estimated expense, or the two sides agree to a price adjustment that reflects it. None of those outcomes is automatic, which is why knowing the real cost range matters before either side names a number.
A standard sub-slab depressurization system, the go-to fix for a basement foundation, typically runs $1,200 to $2,500 installed, with $1,500 a common all-in price for a Front Range home with a finished basement. Crawlspace installs cost more, usually $2,000 to $5,000, because they require a sealed vapor membrane and sub-membrane suction rather than a single point in a slab. Split-level and multi-zone homes sometimes need more than one suction point to depressurize the whole footprint, which pushes the estimate higher still.
Foundation type is the detail that decides which end of that range applies, and it tracks closely with which part of Westminster a home sits in. The finished-basement stock common to Legacy Ridge and The Ranch generally takes a straightforward single-point install. Older homes near historic Westminster, some built over crawl spaces, need the more involved membrane approach. A buyer asking for a flat mitigation credit without knowing which foundation type they're dealing with risks asking for either too little or more than the job actually costs.
New construction doesn't get a pass
Some of Westminster's newer stock, including the transit-oriented development around Westminster Station, comes with passive radon piping already roughed in during construction. That is a head start, not a guarantee. A passive system moves soil gas through a pipe using natural pressure differences alone, without a fan, and it does not always bring levels below the 4 pCi/L action threshold on its own. The only way to know if a passive system is doing its job is to test the finished home, the same as any resale.
A few direct answers
Does a clean disclosure form mean the home has no radon problem? No. It means the seller has no known test result to report. If no test was ever run, "none known" is the honest and legal answer, and it tells a buyer nothing about the home's actual level.
Is one side of Westminster safer than the other? Both Jefferson and Adams County sit in EPA Zone 1, and both show close to half of all tests coming back elevated. The county-level split shows a modestly higher rate on the Jefferson side, but it is not a substitute for testing the specific house, since soil conditions vary lot to lot.
Who has to pay if a test comes back high? Nobody is legally required to. In practice, the cost becomes part of the inspection-period negotiation, typically resolved through a seller-installed system, a closing credit, or a price adjustment, with the final number usually landing somewhere in the $1,200 to $5,000 range depending on the home's foundation type.
Does this apply to rentals too? Colorado's disclosure requirement extends to leases as well as sales. Landlords must provide the same warning statement and known radon history to tenants before a lease begins.
A radon test is a small, fast, well-understood part of a Colorado home purchase or sale. The mistake isn't the test itself. It's assuming the paperwork already did that job. If you're buying or selling in Westminster and want a straight read on how radon, foundation type, and your specific timeline fit together before you're staring down an inspection deadline, Audrey Michel can walk through it with you. Call or email to talk through your next move.
