Radon Disclosure Requirements When Selling a Home in Colorado

Colorado Does Require Radon Disclosure

Colorado law requires sellers to disclose known radon information when selling a home. The legal mechanism is the Colorado Real Estate Commission’s Seller’s Property Disclosure (SPD) form, which is mandated under Colorado Revised Statutes governing real estate transactions. The current SPD form (version CBS2-5-21 and its successors) specifically asks sellers whether they have knowledge of radon testing, the results of any tests, and whether a mitigation system has been installed.

This is not an optional or suggested disclosure. Sellers who have knowledge of elevated radon and fail to disclose it can face legal liability after closing. Colorado courts have found that concealing known material defects, including radon, exposes sellers to fraud and misrepresentation claims. The cost of a mitigation system is almost always less than the cost of post-sale litigation.

What Colorado Law Requires Sellers to Disclose

The SPD form is specific about what must be disclosed:

Known test results: If you have ever had a professional or DIY radon test performed, you must disclose the result and the approximate date it was conducted.
Mitigation history: If a mitigation system was installed, you must disclose the existence of the system, the installer, the installation date, and any post-mitigation test results you have.
System condition: If you know of any defects or changes in the mitigation system’s performance (a failed fan, a broken pipe connection), those must also be disclosed.

The key word throughout is known. Colorado law does not require you to test your home before selling. If you have never tested and have no knowledge of a radon issue, you mark the SPD form to reflect that. What you cannot do is conceal test results you have in hand or deny the existence of a mitigation system that is physically present in the home.

The Strategic Case for Testing Before You List

While testing is not legally required before selling, the Front Range real estate market has made radon testing essentially universal. Denver, Boulder, Fort Collins, Longmont, and Colorado Springs buyers routinely request professional radon tests during their inspection contingency period. If you have not tested and the buyer’s inspector finds a result above 4 pCi/L, you lose control of the situation. The buyer sets the terms: they request mitigation, a price credit, or in some cases walk away.

Sellers who test before listing and mitigate proactively arrive at the negotiating table with documentation in hand. A post-mitigation test showing levels below 2 pCi/L, combined with a transferable system warranty, removes radon as a negotiating point entirely. Mitigation in Colorado costs $800 to $2,500. A buyer negotiating after a surprise elevated result will typically ask for more than mitigation cost alone.

How Colorado Buyers Are Protected

The Colorado contract inspection process gives buyers strong radon protections:

Inspection period: Standard Colorado purchase contracts include an inspection contingency, typically 10 days. Buyers can request a professional radon test during this window at their expense.
Inspection objection: If the radon test exceeds 4 pCi/L, buyers can submit an inspection objection requesting that the seller mitigate or provide a price concession. Sellers are not required to comply, but refusing puts the buyer in position to terminate the contract and recover their earnest money.
Contract termination: Under Colorado’s Inspection Termination deadline, a buyer who cannot reach resolution with the seller over radon can terminate the contract without penalty.

Buyers should insist on a test by a CDPHE-certified radon measurement professional using a continuous radon monitor (CRM). DIY kit results are not accepted for real estate purposes. The test typically runs 48 hours and results are available quickly, fitting comfortably within the standard inspection window.

Colorado Market Reality: Front Range and Mountain Communities

The Front Range Corridor

Denver, Boulder, Colorado Springs, Fort Collins, and the communities between them sit in EPA Zone 1 on uranium-rich granite geology. Roughly half of homes in this corridor test above the EPA action level of 4 pCi/L on initial screening. Radon is a routine inspection issue, not an unusual finding. Experienced real estate agents in these markets advise sellers to address radon before listing as a matter of course.

Homes with pre-installed mitigation systems, documented with a current post-mitigation test result below 2 pCi/L, often close faster than comparable homes without mitigation. Buyers who have lost previous contracts over radon know the process and appreciate when the work is already done.

Mountain and Ski Town Properties

Summit County (Breckenridge, Keystone, Frisco), Eagle County (Vail, Avon), and Pitkin County (Aspen, Basalt) sit on some of Colorado’s highest-uranium granite formations. Radon levels in these communities are frequently very high, sometimes well above 10 pCi/L. Vacation homes and investment properties in these areas face the additional complication that they are often unoccupied for long stretches, during which radon can accumulate to high concentrations.

Sellers of mountain properties should be especially proactive about testing and disclosure. Second-home buyers often have legal counsel and purchase these properties as investments, making thorough documentation more important. For mountain cabins and ski condos, a pre-listing test and mitigation (if needed) signals professionalism and reduces the chance of a failed inspection derailing a seasonal sale.

Seller Strategy: Step by Step

Here is a practical sequence for Colorado sellers who want to handle radon cleanly:

Test before listing. Hire a CDPHE-certified measurement professional. Cost: $125 to $300. Schedule during the heating season if possible for the most conservative result.
If results are below 4 pCi/L, document the test (date, tester name, result, testing method) and attach it to the SPD disclosure. Buyers will likely accept these results without further negotiation.
If results are between 4 and 8 pCi/L, install a sub-slab depressurization system immediately. Cost: $800 to $2,500. Get a post-mitigation test showing levels below 2 pCi/L and include that documentation with the disclosure.
If results are above 8 pCi/L, same action, but prioritize speed. High-level results are more likely to alarm buyers. A documented mitigation result under 2 pCi/L converts a serious liability into a non-issue.
Transfer the warranty. Most Colorado mitigation contractors offer a one-year warranty on their systems. Transferring this to the buyer at closing adds tangible value and reduces post-sale liability.

What to Disclose About an Existing Mitigation System

Many Colorado homes already have mitigation systems in place, installed by previous owners or disclosed in prior transactions. If your home has a system, buyers will want to see:

The name and CDPHE certification number of the installing contractor
The installation date and original post-mitigation test result
The most recent re-test result (CDPHE recommends annual testing after mitigation)
Any service records, warranty documentation, and fan model information

Buyers or their inspectors will also physically inspect the system: checking that the fan runs, that the manometer shows negative pressure (indicating the system is drawing soil gas below the slab), and that all pipe connections and exterior termination points look sound. Make sure the system is operational before listing.

A system that has not been tested in several years is a red flag for buyers. Schedule a current test before listing so you can present fresh documentation.

Find Certified Radon Professionals in Colorado

Whether you need a pre-listing test, a mitigation system installed before closing, or documentation for a buyer’s inspection, our directory lists radon professionals in Colorado who are certified through NRPP or NRSB as required by CDPHE. With 428 listed professionals across the state, you can find a tester or mitigator serving your county, from the Denver metro and Front Range to the Western Slope and mountain communities.

Frequently Asked Questions

Does Colorado require radon disclosure when selling a home?

Yes. Colorado law requires sellers to disclose known radon test results and any history of radon mitigation on the Colorado Real Estate Commission's Seller's Property Disclosure (SPD) form. If you have tested and the results showed elevated levels, you must disclose that to buyers.

What if I have never tested my Colorado home for radon?

You are not required to disclose a result you do not have. If you have never tested, you mark the SPD form accordingly. However, savvy buyers in the Front Range market will request a professional radon test during their inspection period regardless of what the seller discloses. Testing before listing lets you control the narrative and, if needed, mitigate proactively.

Can a buyer walk away from a Colorado contract because of radon?

Yes. Under standard Colorado contracts, a radon result above 4 pCi/L during the inspection period gives the buyer grounds to request mitigation, negotiate a price reduction, or terminate the contract. In practice, most deals do not fall apart over radon because mitigation is straightforward and affordable. Deals are far more likely to close when the seller addresses radon upfront.

Does a mitigation system hurt or help a home sale in Colorado?

A properly installed, warranted mitigation system with post-mitigation test results showing levels below 2 pCi/L is a selling advantage in the Colorado market. It removes radon as a negotiation point entirely and signals to buyers that the seller handled the issue professionally. Buyers who have shopped around in the Front Range are accustomed to seeing mitigation systems; an untreated elevated result is more concerning to most buyers than a system that is already working.

What should I disclose about a mitigation system already on my home?

Disclose the installer name, installation date, the post-mitigation test result, and any service history or warranty documents. If the system has a warranty, transfer it to the buyer. Buyers will also want to see that the system fan is operational and that the manometer (the U-shaped gauge on the pipe) shows negative pressure, confirming the system is drawing properly.