Colorado radon disclosure comes down to one rule. A seller has to give the buyer whatever they already know about radon in the home, plus the state’s radon brochure. Colorado law does not require a seller to test, so if radon gets measured at all, it usually happens when the buyer runs a test during the inspection period.
That’s the whole thing, really. The law asks sellers to share what they know. The buyer’s own test is where radon actually gets decided.
Key takeaways
- Sellers must disclose known radon information and provide the CDPHE brochure.
- Colorado does not require a radon test to sell a house.
- Buyers usually test during inspection and can object if levels come back high.
- The table below shows what each result range tends to mean in a sale.
What the Colorado radon disclosure law actually says
The rule lives in C.R.S. 38-35.7-112. It opens with a simple right: a buyer of residential property gets to know whether the home has been tested for elevated radon.
From there, every residential contract has to carry a bold-faced warning from the Colorado Department of Public Health and Environment (CDPHE). The warning says CDPHE “strongly recommends that ALL home buyers have an indoor radon test performed before purchasing” and recommends mitigation if levels are elevated. It calls radon “the leading cause of lung cancer in nonsmokers and the second leading cause of lung cancer overall.” Then comes the line sellers should read twice: “The seller … is required to provide the buyer with any known information on radon test results.”
Known information. That’s the phrase that matters.
Nowhere does the statute order a seller to test. CDPHE pushes hard for testing, but the legal duty is about disclosure.
The current version came from SB23-206, which took effect August 7, 2023. According to the Division of Real Estate’s broker advisory, the state updated its Commission-approved residential contracts and the Seller’s Property Disclosure to match.
Why bother? CDPHE describes radon as a colorless, odorless, tasteless radioactive gas that comes from uranium breaking down in soil. It also says radon “is common throughout Colorado.” You can’t see it or smell it. Testing is the only way to know.
What a Colorado seller must disclose about radon
Under 38-35.7-112(2)(b), the contract or the Seller’s Property Disclosure has to capture what the seller knows about radon. The statute’s items line up like this:
- Testing: whether radon tests were ever done.
- Records: the most recent test records and reports.
- Results and work done: a description of the radon levels found, or any mitigation or remediation performed.
- Systems: whether a mitigation system is installed.
- Brochure: an electronic or paper copy of the most recent CDPHE radon brochure.
In practice, most of this goes on the Seller’s Property Disclosure. The current residential form was adopted August 5, 2025 and became mandatory January 1, 2026. It has a section labeled “O. RADON” and asks for copies of your most recent radon records. The seller has to complete it personally, based on their current actual knowledge. The broker can’t fill it in for you. If a new adverse material fact turns up later, it has to be disclosed in writing.
The form is direct about why this matters. It warns that a “Seller’s failure to disclose a known adverse material fact … may result in legal liability.” For a walkthrough of the rest of that form, see our guide to the Colorado seller’s property disclosure.
People forget the brochure all the time. CDPHE publishes it as “Radon and real estate transactions in Colorado,” in English and Spanish, on its radon and real estate page.
Do FSBO and bank-owned sales count?
Yes. The statute defines residential real property broadly: single-family homes, manufactured and mobile homes, condos, apartments, townhomes and duplexes. It also specifically names homes sold by the owner, by a financial institution or by HUD (38-35.7-112(3)(b)).
So a for-sale-by-owner listing doesn’t get a pass, and neither does a bank-owned home. A bank that never lived in the house may honestly know nothing about radon. That’s fine. The radon rules still apply to the sale.
Why so many search results are about landlords
If you’ve searched this topic, you’ve probably run into a lot of rental forms. That’s because the same law covers leases. Per the DRE advisory, a landlord has to give the radon warning, any known radon information and the brochure before a tenant signs, and the tenant signs an acknowledgment. That’s useful if you rent, but it won’t help much when you’re buying or selling.
Radon results and what usually happens next
CDPHE’s guidance measures radon in picocuries per liter (pCi/L). Here’s how its recommendations match up with what tends to happen in a Colorado sale. The last column is general because every deal gets negotiated on its own terms.
| Result | What CDPHE suggests | What it often means in a sale |
|---|---|---|
| Under 2 pCi/L, no system | Retest in 5 years | Usually a non-issue. The buyer moves on to other inspection items. |
| 2 to 3.9 pCi/L, no system | Consider a long-term test | Buyer may raise it or plan a long-term test after moving in. |
| 4 pCi/L or higher, no system | Above the EPA action level; confirm and mitigate | Buyer may object during inspection. Seller may agree to mitigate, offer a credit or decline. |
| Under 2 pCi/L, system installed | Retest in 2 years | Good news for the buyer. Seller should hand over the system’s paperwork. |
| 4 pCi/L or higher, system installed | Call a licensed mitigation professional or the original installer | Buyer may object and ask the seller to have the system checked or repaired. |
Two more things from CDPHE. Test even when a mitigation system is already in place. And long-term tests are the best way to see average levels, so a quick test during escrow only shows a short window of time.
Seller checklist before you list
- Look for any old radon test results, even ones from years ago. If you have them, they count as known information.
- Gather the paperwork on any mitigation system, including who installed it, when, and the warranty.
- Fill out the radon section of the Seller’s Property Disclosure yourself. If you never tested, say so.
- Put the CDPHE radon brochure in your disclosure packet.
- Decide whether to test before listing. Any result you get becomes something you have to disclose.
- Think through how you’d answer a radon objection before one shows up.
- If you learn something new about radon after you’ve disclosed, put it in writing.
My take on item 5: if your house has never been tested, don’t panic. “I don’t know” is an honest answer, and the law allows it. Testing early makes more sense if you’d rather have time to collect mitigation bids without a buyer’s deadline hanging over you. Either way works. Just go in knowing that once you have a number, you have to share it.
For buyers: the test is yours to run
The warning in your contract says CDPHE strongly recommends that every buyer test. I agree. The seller’s disclosure tells you the home’s history. Your test tells you what’s in the air now.
Schedule it during your inspection period. CDPHE says hiring a licensed radon measurement professional costs “around $150” and is a good idea in a real estate transaction (CDPHE Radon FAQ). CDPHE also has a lookup for licensed professionals on its radon and real estate page. If you’re putting together an inspection budget, here’s what a home inspection costs for comparison.
DIY kits are an option for radon testing in Colorado too. CDPHE gives out free kits, one per household per year while supplies last. The catch is timing. CDPHE says short-term kits return results within a couple of weeks, which can be tight against contract deadlines.
So what happens if the number comes back at 4 pCi/L or higher? You bring it up through the normal inspection objection process before your inspection objection deadline. Then buyer and seller try to settle on a fix. The seller might install a system, offer a credit or say no.
The law requires disclosure and a radon information notice; it doesn’t require the seller to test or mitigate. Like most inspection findings, it’s a negotiation. For more on that, see which fixes are mandatory after a home inspection. If you settle on a credit, it will show up in your final numbers, so it helps to understand closing costs in Colorado before you ask for one.
Frequently asked questions
Does Colorado require a radon test to sell a house?
No. Colorado law does not require a seller to test. C.R.S. 38-35.7-112 requires sellers to share known radon information and give the buyer the CDPHE brochure. Testing is strongly recommended for buyers, but nobody is required to do it.
What if the seller never tested?
Then the seller says so on the disclosure, and that’s a legitimate answer. As the buyer, you set up your own test during the inspection period. CDPHE puts a licensed professional test at around $150.
How much does radon mitigation cost in Colorado?
CDPHE estimates that typical radon mitigation on an existing single-family home runs about $1,300 to $3,000. Most systems use active soil depressurization, which means a fan and a barrier with a vent above the roofline. CDPHE suggests getting at least three bids and asking for a guarantee below 4 pCi/L, plus a warranty of at least five years on the fan and workmanship. Low-income owner-occupants may qualify for help through CDPHE’s LIRMA program, which requires a test over 4 pCi/L within the past year.
Does a new home with a radon system still need a test?
Yes. CDPHE says to test even when a mitigation system is installed. If the builder put in a passive system, adding a fan to activate it costs about $500. With a system in place, CDPHE suggests retesting every 2 years when results are under 2 pCi/L, and calling a licensed professional or the original installer at 4 or above.
Does the seller have to give me the radon brochure?
Yes. The statute requires an electronic or paper copy of the most recent CDPHE radon brochure. CDPHE posts it in English and Spanish on its radon and real estate page.
This article is general education, not legal advice. If your situation is unusual, talk with a Colorado real estate attorney.
If you’d like a Colorado agent to help with the disclosure paperwork and the inspection back-and-forth, Trelora is happy to talk it through.
Sellers tell buyers what they know. The buyer’s test decides the rest.
David Speers is a seasoned Prop-tech and Real Estate Analyst dedicated to demystifying the complexities of the modern property market.