What the law does
Senate Bill 23-206 was signed June 5, 2023 and took effect August 7, 2023. It applies to contracts and leases entered on or after that date. The signed text added C.R.S. 38-35.7-112 for sales and 38-12-803 for rentals.
The sale law covers single-family homes, manufactured and mobile homes, condos, apartments, townhomes and duplexes, including sales by owners, banks and HUD (subsection 3(b)).
What goes in the contract
Every residential sale contract must carry a bold radon warning, under 38-35.7-112(2)(a). It includes these words: "STRONGLY RECOMMENDS THAT ALL HOME BUYERS HAVE AN INDOOR RADON TEST PERFORMED."
The contract or the seller's disclosure must also cover, under (2)(b):
- Whether radon tests were done, and the latest records or reports.
- The radon levels detected, and any mitigation.
- Whether a radon system is installed.
- A copy of CDPHE's latest radon and real estate brochure.
The Colorado Division of Real Estate updated the state forms to match, including the Contract to Buy and Sell (Residential; Income-Residential; Foreclosure Protection Act version) and the Seller's Property Disclosure. See its broker advisory (August 4, 2023).
If you're selling
The division's advisory says a seller "is required to provide the buyer with any known information on radon test results". So think of this as a law about sharing what's known. If you need to know whether it asks anything more of you in a specific sale, that's a question for your broker or an attorney.
A short checklist before listing:
- Find your old test reports, and at minimum the latest one.
- If a system is installed, find the paperwork, note its labels and confirm the warning device works.
- Look up when the last retest happened. EPA says "It is a good idea to retest your home at least every two years" with a system.
- Find any permit records. The City of Fort Collins notes, about building work generally, "Work without a permit may cause problems when selling a home".
A Colorado state report dated October 15, 2026 says some owners don't learn systems "were installed improperly until they are selling their homes." A check of the system before listing helps you avoid that surprise.
If you're buying
The contract language recommends a test, and that test is yours to arrange. EPA's guide and CDPHE's page describe how to run one; see the testing guide. Keep in mind:
- Read what the seller gave you. Any known test report, levels and system status should be there. A seller who never tested may have nothing to give.
- An old result is an old result. It tells you what was found then. Radon readings for one house can also differ from those next door, per Colorado's notes on its test records.
- If it's a newer home, ask if a system is passive or active. The state report describes homes "marketed to them as having a radon mitigation system installed." See the passive systems guide.
- Numbers for a neighborhood are not numbers for a house. Larimer County lists an area average of 7.8 pCi/L with no year or sample given. Only a test on that house answers the question.
An inspection reading came back high. What now?
What you can ask for, and when, depends on your contract, and that's for your broker or an attorney to answer. On the practical side:
- Look at how the test was run. Closed-house conditions and placement matter. EPA and CDPHE describe them in the testing guide.
- Know the benchmarks. EPA's action level is 4 pCi/L, and it says "Consider fixing between 2 and 4 pCi/L."
- Get a real price. Colorado's published figures are on the cost guide, but your contract talks need a written number for that house. A contractor's look at the house can give you one.
- Ask about the job. The hiring guide lists questions for whoever does the work, and what the contract should cover.
- Plan the test after the work. EPA says "within 30 days of system installation, but no sooner than 24 hours."
A short note for landlords
Under C.R.S. 38-12-803, a landlord gives written radon disclosure before the lease is signed, and the tenant signs a receipt. A tenant may void the lease under the statute in two situations: if those disclosures weren't provided, or if the landlord doesn't make a reasonable effort to mitigate within 180 days of being notified that a radon measurement professional found 4 pCi/L or more.
From January 1, 2026, that lease remedy doesn't apply to leases of one year or less. The City of Fort Collins puts it this way: it "will not apply to leases that are one year or less." The city also says "Landlords are not legally required to install a system." The state's broker advisory words the professional's role differently from the statute, so ask an attorney how this applies to you.
What commonly goes wrong
- Waiting until the deal is live. It leaves little room to test, get a price and do the work.
- Handing over the wrong report. Provide the latest one, along with anything else you know.
- Treating sealing as the fix. "EPA does not recommend the use of sealing alone to reduce radon."
- Calling a passive pipe a working system. Be precise about what's installed.
- Relying on this page in a dispute. It's general information. Get legal advice for your deal.
Who this page isn't for
This page isn't legal advice, and it doesn't replace your broker or attorney. If no sale is involved, start with the mitigation or testing guide.
Permits
Permit rules vary, so ask the building department before work begins. Numbers are on the Larimer County page.
Questions homeowners ask
I'm selling. What do I have to tell the buyer?
What you know: tests done and the latest records, levels detected, any mitigation, whether a system is installed, plus CDPHE's brochure. Your broker can walk you through the form.
Do I have to test or fix before selling?
The law as described in state materials is about disclosing what's known. For a clear answer on your sale, ask your broker or an attorney.
Does this cover condos and townhomes?
Yes. The law lists condos, apartments, townhomes and duplexes, plus manufactured and mobile homes.
I'm buying from a bank. Does it still apply?
The law lists owner, bank and HUD sales.
Where do I get the brochure?
From CDPHE's radon and real estate page.
How this site helps
LarimerRadon.com is a referral service: if a sale calls for a system, a request here goes to one independent local contractor, who quotes and does any work directly with you.
Sources
- Colorado General Assembly, SB23-206 and signed text (2023).
- Colorado Division of Real Estate, broker practice advisory (Aug 4, 2023).
- CDPHE, radon and real estate (© 2026); state radon dataset notes (undated).
- City of Fort Collins, radon page (undated, © 2026) and Building Permit Process.
- Larimer County health page (updated Nov 1, 2026).
- Colorado Department of Regulatory Agencies, radon report (Oct 15, 2026).
- EPA, Consumer's Guide to Radon Reduction and A Citizen's Guide to Radon.