Colorado Radon Law: License and Disclosure Guide
This is a summary, not legal advice. For your specific situation, read the statutes linked below and talk with a qualified professional or attorney.
Colorado treats radon as three separate problems at once: a public health issue, a licensing issue, and a real estate disclosure issue. For homeowners, buyers, sellers, landlords, and tenants across Northern Colorado, that means three different sets of rules can touch a single property. This guide pulls them together for Larimer and Weld Counties, with links to the primary sources so you can check anything yourself. When you are ready to act, we connect you with an independent, Colorado-licensed radon professional. NoCo Radon Pros is a matching service and does not hold a radon license or perform radon work.
The geology: why Northern Colorado sits in the highest radon zone
Radon is a naturally occurring radioactive gas produced as uranium in soil and rock breaks down. It moves up through the ground and can collect inside homes. Both Larimer County and Weld County are classified as EPA Radon Zone 1, the highest of the three zones, which the EPA defines as a predicted average indoor screening level above 4.0 pCi/L. You can see the county classifications on the EPA Map of Radon Zones for Colorado.
Statewide, about half of Colorado homes test above the action level, according to the Colorado Department of Public Health and Environment (CDPHE). The health stakes are the reason the state pays attention: CDPHE reports that radon is the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked (CDPHE, radon and your health). Zone 1 describes a county average, not your specific address, so the only way to know a given home is to test it. Start with our radon testing overview or the Northern Colorado radon levels guide.
The licensing regime: who must be licensed, and how to check
Colorado is one of a small number of states that license radon professionals. Under House Bill 21-1195, the Regulation of Radon Professionals, signed June 30, 2021, the state created a licensing program administered by the Department of Regulatory Agencies (DORA) through its Division of Professions and Occupations. Since July 1, 2022, it has been mandatory: performing radon measurement or mitigation for pay without a license is prohibited and subject to penalties. You can read the bill at leg.colorado.gov.
There are two license types, a radon measurement professional license and a radon mitigation professional license. The practical takeaway for a homeowner is simple and it costs nothing: before you hire anyone, verify the license. The program page is dpo.colorado.gov/RadonProfessionals, and you can look up an individual or business on the DORA license lookup. Every professional we match you with is independent and state-licensed, and the license belongs to that professional. The program is scheduled to sunset on September 1, 2027 unless the legislature renews it.
The disclosure law: SB23-206 for sales and leases
Senate Bill 23-206 took effect August 7, 2023 and added radon disclosure duties to both home sales and residential leases. The full signed act is on the Colorado General Assembly site. One point trips up a lot of summaries: the sale rules and the lease rules live in two different statutes. Do not assume they are the same.
For home sales: C.R.S. 38-35.7-112
The sale disclosure requirement is codified at C.R.S. 38-35.7-112. Every contract for the sale of residential real property must contain a bold-faced, clearly legible warning recommending that buyers have an indoor radon test performed before purchasing and that elevated levels be mitigated. The seller must disclose what they know about the property's radon: whether a test has been conducted, the most recent records and reports, a description of any concentrations detected or mitigation performed, and whether a mitigation system is installed. The seller must also provide the current CDPHE brochure on radon in real estate transactions. Our radon at home sale page and the selling with high radon guide walk through the timeline.
For leases: C.R.S. 38-12-803
The lease and tenant provisions are separate, at C.R.S. 38-12-803. Before a prospective tenant signs a residential lease, the landlord must provide in writing a bold-faced radon warning, any knowledge the landlord has of the property's radon (including tests, the most current records and reports, and any concentrations or mitigation), and the current CDPHE brochure.
The statute also gives tenants a remedy. A tenant may void the lease and vacate the premises, in accordance with C.R.S. 38-12-507, if the landlord fails to provide the required disclosures, or fails to make a reasonable effort to mitigate within 180 days after being notified that a radon measurement professional determined the air concentration of radon is 4 pCi/L or more. A recent change matters here: on or after January 1, 2026, this void-lease remedy no longer applies to leases of one year or less, so as of 2026 it reaches longer leases rather than short ones. Separately, a landlord's failure to comply with the disclosure requirements is a breach of the warranty of habitability. Landlords and tenants can read more on our radon for rentals page and the landlord radon obligations guide.
Testing: what a compliant measurement looks like
Testing is the front door to every other decision. There are short-term tests (commonly two to seven days), long-term tests (90 days or more), and continuous radon monitors used by professionals. For a real estate transaction, a measurement done under closed-house conditions by a licensed measurement professional carries more weight than a loose do-it-yourself kit, because placement, timing, and tamper-resistance all affect the result. If a first short-term test comes back near the action level, a follow-up test is often the sensible next step. Our test results explained guide covers how to read a number and when to retest.
Mitigation: what licensed professionals actually do
The standard fix for an elevated home is an active soil depressurization system. A licensed mitigation professional installs a sealed vent pipe and an in-line fan that draws radon-laden soil gas from beneath the slab and routes it above the roofline, where it disperses. The design changes with the foundation. Slab and basement homes usually get a sub-slab system; crawl space homes often get a sub-membrane system that seals a vapor barrier over the crawl space floor and draws gas from underneath. After installation, the professional runs a post-mitigation test to confirm the level has dropped. See radon mitigation and crawl space and basement radon for the details.
Costs in Northern Colorado
For most homes, CDPHE and Larimer County put a mitigation system at about $1,000 to $2,000, and a common retail figure for a standard sub-slab system is around $1,500. The fuller market range runs to about $2,500, and some Fort Collins installers advertise flat rates near $1,200. Crawl space and multi-foundation homes cost more, roughly $2,000 to $5,000, and difficult crawl spaces can run higher. Colorado also has a low-income assistance program that can cover up to 100 percent of a certified system, and some utilities offer small rebates. The full breakdown is in our radon mitigation cost guide, drawn from CDPHE and Larimer County.
New construction
Colorado has no single statewide mandate for radon-resistant new construction. Residential building codes are adopted locally, and radon control is an optional appendix to the International Residential Code that a jurisdiction must choose to adopt. Local jurisdictions in Northern Colorado, including Fort Collins, require passive radon-resistant systems in new residential construction through their adopted codes, which is separate from any duty to test or mitigate an existing home. The EPA explains the code framework in its building codes for radon-resistant new construction resource. If you are buying new, a passive rough-in can often be activated with a fan for a modest cost.
Quick checklists by audience
Buyers
Ask for the seller's radon disclosure and any prior test reports. Have a test performed under closed-house conditions before closing. If the result is 4.0 pCi/L or higher, get a mitigation quote and factor it into the deal.
Sellers
Complete the required disclosure and provide the CDPHE brochure. Consider testing early. A finished mitigation system with a confirming retest can remove a common negotiation snag.
Landlords
Provide the written warning, your known radon information, and the CDPHE brochure before the tenant signs. If a measurement professional finds 4 pCi/L or more, make a reasonable effort to mitigate within 180 days of notice. Remember the January 1, 2026 change to the void-lease remedy for shorter leases.
Tenants
You are entitled to the written disclosures before signing. If a professional finds an elevated level and the landlord does not act within the statutory window, you may have a remedy under C.R.S. 38-12-803. Read the statute and consider professional advice.
Where NoCo Radon Pros fits
We are a free matching service for Northern Colorado. Tell us your city in Larimer or Weld County, your situation, and your test result if you have one, and we connect you with an independent, Colorado-licensed radon professional who handles the measurement or the mitigation. We do not perform the work and we do not hold a license. See how we make money or contact us to get started.