Radon Mitigation for Rentals in Northern Colorado

Owning a rental in Larimer or Weld County comes with a written radon step that many landlords miss until a tenant asks about it. Since Colorado’s disclosure law took effect, the duty sits with you before the lease is even signed, and the geology under Northern Colorado makes it a real risk rather than a paperwork formality. Both counties sit in EPA Radon Zone 1, the highest-potential category, so elevated readings in a rental here are common enough to plan for.

NoCo Radon Pros is a free matching service. We are not a contractor, we do not test or mitigate, and we hold no Colorado radon license. What we do is connect landlords with an independent, Colorado-licensed radon professional who handles the actual testing and mitigation. This page walks through what the law asks of you and how to get the work done. This is a summary, not legal advice.

The lease disclosure duty under C.R.S. 38-12-803

Colorado’s radon rules for rentals come from SB23-206 and live in C.R.S. 38-12-803. Before a tenant signs a residential lease, the landlord must provide, in writing, three things:

• A bold-faced radon warning.
• Any knowledge the landlord has of the property’s radon status: whether a test was done, the most current records or reports, any concentrations detected, and any mitigation performed.
• The current CDPHE radon brochure.

If you have never tested the unit, you disclose that you have no records. If a radon test was done at some point, the most recent results and any mitigation history have to go to the tenant. The point is that a prospective renter sees the same radon facts you do before committing.

A landlord’s failure to comply with these disclosure requirements counts as a breach of the warranty of habitability. That is worth sitting with. Radon disclosure is not a minor lease addendum in Colorado; it is tied to the baseline legal condition of the rental. You can read the statute directly through Colorado’s revised statutes and the bill history at leg.colorado.gov.

The tenant’s void-lease right, and the 2026 change

The disclosure duty has teeth. Under C.R.S. 38-12-507, a tenant may void the lease and vacate the unit if either of these happens:

1. The landlord fails to provide the required written radon disclosures, or
2. The landlord fails to make a reasonable effort to mitigate within 180 days after being notified that a radon measurement professional determined the radon concentration is 4 pCi/L or more.

Read that second point carefully. The 180-day clock starts when a licensed measurement professional confirms a reading at or above the EPA action level of 4.0 pCi/L and you are notified. From there, a reasonable effort to fix the problem is expected. Sitting on a confirmed elevated result is what exposes you to a tenant walking away from the lease.

Here is the part that changed and is already in effect. On or after January 1, 2026, the void-lease remedy no longer applies to leases of one year or less in duration. In practice, that means the remedy now reaches leases longer than one year. A standard 12-month lease of exactly one year or shorter no longer triggers the tenant’s void-and-vacate right, while longer-term leases still do.

Do not read that carve-out too broadly. It narrows only the void-lease remedy. The written disclosure duties in C.R.S. 38-12-803 still apply to residential leases of any length, and the habitability point still stands. Skipping the bold-faced warning, the known records, or the CDPHE brochure is a compliance problem no matter how long the lease runs. Our landlord radon obligations guide breaks the duties and the remedy apart in more detail, and the broader Colorado radon disclosure law guide covers how the sales side differs.

One more distinction worth keeping straight: the lease rules live in C.R.S. 38-12-803, while the separate rules for selling a home live in C.R.S. 38-35.7-112. They are different statutes for different transactions, so if you both rent out and sell property in Northern Colorado, treat them as two separate checklists. For the full picture across leases, sales, testing, and mitigation in one place, see the Colorado radon law guide.

Why Northern Colorado rentals test high

The disclosure law would matter anywhere, but it matters more here. Larimer County and Weld County are both classified as EPA Radon Zone 1, meaning the predicted average indoor screening level is greater than 4.0 pCi/L. You can see the county classifications on the EPA radon zone map for Colorado.

Statewide, about half of Colorado homes exceed the 4.0 pCi/L action level, according to CDPHE. That is a Colorado figure, not a Larimer or Weld number, but it tells you elevated results are ordinary rather than rare across the state. 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, which is why the disclosure and mitigation framework exists in the first place. The only way to know what a specific rental unit reads is to test it, since two houses on the same street can differ.

If your rental sits over a basement or crawl space, the entry path for soil gas is right there under the living area. Our page on crawl space and basement radon explains why those foundations often need a tailored approach.

How a landlord gets matched with a licensed mitigator

You cannot legally pay an unlicensed person to test or mitigate radon in Colorado. Since July 1, 2022, radon measurement and radon mitigation for pay both require a state license issued through DORA, the Division of Professions and Occupations. That licensing rule is the backbone of how this works, and it is why the brand stays a matching service and never a contractor.

Here is the sequence for a rental:

1. Tell us about the property through our contact page: city, foundation type, and whether the unit has ever been tested.
2. We connect you with an independent, Colorado-licensed radon professional serving Larimer and Weld Counties.
3. That licensed pro tests the unit. If the result is 4 pCi/L or higher, they design and install a mitigation system, typically a sub-slab depressurization setup with a fan and vent stack.
4. You hire and pay the licensed professional directly. NoCo Radon Pros takes no cut from you.

You can and should confirm any professional’s license yourself. The state runs a public DORA license lookup where you enter a name and see current standing. We point every landlord there rather than vouching for anyone, because the license belongs to that professional, not to us. Curious how we stay free to you? Our how we make money page lays it out.

What mitigation usually runs

For most Northern Colorado homes, a standard sub-slab mitigation system lands in the range of $1,000 to $2,500, with about $1,500 common. A rental over a crawl space can run higher, roughly $2,000 to $5,000 depending on size and condition, and an inaccessible crawl space that needs air exchange can reach further. If your unit was built with a passive radon rough-in, activating it with a fan is often much cheaper, in the neighborhood of $500 to $800.

For the 180-day mitigation window, the practical takeaway is to act on a confirmed elevated result rather than waiting. Getting matched, tested, and scheduled early keeps you inside a reasonable-effort timeline. Our mitigation cost guide goes deeper on pricing, and the main radon mitigation service page covers how systems are built.

If you own units across the region, we serve landlords in Fort Collins, Loveland, Greeley, and Windsor, along with the smaller fast-growing towns nearby. Start with the disclosure paperwork, test the unit, and if the numbers come back high, let us connect you with someone licensed to fix it.