A buyer's inspector walks through a five-year-old build in Northfield Village, checks the mechanical room, sees a capped vent pipe running from the slab to the roofline, and moves on. No radon test gets ordered. Nobody flags it in the inspection objection. The logic feels sound: this is new construction, it was built to current code, why spend $150 confirming something the builder already handled.
That assumption is the friction point that catches sellers off guard here more than almost anywhere else in Boulder. Northfield Village sits inside one of the few Colorado jurisdictions that requires new single-family homes to include a radon-resistant system at construction. That requirement is real. It is also not the same thing as a guarantee, and Colorado's disclosure law does not treat it as one.
What "Radon-Resistant" Actually Means at Construction
Boulder County is among a small group of Colorado jurisdictions, along with Larimer County and the City of Longmont, that require radon-resistant construction techniques in new single-family homes. Boulder has required this for over a decade, well before most of the Front Range paid attention to it.
What gets installed is a passive system: a gas-permeable layer beneath the slab, a sealed foundation, and a vent pipe that carries soil gas up through the house and out the roof, with no fan running. Boulder County's own guidance is direct about what that buys you and what it doesn't. A properly installed passive system lowers indoor radon levels by roughly half. The county's own example makes the math plain: a home sitting on soil that would otherwise produce a 10.0 picocurie-per-liter reading gets knocked down to 5.0 by the passive system alone. That's progress. It's also still above the EPA's 4.0 pCi/L action level.
This is the detail that gets lost in a five-second glance at a vent pipe. Radon-resistant construction reduces risk. It does not zero it out, and it does not eliminate the paper trail a seller is legally required to produce.
The Soil Doesn't Read the Building Code
Radon comes up from the ground, not from anything a builder chooses to put in the walls. Colorado sits in the EPA's highest-risk radon zone, and industry guidance citing a Colorado State University study puts the statewide average indoor reading at roughly 6.3 pCi/L, well above the national average. Close to half of Colorado homes test above the 4.0 action level at some point.
None of that changes because a house was framed in 2021 instead of 1971. Two lots forty feet apart in the same Northfield Village phase can produce different soil gas readings depending on fill, moisture, and how the foundation was poured. Boulder County's transaction guidance says this outright: the decision to test should never be based on results from a neighboring home, a zone map, or a survey. Every property gets its own answer.
So the honest way to think about a Northfield Village new-construction radon system is as a head start, not a finish line. A home with the passive system already installed might land comfortably under 4.0. It might not. The only way to know is a test, and the state doesn't let a seller skip the disclosure step just because the system exists.
What Colorado Law Actually Requires You to Hand Over
Senate Bill 23-206 took effect in August 2023 and changed radon from a soft recommendation into a hard disclosure requirement. If you're listing in Northfield Village, here's what the statute actually obligates you to do, regardless of the age of the home:
- Include a bold-typed warning statement in the sales contract about radon risk, using language the state has already written for you
- Disclose any known radon test results, ever conducted, including old ones below 4.0 pCi/L
- Disclose whether a mitigation system exists on the property, active or passive
- Provide the buyer with the current Colorado Department of Public Health and Environment brochure on radon in real estate transactions
The law doesn't require you to test before you list. It requires you to tell the truth about what you already know, and Boulder County's guidance notes that a mitigation system on file is treated as evidence radon existed previously, whether or not you ever ran a fresh test yourself.
A new-construction home with a built-in passive system still needs a disclosure conversation. The system is not a substitute for the paperwork.
The Colorado Division of Real Estate's practice advisory on SB23-206 shows this is still an active area of regulatory attention. The original law included a built-in change to the related landlord disclosure rules, narrowing which lease terms qualify for a tenant's right to void a lease over undisclosed radon, and that narrower rule has been in effect since January 1, 2026. If you're weighing a Northfield Village property as a rental rather than an owner-occupied sale, that shift is already live.
Three Phases, Three Different Radon Files
Northfield Village didn't get built all at once, and that matters for what documentation actually exists on a given property.
Northfield Commons was the first community built here, developed by a mix of builders including Coast to Coast, Markel Homes, Hendricks Fine Homes, and Accent Properties. These are the oldest homes in the footprint, meaning any original move-in radon test, if one was done, is now years old. Boulder County's guidance is specific that even a clean test from years back must be disclosed if it exists, so a seller in this original phase needs to track down whatever paperwork survives from the first owner.
Northfield Village proper is now largely a resale market. That means the current seller may not be the person who ordered the original post-construction test, and tracing that history through prior transaction files or a prior listing agent's records becomes part of prelisting prep, not an afterthought.
Kalmia38 is the newest phase of the community, where new construction has continued more recently than in the other two sections. Here the disclosure conversation is closer to real time. Boulder County recommends testing a new radon-resistant home as soon as possible after move-in specifically to confirm the passive system is doing its job, and if that first test wasn't done, a Kalmia38 seller may be listing a home with zero radon history on file at all, which is its own kind of disclosure gap.
What to Have Ready Before You List
- Any radon test results from any point in the home's history, even if the number was low
- Documentation of the mitigation system itself, including whether it's passive-only or has a fan added
- Receipts or contractor records if a fan was ever added to convert a passive system to active
- A copy of the current CDPHE radon brochure to hand the buyer
- Confirmation of who built the home and when, since that establishes whether radon-resistant construction was mandatory at the time
If you're the current owner of a Northfield Commons or early Northfield Village home and no test exists in your file, that absence is itself worth addressing before a buyer's inspector raises it during the objection period.
What Happens If the Buyer's Test Comes Back High
Even though the law doesn't require you to test as the seller, expect your buyer to order one anyway. A professional continuous radon monitor test runs a minimum of 48 hours, with closed-house conditions required for at least 12 hours beforehand and throughout, and typically costs $125 to $175. Most Colorado agents use the EPA's 4.0 pCi/L line as the trigger point during the inspection objection period. If the result comes back at or above that number, the buyer usually asks for one of two things: a licensed mitigation contractor installs or upgrades the system before closing, with a follow-up test confirming the fix, or the seller offers a closing credit, commonly in the $1,200 to $1,800 range, to cover the work after closing.
A full mitigation system with an active fan typically runs $800 to $2,500 and can cut radon levels by 90 to 99 percent, which is a very different outcome than the roughly 50 percent reduction a passive system alone provides. Knowing which of those two situations your home is actually in, passive-only or already active, changes how that negotiation goes.
Frequently Asked Questions
Does Colorado require me to test for radon before selling my Northfield Village home? No. SB23-206 requires disclosure of what you already know, not a fresh test. But if a buyer's own test during the inspection period comes back elevated, you'll likely be negotiating mitigation or a credit regardless.
My home already has a radon mitigation system from construction. Do I still need to disclose anything? Yes. Boulder County guidance treats the presence of a mitigation system as proof radon existed on the property, and the law requires you to disclose that the system exists whether or not it's ever been tested since installation.
What if I never had my home tested after move-in? That's common, especially in the newer Kalmia38 phase. It isn't a violation on its own, but it means you have less to disclose and less leverage if a buyer's test comes back high. Ordering your own test before listing gives you time to address a result on your terms instead of the buyer's.
Radon paperwork is a small piece of a Northfield Village sale, but it's exactly the kind of detail that stalls a closing timeline when it surfaces late. If you're weighing a sale here and want a clear read on your specific phase, builder, and pricing position, Maureen McCarthy has spent years tracking these blocks lot by lot. Get your instant home valuation and start the conversation with the facts already in hand.
