Selling a home with high radon: disclosure, mitigation, negotiation

When a buyer's radon test on your home comes back at or above 4 picocuries per litre (pCi/L), the EPA Home Buyer's and Seller's Guide to Radon (EPA-402-K-13-002) framework is unambiguous: disclose the result, install a sub-slab depressurization mitigation system (typically $800–$2,500), and confirm the fix with a post-mitigation test before close. The negotiation almost always reduces to who pays (seller-installed pre-close, seller-credited at close, or split), and the right answer is usually the one that puts a working system and a clean post-mitigation test number into the closing file.

The 30,000-foot answer

If the inspection report on your home just came back with a radon reading at or above 4 pCi/L, do not panic and do not get clever. The EPA Home Buyer's and Seller's Guide to Radon (EPA-402-K-13-002) walks both sides through a playbook that has been used in tens of thousands of US transactions: test, evaluate, mitigate if at or above 4 pCi/L, retest. Almost every successful close in a high-radon situation follows that same four-step path; what varies is who pays, who picks the contractor, and whether the system is installed before or after closing.

The EPA framework, in four steps

The EPA's transaction playbook in EPA-402-K-13-002 is unusually clear and we recommend reading it cover-to-cover (it is short). The four steps are:

1. Test the home using a short-term test that complies with the AARST real-estate measurement protocol (more on that below).
2. Evaluate the result against the 4.0 pCi/L EPA action level from the Citizen's Guide to Radon (EPA-402-K-12-002).
3. Mitigate if the result is at or above 4 pCi/L. The EPA Consumer's Guide to Radon Reduction (EPA-402-K-10-005) describes the active soil-depressurization techniques used in roughly 95% of US installations.
4. Retest after mitigation, both for short-term verification (24 hours to 30 days post-install per AARST guidance) and ideally with a follow-up long-term test once the buyer is in occupancy.

Each step ties to a specific EPA or AARST document. There is no judgement call required at the framework level. The judgement calls are all in the negotiation that sits inside step 3.

Why your buyer used a short-term test (and why that is fine)

Long-term tests are scientifically preferable (they integrate seasonal variation and give a true annual average), but they take 90 days or more. A real-estate transaction does not have 90 days. The EPA, AARST, and every state radon program agree that short-term tests are acceptable in the time-compressed window of a sale, provided the protocol is followed correctly. The current measurement standard is ANSI/AARST MAH-2019 (Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes), adopted by every NRPP- and NRSB-credentialed measurement professional.

The protocol gives the tester three valid measurement strategies for a real-estate transaction:

• Two concurrent short-term tests, placed roughly 4–8 inches apart in the same room. The result is reported as the average of the two devices, and the devices must agree within a specified tolerance; if they disagree, both tests are invalid and the home must be retested.
• Two sequential short-term tests, each at least 48 hours, with the second beginning within a defined window of the first. Again the result is reported as the average.
• One short-term test paired with a long-term test, where the long-term test is allowed to run after closing if the buyer accepts that condition.

If your buyer's inspector ran a single uncorroborated short-term test, that is below the AARST protocol bar and you have a legitimate basis to ask for a second test before agreeing to any remediation. For the full short-term vs. long-term comparison, see our guide to short-term and long-term radon testing.

Disclosure: what you must say, and where to get state-specific help

This is the one section we cannot give you a clean universal answer for, because disclosure law varies meaningfully by state. The general principle is consistent across US jurisdictions: if you know about a material defect that affects health or value, you must disclose it on the seller's property disclosure form. A radon test result at or above 4 pCi/L meets that threshold in most states.

A handful of states (Illinois, Florida, New Jersey, Rhode Island, and Maine, among others) have specific radon-disclosure requirements (pamphlet-distribution rules, dedicated radon questions on the seller property disclosure form, or both) that go beyond general material-defect statutes. Other states fold radon into the general seller property disclosure form. A few states have no statutory radon-specific disclosure requirement at all but still allow buyers to sue under common-law fraud or misrepresentation if the seller concealed a known result.

The honest answer is: consult a real-estate attorney licensed in your state. We are not licensed to give legal advice in any jurisdiction, and the specific statutory language, the timing of disclosure (pre-listing vs. pre-contract vs. pre-close), and the form to use all vary. What we can tell you with confidence is that concealing a known test result is the single most expensive mistake a seller can make in a high-radon situation: rescission, damages, and attorney's fees in the worst cases.

Who pays for mitigation: typical patterns

Mitigation cost is negotiable in nearly every transaction. The three patterns we see most often in inspection-driven radon negotiations are:

• Seller pays for mitigation, pre-close. The seller hires an NRPP- or NRSB-certified mitigator, gets the system installed, and provides the post-mitigation test result to the buyer before closing. This is the cleanest outcome and the one most listing agents push for.
• Seller credits the buyer at close. The seller agrees to a closing credit (typically toward the higher end of the EPA cost range so the buyer can choose their own mitigator and any cosmetic options), and the buyer arranges installation after taking possession. Slower, but gives the buyer choice.
• Split the cost. Less common; usually negotiated when the test result is borderline (3.8–4.5 pCi/L) and there is genuine ambiguity about whether mitigation is strictly required versus prudent.

The EPA Consumer's Guide to Radon Reduction (EPA-402-K-10-005) quotes a national cost range of $800 to $2,500 for an active soil-depressurization installation on a typical single-family home. Costs at the high end of the range reflect houses with multiple suction points, complex foundations (combination basement-plus-crawlspace), aesthetic routing through finished space, or high-radon situations needing larger fans. Costs at the low end reflect newer construction with a passive stub already installed, where the contractor only needs to add a fan and wire it. For the engineering detail on what the contractor is actually installing, see our guide to sub-slab depressurization.

What the credit number usually looks like

When sellers credit the buyer rather than installing pre-close, the credit is almost always at the upper end of the EPA range (typically $2,000 to $2,500) for a simple reason: the buyer has no bargaining power to find the cheapest mitigator after close, and the seller wants the negotiation closed without a counter. Crediting the median cost ($1,500-ish) tends to invite a counter; crediting the top of the range tends to close the issue in one round. From the seller's perspective the credit also reduces sale proceeds, which has tax implications worth discussing with a tax advisor for your specific situation.

Timing: install pre-close, or credit at close

The two paths produce slightly different practical outcomes:

One pattern to avoid: do not install mitigation after the buyer has taken occupancy without a follow-up test under the new occupancy pattern. The buyer's HVAC habits, window-opening behaviour, and basement use can shift the radon level in either direction, and a post-mitigation test under the old occupancy is not a clean record of system performance in the home as actually lived in.

Framing the negotiation

Radon mitigation is a small, well-bounded cost relative to a home transaction. On a $400,000 sale, a $1,500 mitigation system is 0.4% of the price. On a $1,000,000 sale it is 0.15%. Framed as a percentage of price, it is almost always smaller than a furnace replacement, a roof patch, or a structural pier; and unlike those line items it comes with a verifiable post-install test number that proves the problem is fixed.

The cleanest negotiation framing we see, from listing agents who handle high-radon transactions routinely, is some version of: “We'll install a certified sub-slab depressurization system and provide a post-mitigation test under 2 pCi/L before close.” That moves the conversation from a vague liability to a specific deliverable. It also, in practice, often drops the system performance well below 4 pCi/L: a properly designed SSD system can take a 10 pCi/L house to under 1 pCi/L, which is a stronger result than the buyer would have received from any alternative remedy. The risk-numbers context behind the 4 pCi/L action level is broken down in radon and lung cancer risk numbers.

Buyer protections: the post-mitigation test and warranties

Per ANSI/AARST MAH-2019 and the EPA Consumer's Guide, mitigation contractors are expected to perform a post-mitigation diagnostic test, typically between 24 hours and 30 days after the fan is energised. The 24-hour minimum allows the soil-gas equilibrium under the slab to reach a new steady state with the fan running; the 30-day maximum prevents the test from drifting into seasonal-variation noise.

Reputable mitigators back their work with two warranties: a fan warranty of typically 5 years from the manufacturer (the fan is the only mechanical component, and modern inline radon fans are extremely reliable), and a system warranty of typically 1 to 2 years from the contractor covering joints, manometer function, and labour. A few contractors offer a performance warranty tying the post-mitigation result to a specified pCi/L target (commonly under 4, sometimes under 2); these are worth paying a small premium for in genuinely high-baseline situations. Mitigators should be NRPP- or NRSB-certified; both organisations publish online directories searchable by ZIP code.

What NOT to do

• Do not open the windows during the test. AARST-compliant tests detect it (see callout above), the test is invalidated, and in some states the attempt itself becomes a disclosure event.
• Do not tamper with the kit. CRMs log every hour; charcoal canisters can be analytically checked for handling artefacts.
• Do not claim “we don't have a basement so radon doesn't apply.” Slab-on-grade homes and homes over crawlspaces can and do test above 4 pCi/L; the EPA Citizen's Guide is explicit that every home should be tested regardless of foundation type. The county-level base rates are explained in our EPA radon zones guide.
• Do not “let it air out” before the buyer retests. The conditions during the second test must match the AARST closed-house protocol or the test is invalid; opening up the house briefly does nothing for the long-term result, and the buyer's tester will know.
• Do not install a DIY system to save money. Buyers and their attorneys will, reasonably, want NRPP or NRSB certification on the install. Owner-installed mitigation can become a re-disclosure issue on the next resale.
• Do not skip the post-mitigation test. An installed system without a verified post-mitigation result is worth less to the buyer than the same install with a clean test number in the file.

Scenario table: what to do at each result level

Costs above are anchored to the EPA Consumer's Guide to Radon Reduction (EPA-402-K-10-005) national range; regional cost variation can push numbers up in high-cost metros and down in rural areas. The mitigation engineering for any of these scenarios is described in sub-slab depressurization explained.

After the deal: retesting cadence

Whether mitigation was installed by the seller, the buyer, or not at all, the EPA recommendation post-close is consistent: retest every two years, and after any major renovation, HVAC change, or foundation modification. Renovations are an under-appreciated trigger: sealing a crawlspace, finishing a basement, adding a basement bathroom, or installing a high-CFM range hood can all shift a previously safe home above 4 pCi/L. See our guide to retesting after renovation for the specific triggers. For the homeowner-side testing protocol the buyer should follow once they take possession, see our DIY radon testing guide.

The spine of a clean close

Test. Evaluate. Mitigate. Retest. Disclose the result on the seller’s property form. Install or credit a certified sub-slab depressurization system in the $800–$2,500 band. Put a clean post-mitigation number in the closing file. Every negotiation on this page is a variation on that spine.

Sources

• EPA Home Buyer's and Seller's Guide to Radon. EPA-402-K-13-002. The federal four-step transaction playbook (test, evaluate, mitigate, retest) used throughout this guide.
• EPA Citizen's Guide to Radon. EPA-402-K-12-002. Source for the 4.0 pCi/L action level and the “consider action” band between 2 and 4 pCi/L.
• EPA Consumer's Guide to Radon Reduction. EPA-402-K-10-005. Source for the $800–$2,500 national mitigation cost range and the active soil-depressurization technology baseline.
• ANSI/AARST MAH-2019. Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes. Source for the real-estate measurement strategies (two concurrent, two sequential, one short plus one long) and the closed-house protocol.
• ANSI/AARST SGM-SF-2017. Soil Gas Mitigation Standards for Existing Homes. Source for the post-mitigation diagnostic test window (24 hours to 30 days) and SSD system design requirements.
• NRPP and NRSB certification directories (National Radon Proficiency Program and National Radon Safety Board): ZIP-code-searchable directories for finding a certified measurement professional or mitigator.

Related guides

• No. 03 · Testing protocolShort-term vs. long-term radon testing: which one to use and when
• No. 07 · MitigationSub-slab depressurization explained: how a radon mitigation system works
• No. 09 · RetestingRetesting radon after renovation: triggers and cadence
• No. 01 · Zone basicsEPA Radon Zones explained: what each zone means for your home