A radon result can arrive at an awkward time for a homeowner: after a monitor has been running in the basement, but just before the property is supposed to be listed. The number may be below Canada’s guideline, above it, or based on a test that was too short to support a sound conclusion. For someone selling a Richmond Hill home after a radon test, the best next step is not to minimize the result or turn it into a dramatic marketing claim. It is to establish what was measured, preserve the evidence and make a deliberate plan for testing, mitigation, disclosure and pricing.
This guide explains a practical seller workflow for Richmond Hill, including Mill Pond, Oak Ridges, Jefferson and Rouge Woods. It is general real-estate information, not medical, engineering or legal advice. A qualified radon professional should assess testing and mitigation, while an Ontario real-estate lawyer should advise on disclosure and agreement wording.
What a radon result actually tells a seller
Radon is a naturally occurring radioactive gas that can enter a building from the ground. It cannot be seen, smelled or tasted. Health Canada’s current guideline is 200 becquerels per cubic metre (Bq/m³), based on the average annual concentration in a normally occupied area. Health Canada recommends corrective action when the long-term average is above that level, with faster action as the concentration rises.
The result belongs to the tested home—not to a postal code, neighbourhood or house style. York Region conducted a residential study using measurements of at least 91 days between November 2017 and August 2018. That study helps establish local relevance, but it cannot predict the level in a particular Richmond Hill property. Foundation details, soil conditions, ventilation, renovations and how the home is used can all affect the reading. The only reliable way to understand an individual home is to test that home properly.
First verify whether the test is decision-grade
A number displayed after a few days can be useful for screening, but it is not equivalent to a long-term result. Health Canada’s 2026 residential measurement guide recommends testing for three to 12 months. At minimum, a long-term test should run for more than 91 days, with at least 91 days during the heating season; otherwise, additional testing may be needed because a shorter warm-season test can underestimate the annual average.
Before making a repair or listing decision, collect these details:
- the device make, model and serial number;
- the start and end dates;
- the room and exact placement location;
- whether the room is normally occupied;
- whether the monitor was moved, unplugged or obstructed;
- the laboratory report or exported monitor record; and
- any renovation, ventilation or HVAC changes during the test.
Test the lowest level that is normally occupied, following the device and Health Canada placement instructions. A finished basement family room, office, bedroom or apartment can matter; a seldom-used furnace room may not be the correct location. If a service provider performs the measurement, Health Canada recommends using a professional certified through the Canadian National Radon Proficiency Program (C-NRPP).
Four common pre-listing situations
1. You have not tested and the sale is months away
Start a properly placed long-term test early enough to finish before listing. A seller who begins in the fall has a better chance of including at least 91 heating-season days. Preserve the unopened kit receipt, placement photograph, dates and final report. Do not promise a future result in advertising.
2. A valid long-term result is below 200 Bq/m³
Keep the complete report and describe it precisely: the measured average, dates, device and location. A result below the guideline can answer buyer questions, but it is not a lifetime certificate. Radon varies over time, and Health Canada recommends retesting after major renovations or energy-efficiency work that changes the building envelope or airflow.
3. A valid long-term result is above 200 Bq/m³
Contact a C-NRPP-certified mitigation professional rather than selecting a system from an internet diagram. The most common approach is active sub-slab depressurization: a fan and pipe draw soil gas from beneath the foundation and exhaust it outdoors. The design, discharge location, sealing work and electrical connection must suit the building. Health Canada says these systems are effective in most homes, but the seller should obtain property-specific recommendations instead of assuming a standard solution.
Retain the proposal, invoice, system details, photographs, warranty and post-mitigation test. A fan running on listing day is not proof of the achieved radon level. The evidence is the completed work plus a properly conducted follow-up measurement.
4. The listing date arrives before a long-term test is complete
Do not convert an incomplete average into a definitive “safe” or “high” statement. Record the elapsed test period and current status, then discuss the facts with the listing agent and lawyer. Options may include continuing the test, delaying the listing, disclosing that testing is in progress, or using transaction wording prepared by the lawyers. Health Canada notes that some transactions use a holdback so long-term testing and, if required, mitigation can occur after closing. A holdback is a legal and financial mechanism—not a do-it-yourself clause.
Should you mitigate before selling?
For a confirmed result above the guideline, completing appropriate mitigation before marketing can reduce uncertainty. Buyers can review a defined system, an invoice and a post-mitigation result instead of estimating an unknown scope. It may also prevent the radon issue from dominating negotiations or becoming a last-minute condition.
Pre-listing work is not automatically the correct choice in every case. Timing, test validity, contractor availability, basement finishes, the proposed route for the pipe and fan, and the seller’s broader moving plan all matter. If the result is based only on a short screening period, first ask a qualified professional whether additional measurement is the proper next step. Spending money before confirming the evidence can create confusion rather than confidence.
If mitigation is recommended, compare written scopes—not only prices. Ask who is certified, what diagnostic testing will be done, where the pipe and fan will be located, how the system will be labelled, what electrical work is involved, how finished areas will be protected, what warranty applies and how follow-up testing will be documented. Costs vary by foundation and system design, so this article does not provide a universal Richmond Hill estimate.
Build a buyer-ready radon file
A clean evidence package is more persuasive than a vague reassurance. Assemble:
- the original long-term test report and device information;
- a placement photograph and floor-plan note;
- the mitigation assessment and written scope, if applicable;
- the contractor’s C-NRPP certification details;
- the paid invoice, warranty and operating instructions;
- the post-mitigation test report;
- relevant permits or electrical documentation identified by the contractor; and
- a short chronology of testing, work and retesting.
This file can be reviewed alongside a broader pre-listing inspection strategy. It also helps the seller and lawyer give consistent, fact-based answers instead of relying on memory during negotiations.
Disclosure: facts are safer than conclusions
Ontario disclosure obligations depend on the facts. RECO distinguishes patent defects from latent defects and advises sellers and agents to obtain legal advice when there is uncertainty. A known elevated radon result, incomplete mitigation or a disputed report should not be hidden or described misleadingly. At the same time, a seller should not improvise a legal conclusion or overstate what a test proves.
Review the site’s detailed Ontario seller-disclosure guide, then give the radon records to the lawyer handling the transaction. Clear statements include the measurement period, average result, tested location, mitigation date and post-mitigation result. Avoid labels such as “radon-free,” because all homes contain some radon and levels can change.
How radon can affect pricing and marketing
Radon does not create a standard percentage adjustment to a home’s value. The effect depends on whether the result is reliable, whether mitigation is complete, whether the system is documented, how the basement is used, and how much uncertainty remains for the buyer. A confirmed high result with no plan creates a different pricing question from a documented system followed by a long-term result below the guideline.
Comparable-sale analysis should still focus on location, lot, condition, size and features. The radon file is a property-specific risk and confidence factor, not a neighbourhood-wide discount. For a customized comparison of competing listings, relevant sales and property condition, request a complimentary English home evaluation.
Hypothetical Richmond Hill example
This example is illustrative, not a client story. A Mill Pond owner plans to list in four months and has used a digital monitor for 18 days in a finished basement office. The displayed average is 236 Bq/m³. The owner should not advertise 236 as the annual level or assume that opening windows solves the issue. A sensible sequence is to preserve the monitor data, confirm correct placement, consult a certified measurement or mitigation professional and continue a qualifying long-term test. If the completed test confirms a result above 200 Bq/m³, the owner can evaluate mitigation, retain the installation file and complete follow-up testing before setting the final listing strategy.
If the property must be sold sooner, the owner, agent and lawyer can plan how to present the known facts and handle buyer requests. The goal is not to eliminate every question; it is to prevent avoidable surprises and give buyers a credible record.
A practical seller timeline
- Three to six months before listing: start or validate the long-term test and collect property records.
- After a completed result: obtain certified advice if the average exceeds the guideline.
- Before photography: complete chosen work, tidy visible system components and prepare the document file.
- Before listing: review disclosure and offer wording with the lawyer and align the pricing strategy with the evidence.
- During showings: give consistent factual answers and make supporting records available through the agreed process.
For a coordinated Richmond Hill selling plan, see Richard Wang’s Richmond Hill home-selling service or work through the GTA Home Seller Plan. The earlier testing, documentation and marketing decisions are aligned, the less likely radon is to become a closing-week surprise.
Sources reviewed
- Health Canada, Guide for Radon Measurements in Homes, current guide reviewed October 27, 2026.
- Health Canada, Government of Canada Radon Guideline, reviewed October 27, 2026.
- Health Canada, Reducing Radon Levels in Your Home, reviewed October 27, 2026.
- Health Canada, Radon in Real Estate, published December 18, 2019 and reviewed October 27, 2026.
- York Region Public Health, Radon: Test Your Home, including the 2017–2018 York Region study, reviewed October 27, 2026.
- Real Estate Council of Ontario, Bulletin 7.4, reviewed October 27, 2026.
Information is current to October 27, 2026 and applies generally to Ontario homes. Requirements and professional recommendations can change. Confirm property-specific testing, construction, electrical, contractual and disclosure questions with the appropriate qualified professionals.