Test for radon before you finish your basement. Here is Health Canada's 200 Bq/m³ guideline, when and how to test in the GTA, and what fixing a high reading costs.
If you are planning to finish your basement in the GTA, test it for radon first, and start the test now. Health Canada's guideline is 200 becquerels per cubic metre (Bq/m³), and the only way to know where your house sits is a long-term test of at least three months, run in the heating season on the lowest floor you live on. Health Canada puts a test kit at about $30 to $60 and fixing a high reading at $2,000 to $4,000, and that work is simpler and tidier before the drywall goes up than after.
Radon is a radioactive gas that comes out of the soil. You cannot see it or smell it, and Health Canada calls it the leading cause of lung cancer in people who have never smoked. It gets into a house wherever the house touches the ground: cracks in the slab and foundation walls, the joint where the floor meets the wall, gaps around pipes, floor drains and the sump pit. That is why levels tend to be highest in basements.
An unfinished basement is a room you pass through. A finished one is where your kids sleep over, where someone works from home eight hours a day, where the guest room is. The Health Canada guideline is measured in the lowest lived-in level, meaning the lowest floor where someone spends more than four hours a day. The day you finish the basement, that level often becomes the basement itself. A house that was never worth worrying about upstairs can read differently once the measurement moves down a floor.
Less common than in some parts of Ontario, but common enough that guessing is a bad plan. The City of Toronto estimates that about 7% of buildings in Toronto, homes, workplaces and schools included, have radon levels that may pose a risk. Across Ontario, Health Canada's cross-Canada survey found about 8% of homes above the guideline, with some regions much higher. Radon varies from one house to the next on the same street, so a neighbour's result tells you very little about yours. Only a test does.
Use a long-term test that runs for at least three months. Short digital readings bounce around from day to day, and public health units advise following any high short-term reading with a long-term test before deciding anything. Fall and winter give the most honest number, because windows and doors stay shut and the house holds its air. Late October is about the best time of year to start.
Place the detector on the lowest level you use or plan to use, which, if you are reading this, is probably the basement you are about to finish. Follow the kit's instructions on height and placement, leave it alone for the full period, then mail it to the lab. Kits are sold online and at home improvement stores, and the Take Action on Radon program lists Health Canada-approved tests.
Three months sounds like a delay. It usually is not. Design, drawings and the permit take time of their own, which we lay out in how long a basement renovation takes. Start the test the week you start talking to contractors and the result tends to arrive around the time the permit does.
The 2024 Ontario Building Code tightened soil gas rules for new homes. Every new house now needs a radon rough-in pipe under the slab, at least 100 mm in diameter, sealed and labelled for radon, plus an air barrier that runs under the basement floor. The idea is that if a future test comes back high, a fan can be added to that pipe without breaking up the floor.
Here is the honest part. That rough-in rule is written for new construction. A renovation inside an existing house is not clearly covered by it, and the Ontario guidance we have read does not say it is. Your municipality's building department is the authority on what your permit requires, and we cover the wider picture in our guide to what the building code actually requires for a basement in 2026. Our view is simpler than the code question. If your test comes back high, deal with it while the basement is open, whatever the permit asks for.
Below 200 Bq/m³, you finish the basement as planned. Keep the slab sealed where you can, and retest after the renovation, since any major change to a house can change how air moves through it.
Above 200 Bq/m³, hire a mitigation professional certified under the Canadian National Radon Proficiency Program (C-NRPP). The usual fix is sub-slab depressurization: a pipe through the slab and a small fan that pulls soil gas from under the floor and vents it outside. Health Canada's cost range for this work is $2,000 to $4,000, depending on the house. The City of Toronto also notes that simpler steps, like sealing the sump pit and filling foundation cracks, can lower levels on their own. A certified professional will tell you whether that is enough for your reading.
Timing is where the renovation helps you. Before framing, a mitigator has a bare slab to work on, open walls to route the pipe through and no finished ceiling to patch. Sealing cracks and the sump is the same kind of work you want done anyway, and it fits naturally alongside the moisture side of the job we describe in our basement waterproofing guide. After the drywall, every one of those steps means cutting into a room you just paid to finish.
If your home is under seven years old and your test is above the guideline, contact your builder first. Mitigation for newer Ontario homes can fall under the Tarion new home warranty. The City of Toronto also points older homeowners to the Ontario Renovates program and the Lung Association as possible sources of help with the cost.
A test is the cheapest line in any basement project. Mitigation, if you need it, is a real cost, but it is a fraction of what a finished basement runs, and our Toronto basement finishing cost guide shows the full range. We would rather you spend $50 and three months learning the number than discover it after the media wall is installed.
If you are planning a basement this winter, book a free in-home consultation. We will design the space in 3D at no charge, give you an itemized quote, and plan the build around your test result so nothing gets torn out later.
Yes. A finished basement usually becomes the lowest lived-in level of the house, which is where Health Canada says radon should be measured. Testing first lets any mitigation happen while the slab and walls are still open.
Health Canada's guideline is 200 Bq/m³ as an average in the lowest lived-in level. Above that, Health Canada recommends reducing it. Health Canada also notes there is no risk-free level.
A long-term test should run for at least three months, ideally in fall and winter when the house is closed up. Short-term digital monitors give quick readings, but a high short-term result should be confirmed with a long-term test.
Health Canada estimates $2,000 to $4,000 to reduce high radon in a typical home, depending on its size and design. The usual method is sub-slab depressurization, installed by a C-NRPP certified professional.
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