If you have been researching a basement project lately, you have probably run into a claim that goes something like this: the 2026 Ontario Building Code now makes a radon rough-in mandatory for any basement renovation that creates habitable space. It appears on contractor blogs, in forum threads and in a lot of recycled advice.
Most of it is wrong, and the errors matter, because they change what homeowners think they are obligated to do. Here is what the radon rules actually say, who they actually apply to, and what we would genuinely recommend if you are finishing a basement in the GTA this year.
First: there is no “2026 Ontario Building Code”
Ontario’s Building Code is not updated annually. As of the date this article was published, the edition in force is the 2024 Ontario Building Code, which came into effect on January 1, 2025. The province’s own building code updates page lists the 2024 Code and its amendments alongside the older 2012 Code, and no 2026 edition.
So when an article refers to “2026 code changes,” it is almost always describing the 2024 Code — requirements that have already been in effect for well over a year. If you have been putting off a project because you were waiting for new rules to land, they landed in January 2025.
What the radon requirement actually says
The 2024 Code introduced meaningful soil gas provisions for Part 9 residential construction, which is the category most houses fall under. In plain terms, a floor-on-ground has to be built so that radon and other soil gases are resisted, and so that an active mitigation system can be added later without tearing the house apart.
In practice, that generally means three things working together:
- A soil gas barrier under the slab — typically 6 mil polyethylene, lapped generously at the seams.
- Sealed penetrations and perimeter — the slab edge, service penetrations and joints sealed with a flexible sealant so soil gas has no easy path indoors.
- A capped, labelled rough-in for subfloor depressurization — a granular layer beneath the slab and a pipe stubbed up and left capped, ready to be connected to a fan if a later radon test comes back high.
The important nuance is that the Code requires the rough-in, not an operating mitigation system. Nobody is being told to install and run a radon fan. The requirement is to build in the capability cheaply now, so that fixing a problem later is straightforward instead of destructive.
Who it actually applies to
This is where the circulating advice goes furthest off course. The soil gas provisions are written for new construction — new houses and new additions with a floor-on-ground. Municipal building departments across Ontario have issued their own bulletins describing the requirement in exactly those terms, aimed at builders pouring new slabs.
If you are building a new home, or adding an addition that includes a new slab, crawl space or basement, expect the radon rough-in to be part of your permit and inspection process. Your builder should already be pricing it.
The part everyone gets wrong: does it apply to finishing an existing basement?
Here is the honest answer, and it is more useful than the confident one you will read elsewhere.
Finishing an existing basement — framing, insulating, drywalling and flooring over a slab that is already there — does not involve constructing a new floor-on-ground. On a plain reading of the Code, that work does not trigger the rough-in requirement, and we are not aware of any municipal bulletin that says otherwise.
Where it becomes genuinely less clear is when a basement project does create new slab. Underpinning to gain ceiling height, bench footing, lowering a floor, or pouring slab in a previously unexcavated area all mean new concrete going down over soil. That is much closer to the situation the requirement was written for, and some building departments will treat it accordingly.
Building code interpretation is made locally by your municipality’s building department, and they are not obliged to read a marginal case the same way as the next municipality. If your basement project involves underpinning or any new slab, ask your building department directly at the permit stage — or use a contractor who asks on your behalf. What you should not do is assume the answer based on a blog post, including this one.
Why radon deserves attention in Ontario regardless
The code question is narrow. The health question is broader, and it applies to houses of every age.
Radon is a naturally occurring radioactive gas produced by uranium decaying in soil and rock. It disperses harmlessly outdoors, but it accumulates in enclosed spaces below grade — which is precisely what a basement is. Health Canada attributes roughly 16 percent of lung cancer deaths in Canada to radon exposure, amounting to more than 3,000 deaths a year, and it is the leading cause of lung cancer among people who have never smoked.
Health Canada’s guideline is 200 becquerels per cubic metre (Bq/m³). Above that level, the recommendation is to take action to reduce it. According to the 2024 Cross-Canada Survey of Radon, roughly one in five Canadian residential buildings sits at or above that guideline.
Ontario’s average is comparatively moderate — the survey puts the provincial geometric mean around 72 Bq/m³ — but averages are close to useless at the level of an individual house. Radon depends on the geology directly beneath your foundation, how your home is sealed, and how it is ventilated. Two houses on the same street can test very differently. The only way to know your number is to measure it.
Why finishing a basement changes the calculation
An unfinished basement is usually a space people pass through. A finished basement is a family room, a bedroom, a home office or a rental suite — somewhere people spend hours every day, often including sleeping hours.
Radon risk is a function of concentration multiplied by time. Finishing the basement does not necessarily raise the concentration, but it dramatically raises the exposure, because someone is now down there for a third of their life rather than ten minutes a week. That, rather than any code obligation, is the real argument for dealing with radon as part of a basement renovation.
There is also a sequencing argument. Once the slab is covered with subfloor and finished flooring, and the perimeter is framed and drywalled, the cheap options are gone. Sealing the slab, running a sub-slab pipe or building in a rough-in is inexpensive while the floor is still bare. Afterwards, it means cutting finished work apart.
What a rough-in costs now versus mitigation later
Installed during construction, while the slab is exposed and trades are already on site, a radon rough-in is a modest line item — typically a few hundred dollars in materials and labour for the granular layer, pipe, sealing and capping.
A full active mitigation system retrofitted afterwards is a different proposition. It normally means core-drilling the slab, excavating a suction pit beneath it, running pipe up through finished space to roof level and installing a continuously running fan, plus sealing work. Retrofit mitigation in Ontario commonly runs in the low thousands, and it costs more when it has to be routed through finished rooms.
That gap is the entire logic of a rough-in. Even where it is not required, if your floor is already open, it is one of the better value decisions available in a basement project.
How to test your home properly
Testing is inexpensive and the protocol matters more than the device. Health Canada recommends a long-term test of at least three months — more than 91 days — ideally during the heating season, when homes are closed up and radon levels are typically at their highest.
Short-term tests, and tests run over the summer, tend to understate a home’s true annual average, sometimes substantially. They are useful as a quick screen, not as a basis for deciding you have no issue. Place the detector in the lowest lived-in level of the house, and if the result comes back at or above 200 Bq/m³, have mitigation done by a certified professional.
If you are renovating, the practical sequence is: test now if you can, build in the rough-in while the floor is open regardless of the result, then test again after the space is finished and in use.
What this means for your GTA basement project
- Do not delay a project waiting for “2026 code changes.” The current requirements took effect January 1, 2025.
- New build or addition with a new slab? The radon rough-in applies. It should already be in your builder’s scope and pricing.
- Straightforward basement finishing over an existing slab? The rough-in requirement is written for new floors-on-ground, so it generally does not apply — but the health case for addressing radon still does.
- Underpinning or any new slab? Ask your municipal building department at the permit stage. This is the genuinely grey case, and local interpretation governs.
- Test regardless of the code. Long-term, heating season, lowest lived-in level. It is the only way to know your own number.
If you are planning a basement renovation and want the code questions handled properly, explore our basement renovation services or book a free consultation. We manage permits and building department questions as part of every project.
This article is general information, not code advice. Building code interpretation is made by your local municipal building department, and requirements change. Confirm the requirements for your specific project with your municipality or a qualified designer before you build.
Ontario Building Code and radon FAQs
Is there a 2026 Ontario Building Code?
No. As of publication, the edition in force is the 2024 Ontario Building Code, which took effect on January 1, 2025. Ontario’s official building code updates page lists the 2024 Code and the older 2012 Code, with no 2026 edition. Articles referring to “2026 code changes” are almost always describing the 2024 Code.
Does the Ontario Building Code require a radon rough-in for basement renovations?
The requirement is written for new construction and additions that include a floor-on-ground. Finishing an existing basement over a slab that is already in place does not construct a new floor-on-ground, so it generally does not trigger the rough-in requirement. Projects that pour new slab, such as underpinning, are a grey area decided by your local building department.
What exactly is a radon rough-in?
A gas-permeable granular layer beneath the slab, a soil gas barrier such as 6 mil polyethylene with lapped seams, sealed penetrations and slab perimeter, and a capped, labelled pipe stubbed up from below the slab. If a later test shows elevated radon, a fan is connected to the existing pipe rather than a whole system being retrofitted.
Does a rough-in mean I have a radon problem?
No. A rough-in is preventive infrastructure, not a remedy. It makes future mitigation cheap and simple if testing ever shows it is needed. Many homes with a rough-in never connect a fan to it.
What radon level is considered too high in Canada?
Health Canada’s guideline is 200 becquerels per cubic metre. At or above that level, the recommendation is to reduce it. Roughly one in five Canadian residential buildings measures at or above the guideline.
How should I test for radon?
Use a long-term test of at least three months, more than 91 days, ideally during the heating season, placed in the lowest lived-in level of your home. Short-term and summer tests tend to understate the annual average and should be treated as a screen rather than a conclusion.
How much does radon mitigation cost in Ontario?
A rough-in installed while the slab is exposed is typically a few hundred dollars. A full active mitigation system retrofitted into a finished home usually runs into the low thousands, because it involves core-drilling the slab, excavating a suction pit, routing pipe through finished space and installing a fan.
Should I add a radon rough-in even if it is not required?
If your slab is already open, it is usually worth it. The cost while trades are on site is small, and it converts a potentially disruptive future retrofit into connecting a fan. Finishing a basement also substantially increases how many hours a day people spend below grade, which is what actually drives radon exposure.