The radon rough-in rule applies to new houses, additions and new dwelling units, not to finishing an existing basement. What the Ontario Building Code really asks of a basement renovation in 2026.
If you are planning a basement renovation in 2026, you have probably read that a new Ontario Building Code rule requires a radon rough-in in every finished basement. A version of that claim has circulated widely, including in an earlier version of this article, and it is not correct. Here is what the Code actually says, what changed and when, and what a basement permit in the Toronto area has to account for this year.
“In the past decade, the building code has been changing at a faster rate than ever. Now in 2026, it is important to know what the OBC requires so you can avoid any surprises.” Kam Oveisy, Owner and Principal Designer, Desire Basements
Ontario adopted a new Building Code in 2024. It came into force on January 1, 2025, with a grace period to the end of March 2025, and every permit application since then has been reviewed under it. The amendments filed in 2026 are housekeeping: each one updates the date of the amendment document the Code adopts, alongside items such as reference standards and fee schedules. None of them touched radon, and there is no separate 2026 Ontario Building Code.
Since January 1, 2025, Ontario's 2024 Building Code has required every new house, addition and additional dwelling unit to be built with a soil gas barrier under the slab and a capped, labelled radon rough-in pipe so a mitigation fan can be added later; finishing or converting an existing basement does not trigger that rule, and no Toronto-area building department currently requires a radon rough-in or radon test on a basement finishing permit. Health Canada still recommends that every home, new or old, be tested with a long-term kit for at least three months, and a basement renovation is a sensible time to do it, since any reading over 200 Bq/m3 should be fixed within a year.
The Code regulates new construction. A basement finish or a basement secondary suite in a house more than five years old falls under Part 11, which asks the new work to comply and does not ask you to upgrade the slab or the foundation you already have. The radon rough-in requirement attaches to a floor-on-ground being built, so if your slab stays, nothing in the Code asks anyone to cut it open to add a pipe or a soil gas barrier. Building departments in Whitby and Guelph have said so in writing, and the Toronto, Markham and Mississauga permit guides for secondary suites make no mention of radon.
One grey zone remains. If your renovation pours a new slab, through basement lowering, underpinning with a new floor, or an addition on a new foundation, that is a new floor-on-ground, and several Ontario building departments treat it as triggering the rough-in. Whether a GTA plans examiner asks for one on a new slab inside an existing footprint depends on the municipality, so we confirm it with the local building department at permit application rather than assume either way.
Radon is a naturally occurring radioactive gas that seeps up from the ground through cracks and gaps in a foundation. It is invisible and odourless, and Health Canada identifies it as the leading cause of lung cancer among people who have never smoked. It collects in the lowest level of a house, which in most GTA homes is exactly the space being renovated. Health Canada's guideline is 200 Bq/m3 as an annual average; it recommends every home be tested with a long-term test of at least three months and that levels above the guideline be reduced within one year.
So the sensible position is the one the Code does not force on you. Test, ideally over a heating season, and if you are about to finish the basement anyway, that is the cheapest moment to seal the slab penetrations or leave a capped sleeve for a future mitigation fan. We price either as its own line on the quote if you want it. Neither is required to pass inspection.
The list that actually governs a GTA basement finishing permit has not changed much: permit drawings prepared and stamped by a licensed structural engineer, an egress window for any bedroom, interconnected smoke and carbon monoxide alarms, insulation and air barrier to current values wherever a wall is opened, ESA inspection of the electrical work, and, for a basement apartment, the fire and sound separation between units. Our permit guide and our legal basement checklist cover each one.
No. The radon rough-in rule came in with the 2024 Code on January 1, 2025, and it applies to new floors-on-ground in new houses, additions and additional dwelling units. Finishing or converting an existing basement does not trigger it, and no Toronto-area building department currently asks for a radon rough-in or a radon test on a basement finishing permit.
Either works, and a long-term test of at least three months over the heating season gives the most reliable reading. If the result is over 200 Bq/m3, Health Canada recommends reducing it within a year, and a mitigation system can be added to a finished basement.
Yes. It is not required on a basement finishing permit, so it is not in the standard quote, but a capped sleeve and sealed slab penetrations can be priced as their own line before you sign.
If the house predates the 1960s, the Code is only half the story, and our guide to renovating the basement of an older Toronto home covers the low ceilings, old foundations and wiring that shape those permits.
Getting quotes and want to know what Ontario's Building Code actually requires of your basement? Book a free in-home consultation. Every Desire Basements quote is itemized, permit-ready, and locked before you sign. If you are budgeting as well, the basement cost calculator prices a permitted build for your square footage.
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