Almost certainly — and Ontario doesn’t publish one province-wide exemption, so the actual threshold is set by your own municipality.
Short answer
In most cases, yes. Ontario’s general rule requires a permit before constructing a new structure, and the province’s own guide to that rule does not carve out a blanket exemption for a deck by size or height. What counts as small enough to skip a permit is set municipality by municipality, so the safe default is to ask before building, not after.
The starting point is Ontario’s general construction trigger, not a deck-specific rule. The province’s citizens’ guide states that “you must obtain a building permit before you construct any new building over ten square meters in area or place another structure, such as a mobile home, on your property.” That is the baseline the guide sets out, and nothing in it carves out an exception for a deck by size, height above grade, or whether it’s attached to the house.
Unlike some structures with a clear province-wide size cut-off, whether a small deck needs its own permit is typically decided municipality by municipality, usually turning on some mix of attachment to the dwelling, height above grade, and footprint. A deck that’s exempt in one city can require a full permit application in the next, and no single published number applies everywhere in Ontario. The only reliable move is to confirm with your own local building department before starting — not to copy what a neighbour, a contractor from a different municipality, or an online forum assumed applied.
Treating a deck as too small to matter carries the same enforcement exposure as a much larger unpermitted project. Building Code Act fines — “up to $50,000 for a first offence and up to $100,000 for subsequent offences” for an individual, more for a corporation — don’t scale down for a small structure, and neither does a municipality’s power to order it brought into compliance or removed. See what happens if we build without a permit for the full penalty picture.
The two recurring mistakes are assuming “it’s just a deck” is a permit exemption on its own, and assuming a neighbour’s exempt deck sets a precedent for yours. Height above grade and attachment method can differ enough between two decks that look similar to change the answer completely, and the person who built next door may simply have skipped the permit without consequence so far — not proof it was actually exempt. See also how long a building permit stays valid once you do apply for one.
A 30-minute call is enough to tell you whether your project needs a permit application first.
