When does a building need Part 3 design instead of Part 9 in Ontario?
You must design to Parts 3 to 6 when a building exceeds 3 storeys or 600 m2 in building area, or contains assembly, care, detention, or high-hazard industrial occupancies, which Part 9 never covers.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
Crossing any single threshold pulls the whole building out of Part 9: a fourth storey, a footprint over 600 m2, or one restaurant, daycare, or care occupancy inside. Part 3 brings fire-resistance ratings, spatial separation calculations, and barrier-free design; Part 4 requires engineered structural design; Part 5 governs the building envelope. Budget and schedule change significantly, so confirm classification with your municipality early. Even within Part 9, individual elements outside prescriptive limits still need Part 4 engineering.
If your building gets too big, too tall, or includes uses like a restaurant, daycare, or care home, the simplified house rules no longer apply. You then need the full code and licensed designers or engineers.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.