If you've spent any time around a residential construction project in Ontario, you've probably heard someone say "it's a Part 9 building." It sounds like jargon — and it is — but it's jargon worth understanding, because whether your project falls under Part 9 or Part 4 of the Building Code changes how it gets designed, who can design it, and what your permit drawings need to show.
Part 9, in plain language
The Ontario Building Code is organized into parts, and Part 9 — "Housing and Small Buildings" — covers buildings that are three storeys or less, 600 square metres or less in footprint, and used for certain occupancies like homes, small offices, and small retail. For these buildings, the code provides prescriptive rules: span tables for joists and rafters, minimum footing sizes, stud spacing, and so on. Follow the recipe, and the structure is deemed to comply — no calculations required.
Part 4: when the recipe runs out
Part 4 — "Structural Design" — is the engineered path. Anything bigger than Part 9 limits lives here, but so does anything within a small building that steps outside the prescriptive rules. And that's the part that surprises people: most modern homes are Part 9 buildings full of Part 4 elements.
Part 9 is a recipe book. The moment your design isn't in the book — a long span, a tall wall, a big opening — someone has to do the math. That someone is an engineer.
Where Part 9 homes need Part 4 engineering
The prescriptive tables assume conventional construction: modest spans, regular layouts, walls stacked on walls. Today's designs routinely exceed them. Common triggers we see every week: open-concept main floors with beams beyond the span tables, tall foundation walls and walkout basements, large window openings that interrupt braced walls, steel beams and columns (steel design is always engineered), floor trusses and engineered joists at long spans, decks beyond prescriptive limits, and any renovation that removes a bearing wall. Each of these needs engineered design and, for a permit, sealed drawings — even though the house as a whole is still "a Part 9 building."
Why this matters for your permit
Building departments review Part 9 projects against the tables. When your drawings show something the tables don't cover, the examiner will ask for an engineer's design — and that request mid-review can cost you weeks. The smoother path is identifying the engineered elements up front and submitting with sealed drawings the first time. It's exactly the gap our residential practice fills: we engineer the parts of your Part 9 building that the recipe book can't, and get the drawings to you within 5 business days for most projects.
Not sure whether your project stays inside the tables? Send us your plans — we'll tell you what needs engineering and what doesn't, and quote it the same day.