What size does an egress window need to be under the Ontario Building Code?

The Ontario Building Code, Div. B 9.9.10, requires each floor level with a bedroom, including a basement secondary suite, to have at least one window with an unobstructed opening of at least 0.35 sq m, with no single dimension under 380 mm, and a maximum sill height of 1,000 mm above the floor for above-grade rooms. A door on that level that leads directly outside can take the place of the window. Where the window sits below grade, the window well needs at least 550 mm of clearance in front of the open sash.

A sliding window opens only half its frame, so a slider has to be larger than a casement to give the same opening. If a basement window is too small, enlarging the opening and adding a well usually costs less before finishing than after. This is a common reason an addition fails inspection on the first pass, which is why we size it into the design from the earliest drawings.