How deep do footings need to be for a home addition in Ottawa?

For most Ottawa additions, the starting point is the Ontario Building Code rule for clay or soils that are not clearly defined: footings must be at least 1.2 m deep, and not less than the depth of frost penetration. There is no official City of Ottawa frost-depth number to quote, because frost depth is set by local experience, so the real depth for your lot is confirmed by your designer or engineer and checked by the City's inspector before concrete is placed.

Why it matters: clay and silt soils are frost-susceptible, meaning they heave when they freeze. Sands and gravels generally don't. A footing that sits too shallow on a frost-susceptible soil can move, cracking the foundation and stressing the joint between the addition and your existing house. That is why depth is checked so carefully.

Soil type changes more than depth. Much of the Ottawa area has sensitive marine clay, known as Leda clay. Under the Code, footings on filled ground, peat or sensitive clay, including Leda clay, must have their sizes engineered rather than taken from the standard tables. If your lot has these conditions, expect an engineer to be involved, and a geotechnical investigation is often how they confirm what is underground. Other lots may run into rock at a shallow depth, which raises its own excavation questions. Either way, ask your designer early what they know about your soil.

Footing width, thickness and reinforcement depend on the loads (one storey or two) and the soil, and your designer or engineer sizes them on the drawings. Ask any contractor to show you how their quote follows the engineered design rather than a generic footing.

Cost depends on depth, soil, access for equipment and whether engineering is needed, so ask for an itemized foundation line in each quote rather than relying on a typical number. Winter pours are possible but need protection for the concrete, which adds cost and should be listed separately in the quote if your schedule pushes into cold weather.

The footing stage is one of the first inspections on an addition, so make sure your contractor books it before any concrete is poured. For site-specific questions, contact the City's Building Code Services at buildingpermits@ottawa.ca. To compare contractors who handle foundations for additions, browse the Ottawa Construction Network directory.

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