How deep must footings be in undisturbed soil?

• A. 1.2 m and not less than frost line
• B. 2.0 m
• C. 0.9 m
• D. 1.0 m

Answer: (A)

Footings must reach below the frost line and rest on undisturbed soil so they don’t heave from freezing and have a solid bearing surface. In cold climates, frost depths drive how deep you need to go, and a practical minimum is about 1.2 meters (4 feet). Saying the depth must be at least 1.2 m and also not less than the frost line ensures you’re both below the frost zone and on stable soil, which protects the structure from movement. The other options either stop short of frost protection or are deeper than necessary without added benefit, so they don’t reliably meet both the frost depth and undisturbed-soil requirements.