How deep do exterior footings have to be below the ground?
Exterior footings must sit at least 12 inches below undisturbed ground, and below the frost line if that is deeper (IRC R403.1.4).
All exterior footings must be placed a minimum of 12 inches (305 mm) below the undisturbed ground surface. This bears the footing on firm, undisturbed soil rather than loose fill or topsoil.
Where the local frost line is deeper than 12 inches, the frost depth governs and the footing must reach below whichever is deeper. Frost protection itself is covered separately under R403.1.4.1.
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Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.