The depth of a structure's foundations is the total vertical distance from that structure's base level down to the lowest continuously joined member of its foundations underground or underwater. It includes the distance from the top of the foundations (lowest floor level) up to the base, and is therefore a greater distance than the actual length of the foundation member(s).

This data field is the underground equivalent to the tip height of a structure.

The lower endpoint can lie below soil or water. A deep pile contributes to foundation depth even when there is no accessible interior space alongside it at that depth.