The total interior depth of a structure or module is the distance from its base level down to the deepest extent of enclosed building space within it. This includes elevator shafts, pits, and wells or other shafts accessible to humans. It does not include thin tubes or conduits for wires, plumbing, or similar utilities.

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

An accessible elevator pit can extend the measurement below the lowest floor depth. A foundation pile extending deeper into the ground does not add interior depth when it encloses no accessible building space.