The lowest floor depth of a structure or module is the vertical distance from its base level down to the upper surface of the lowest structural floorplate within it. This does not include the depths of areas which are not generally utilizable as floor space, such as elevator pits, wells, drainage trenches, etc.

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

A shaft or pit can extend below the lowest usable floor without increasing this depth. Such enclosed spaces can contribute to interior underground depth, whose lower endpoint is different.