Headroom is the space that lies between the head and sill of a doorway or between the ceiling and floor, allowing passage of a person standing upright. Headroom is also the height above the nosing of a tread to the ceiling above it.
According to the "Building Code of Australia" :
"A room or space within a building must have sufficient height suitable for the intended function of that room or space." "The requirement is satisfied if the ceiling height is not less than -
In a class 1, 2 or 3 building a habitable room excluding a kitchen - 2.4 meters & in a kitchen, laundry or the like - 2.1 meters and in a corridor, passageway or the like 2.1 meters."
Otherwise the room is deemed to be a utility room.