An underwater building is an enclosed, inhabitable building which is permanently rooted or anchored in place, and which is at least 90% under the surface of a body of water.

The enclosed space is inhabitable despite its submerged position. The definition concerns that occupiable space, whose underwater share must meet the stated 90% threshold.

Permanent rooting or anchoring distinguishes the building from a vessel moving through the water. A structure merely standing beside water, or having only a small submerged room, does not meet the stated underwater condition.