A basement floor is a fully underground floor level at least 2 meters in height, with sufficient room for persons to move about freely (not counting any wall barriers that may impede this). All fully underground levels of regular height are counted as basements, whether they are occupied, empty, hold mechanical equipment, are sloped or ramped, or extend to the full building perimeter. Therefore basement mechanical floors and basement mezzanines are also counted as basement floors.

The level must be fully underground and at least 2 meters high. A partly sunken level belongs to the garden-floor form, while inadequate headroom can make an underground space a crawl space.

Full underground position takes precedence over the usual above-ground distinctions for mechanical, ramped, and mezzanine floors. Such levels still count as basements when they meet the height and movement conditions.