Idle volume
The band of unused air above head height (roughly above 2 m) across a room's whole footprint — floor you have already paid for but never stand on. The raw material of the vertical Extend lever; the taller the ceiling, the more of it.
Paid-for air, doing nothing
A 100 sq ft room with a 3 m ceiling has ~100 cubic feet of idle air above reaching height. Claim it as high storage, a loft or a deck — the volume above your head is a small home's last frontier.