Mezzanine
A partial intermediate floor you can stand on, overlooking the space below — the most ambitious vertical move. Needs true double-height volume (~4 m+), so it suits heritage, industrial-conversion or purpose-built tall spaces, rarely a standard flat.
A stand-up half-floor
A mezzanine is architecture, not carpentry — structural design, a proper stair, often permissions. Worth it only where genuine double height exists; forced into an ordinary flat it fails on every clearance.