Measure more than the floor

Ceiling height changes what furniture reads as correct. A low sofa in a tall room looks marooned; a high-backed one in a low room makes the ceiling feel closer. The same piece can be right in one room and wrong in the next for no reason other than the height above it.

Window reveals matter because they decide where curtains and blinds can physically go, and because a radiator under a window rules out floor-length anything. Measure the depth of the reveal, not just the width of the opening.

Door swings are the classic omission. A door that opens into the room removes a quarter-circle of floor permanently, and that arc is almost always where somebody wants to put a chair. Draw it on the plan.

Sockets decide where lamps can live, and lamps decide where the room is comfortable. Mark every socket before choosing anything, because an extension lead across a walkway is the compromise nobody plans for and everybody ends up with.

Finally, measure the route in. The doorway, the hall, the turn at the top of the stairs. Sofas that will not go in are a genuinely common and entirely avoidable disaster, and the measurement that matters is the diagonal of the tightest turn.