Clearance
The gap a piece of furniture needs to actually work — to open, to pass, to sit — over and above the floor it stands on. Measuring the room is half the job; knowing clearances is what makes a plan livable instead of just drawable.
What makes a plan livable
Useful Indian minimums: ~600mm to walk past, 750–900mm to open a wardrobe or pull out a chair, 1050mm for two to pass in a joint-family kitchen, 700mm in front of a WC. Ignore them and the plan fits on paper but fails in life.