Draw a readable floor plan
Sketch each relevant room from above. Paper is sufficient if walls, openings and windows are clear. Measure wall sections individually and write the unit beside the figures. Do not rely only on an overall width when a recess or projection interrupts the space where the furniture would actually stand.
Add heaters, sockets and fixed fittings. Mark the directions in which doors and windows open. For tall furniture, measure height at the intended position: a sloping ceiling can change clearance within a short distance. Date the drawing so later layout changes are not confused with measurements taken before the room was rearranged.
Include the space for use
Measure more than a cupboard's width and depth. Allow for open doors and extended drawers. A desk also needs room for its chair and the person sitting down. Consider who needs to pass while the furniture is being used.
Temporarily mark the proposed outline on the floor using something suitable that removes without damage; paper strips may also help. Walk through the room and rehearse normal movements. Open existing doors fully.
Check the delivery route
Compare delivery dimensions with door openings, corridors and the transport route. Packaged or partly assembled furniture may differ substantially from its final dimensions. Use the actual product information as your starting point. A dismantlable item only helps if dismantling and reassembly are intended and practical for that particular piece.
Ask someone to check important figures, or measure them again later. Mark uncertainties instead of guessing. Keep the room sketch and product dimensions together. When planning several purchases, check them as a group: the first item that fits must not quietly consume the space intended for the next one.
