Measure the walls, then the room
Take each wall length at worktop height and again near the floor; older rooms are rarely square, and the smaller number is the one your cabinets must fit. Note the ceiling height in two corners, and the position of any boxed-in pipes or sloping ceilings that steal cabinet space. Enter the wall lengths as the room size and treat diagonal checks as a warning, not a decoration.
Fix every opening on paper
Measure doors and windows from a shared corner, including architraves and sill height, and mark which way each door swings. A window above the sink position decides your wall cabinet run; a door swing decides where tall units cannot go. Add these openings in the editor before placing cabinets, so the checks can warn you about real obstructions.
Record services and sockets
Mark the existing water, waste, gas and extractor points and every socket you intend to keep. Moving a sink along the same wall is usually simpler than crossing the room, and the hob position follows the extraction route. Photograph each wall as a backup; a photo settles arguments that a sketch started.
Questions before you start
How accurate do my measurements need to be?
Work to the nearest few millimetres for the final order, and re-measure after plastering or flooring changes. For a first layout, centimetre accuracy is enough to test whether the arrangement works.
What if my walls are not square?
Use the smaller wall-to-wall measurement for cabinet runs and expect filler pieces at the ends. Flag the discrepancy for your installer rather than forcing the plan to pretend the room is square.
