A fixed cockpit only needs room for the frame and the driver getting in and out. A future motion platform adds another question. Which parts of the cockpit will move, and what could they reach at the extremes of that movement? Answer it before you pick a monitor position or drill a cable pass-through.
Draw the parts that stay put
Get the installation guide for the platform you're considering. Confirm the manufacturer's mounting requirements and motion clearances for your planned setup. On a quick sketch, mark the screen stand, PC, walls and furniture as fixed obstacles. Then mark the seat, wheel deck, pedal tray and accessories that would move with your configuration. A microphone arm or the back of a reclining seat can extend beyond the footprint of the frame. If you are also choosing a frame, check the listed cockpit dimensions as you compare sim rigs.
Follow the cables across the boundary
Trace the power, display and USB cables between fixed and moving parts. Plan for the slack and attachment points the manufacturer specifies. A cable crossing that boundary needs room to travel without sagging into the mechanism or dragging against a bracket. Leave enough access to disconnect equipment according to the manual. If the instructions don't specify a clearance you can verify, ask the platform maker before installing.
Use the manufacturer’s setup and clearance checks once you install the system. Keep people and loose objects clear of the cockpit whenever it is powered and may move. For an example of the details to look for, see the Next Level Racing Motion Platform V3 manual.