The best time to plan your AC is before construction is finished. Changes made later are slower and more expensive.
Your checklist
List every room with its size and use. Decide on the AC type for each area. Plan the position of outdoor units so they have good airflow and are easy to reach for service. Mark the ducting and drain routes. Check that the electrical capacity can handle the full load.
Think about the future
Leave space for servicing and for any later extension. A well-planned system is quieter, cheaper to run and easier to maintain.
