The information in this article applies to Chief Architect X3 and prior:
A room marked as Open Below in my plan has a floor instead of an opening to the floor below.
The Open Below room may be seen by the program as an independent building, separate from the rest of the plan.
In Chief Architect X4 and newer, the program will automatically connect "island"
rooms to the rest of the structure by generating an invisible wall or room divider.
To make the program recognize the Open Below room as a room within the larger plan, make sure that at least one of the walls defining this room is attached to another wall in the plan.