When you use design options, Revit computes room volumes using the following rules:
- To define the perimeter of the room, Revit uses the walls and room separation lines that are room-bounding for the design option. (See Design Options and Rooms.)
- To define the upper and lower boundaries of the room, Revit uses ceilings and floors defined in the current design option, in primary options of other option sets, and in the main model.