This statement needs updating to accommodate the new IfcFacility. In addition, I would question the implicit mandatory requirement of an IfcBuilding. Why is it invalid to simply have an IfcSite with some civil pipes in the ground? Or a heritage IfcSite with no real “building” per-se.
4.4.1.4 Spatial Composition
The order of spatial structure elements being included in the concept for builing projects are from high to low level: IfcProject, IfcSite, IfcBuilding, IfcBuildingStorey, and IfcSpace with IfcSite, IfcBuildingStorey and IfcSpace being optional levels. Therefore an spatial structure element can only be part of an element at the same or higher level.