Qto_BuildingElementProxyQuantities Quantity set for Building Element Proxies. IfcBuildingElementProxy IfcBuildingElementProxy NetSurfaceArea Net surface area of the object, normally generated as perimeter * length + 2 * cross section area taking into account possible processing features (cut-out's, etc.) or openings and recesses. Q_AREA NetVolume Total net volume of the object, taking into account possible processing features (cut-out's, etc.) or openings and recesses. Q_VOLUME