public ElementId BottomExtensionPlaneId { get; set; }

Public Property BottomExtensionPlaneId As ElementId
	Get
	Set

public:
property ElementId^ BottomExtensionPlaneId {
	ElementId^ get ();
	void set (ElementId^ value);
}

Analytical model elements for walls support extension options, while those for floor or slabs do not. This Plane identifies a Level, a Grid, or a Ref Plane. To validate plane use [!:Autodesk::Revit::DB::Structure::AnalyticalModelSurface::isPlaneValidForExtension] method. To chcek if analytical model support extension use HasExtension property. If used on analytical model that do not support extension, exception InvalidOperationException will be thrown.