According to SIA ordinance 416, the construction area (Konstruktionsfläche or KF) is the internal expanse of the floor area (Geschossfläche or GF) that is encompassed by the exterior perimeter walls and interior supporting walls together with any pillars, arches or buttresses.
 

Building elements such as moveable partition walls and built-in cupboards do not count as perimeter walls or internal supports in the context of this norm.
 

There is still a distinction made between load-bearing and non-load bearing construction elements.