The useful surface, as its name indicates, is the one that we can use. Partitions, structural pillars are discounted but cabinets or tables are not discounted.
The definition according to the Habitability Decree 141/2012 is the following.
“Interior useful surface: It is the area within the perimeter defined by the internal face of the enclosures of each habitable space. From the computation of the useful area, the area occupied by the interiors of the home, whether fixed or mobile, will be excluded. by the structural elements and by the pipes or ducts with a section horizontal greater than 0.01 m², as well as the surfaces of the areas with a free height less than 1.90 m under horizontal ceiling or 1.50 m in the spaces under cover with slope equal to or greater than 45º. When the house is developed in more than one floor, Although there are means of mechanical circulation, these must communicate always e by an interior stair and the surface occupied by the interior stair will be counted as a useful surface . “
It must be said that many architectural surfaces that are free are not useful. And on the other hand, there are many surfaces that are not accessible and are much more useful than those that we can walk on. But ask Jorn Utzon contemplating the Sea from his home in Mallorca.