The useful surface is equal to the living area of the dwelling plus 50% of the surface area of the private annexes. Example: The annexes included in the calculation of the useful surface are the following: Cellars and basements, Attics and convertible attics, Sheds and workshops, Outdoor dryers and cellars, Balconies and loggias, Verandas, Accessible terraces on the floor or semi-buried.
The HABITABLE surface is equal to the built floor area, after deduction of the surfaces occupied by walls, partitions, steps and stairwells, sheaths, doorways and windows.
NB: It can be calculated by the owner, but it is possible to have it done by a professional (real estate agency, surveyor…). In general, an error of 5% or more in the measurement engages the responsibility of the person who made the calculation. The annexes can be defined as secondary rooms, external and unfit for habitation.