I’m working on a certification on a very old building with walls having an average width of 120 cm (4 ft). The GFA calculation is therefore extremely high in relation to the net floor area. I would like to know whether in this case too I have to calculate the GFA gross floor area from the external profile of the wall or is there a specification for cases like this. I would like to know if there are specific GFA calculation methods for this case or I have to measure from the exterior face anyway

E.g. the Italian regulations define specific cases precisely for buildings of this type

Thanks in advance,
Have a good day!