yeah, 156 sq m is 1680 sq ft... thats huge!!
the stud walls are usually at 1.2 m but there is not fixed regulation as to what they have to be at.... it all comes down to retaining the structural stability of the attic....
are you going from a cut roof to a dormer, or from a truss roof to a dormer???? i hope you have an architect / engineer engaged to inspect the work....
remember you also have to comply with fire regs if you are going to have bedrooms up there... (actually whatever you are going to use the rooms for, i would advise having the comply with escape regulations)...