Not a substantial material cost overall for wood buildings (concrete is another discussion), more time and logistics to tie them in than anything, but the VAST majority of multi-family dwellers consider a balcony or 'outdoor space' as a top priority, even though usage rates clearly indicate otherwise. I suggest that it is mind over matter, given that folks perceive the need, but in reality rooftop social spaces with lounges, Bbqs, kitchens etc. are much better options and/or simply heading down for a walk to the park or patio.