Roof top construction in bahria phase 8

I have noted that the map for my planned 10 marla house in phase 8 rawalpindi has a very small servant quarter on roof top (just 10 x 9 feet); and i want it to be bigger (13 x 13 if possible)
I am working out of pakistan and therefore can not approach bahria for this and my contractor is not sure if we can increase the covered area of mumti or top floor.
Can some respected members guide me on this? Thanks