Builder changed size of property and asking money for added space

Hello sir, I have booked an under construction flat in lohegaon, pune in march 2015. Flat size(saleable) is 705 sq ft in agreement. now its nearing possession. but builder has altered structure mentioned in agreement and added new patch of 10 sq ft and he is asking to pay for that newly added space. he is also ready to make a new registration and bear all legal expenses. he need cost of that extra space only as per rate at the time of booking. however, he did not informed this before making this change. Now when I asked him why he has made this change anyway? he answered that "initially building plan (at the time of registration) was sanctioned by local authorities and lohegoan pune was a Grampanchayat. now lohegoan comes under PMRDA(new authority) and PMRDA has suggested this change. and if I don't pay I would not get completion certificate. and things can get complecated" also in my agreement with builder it is mentioned that : builder can change the structure of property with or without consent of buyer. however it not mentioned anywhere that builder can change size of property or buyer need pay for any kind of change made by builder. pls suggest. is it possible that one government body can force such kind of changes? am I bind to pay for extra amount? what legal options I have do deny paying extra money; if builder's demand is legally wrong?