Pagadi property sold to builder

Pagadi Property sold to builder by owner. 15 tenants . Builder reconstructing property. Ready to give 50 sq MTR more(free). Average area is 250 extra given 50 equals to 300. Builder asking for construction cost of 1800/- and extra for any area to be increased as per the market rate. Builders challenge is of FSI he says he will not give 50area if not possible. A notice by municipal corporation is already displayed to vacate the building. My question is 1) can the builder charge construction cost 2) how much area he is liable to give free of cost under the reconstruction law 3)what is the HOST in this case