Gst applicability in case of self development of residential society

if residential society developed it's society themselves by taking necessary BMC permissions towards premiums, FSI, tdr & pays for the same. Developer appointed by society, only execute the project & raise his bill to society. New flats will be sold by society only by executing sale agreement between society & prospective buyer. developer is not a part of new sale agreement. Then, 1. is society liable to pay gst on free flats for the area ( existing area + new free area) tobe be given to existing members of the society? 2. if yes how the value of flats should be determined, 3. what is rate of gst? 4 can society utilise ITC of purchase bills, bills raised by developer? 5 is society suppose to charge gst to their new buyer, at what rate of gst,?