Gst on flat already booked

I have booked a flat with builder for which i have paid 30%on booking and on which composite service tax has been paid @ 3.05%. 1% vat was payable on agreement to be entered and registered. However builder says that 1% vat is payable now on the 30% advance paid as there is change in law wef 1st May 2017. Builder says that post july gst will be payable @ 12% on balance 70% payable. Presently i am paying 3.05% service tax & 1% vat which will become 12% gst.i.e. additional 7.95% on the 70% i.e. 5.5% of flat cost. Is this correct? If so what about the input credit available to builder shuld he not pass on this benefit to buyers. What if he says the flat cost is already agreed in allotment letter & doesn't reduce. As i understand in gst the seller is not suppose to take advantage but pass on the benefit to buyer. Please advice