Is GST applicable on builder flat possession balance amount?

Hello Sir, Greetings! I have booked a under construction flat in Noida Ext, May 2016 with plan of (30:70), 30% I had to pay early and 70% on possession. I get sectioned loan from SBI 80%, but in 30% payment I need to pay 10% and 20% should get disburse from bank. By 2th July, builder demand for balance 20% and same was disburse by 30th July 2016. Builder mentioned 4.5% ST on 30% payment. I got offer possession on 1st AUG 2017 of total 70% and on that amount he is charging 12% GST, which is huge for me, I tried to research on it and found builder got OC (Occupancy Certificate) on 17 Aug 2016 and after research over internet I found that they cant's even charge ST after getting OC, but they are charging GST. When I asked them they are saying you booked your flat before OC and you have to pay tax. I asked them to provide me rebate of 12% on 70% payment but they are saying we can only provide you 4.5% discount so you have to pay 7.5% tax on it. Please help me to understand if I am liable to pay GST/ST for his 70% balance payment while he got OC on AUG 2016? It would be great help. Thanks,