GST for under construction flat

I have purchased a new flat and registered it in February 2017. I have paid all taxes as per agreement value(Full agreement). The project hasn't received OC yet. Bcoz of this the bank has not given the balance loan amount. We have already paid tax on this balance loan amount too. Now the builder is saying I have to pay GST on this balance amount if paid after 1st July,2017.
What does GST rules say about this?
Please Help