is 12% gst is applicable for a residential duplex flat that have C .C.before March 2019

I have booked a duplex flat in kolkata in 2016 and I have submitted 80 percent money before the starting of G.S T with 4.5 percent service tax.i now opt to register my flat .the duplex have received c.c.in the month of March 2019.do I have to pay 12 percent money on the balance money .