Gst on under construction property for which vat is paid

Hi, I bought an under construction flat and paid VAT, stamp duty, registration etc charges in 2016. I have also paid service tax on the  installment amount paid to the builder till date. Possession of my flat is in Dec 2017 and till that time, I have to pay 4 more installments Q: Do i need to pay GST i.e. 12% on remaining installment or service tax @ 4.5%? If GST, it means I will be paying VAT as well as GST which is not the principle approach for GST implementation and may increase the burden on my pocket by 100k+ rs. Pls help me in sorting out the query.