Gst on under construction property for which vat is paid

No, you will not be liable to pay both the taxes, as when GST will be applicable, VAT & ST will not be charged.

Secondly, there is no clear cut directive for any such cases, and same is expected from CBEC; but if your builder is liable to pay GST over the remaining incompleted work, he is definately going to charge you 12% GST. How much he reduces the amount from the credits available to him under ITC, is again uncertain.

Thank You Mr. Rambhia!

Actually VAT was charged as a lumsum amount during registration of agreement, hence if I have to pay GST, then it will be like pay GST over and above already paid  VAT.

Any idea when these directives may come from CBEC? My builder is insiting on paying couple of installments now to avoid GST, but I am resistant as it may release his pressure to complete the work on time.

"Actually VAT was charged as a lumsum amount during registration of agreement......."

Morally liability to pay any taxes doesn't arise (already discharged). Secondly, builder can pay off the amount to VAT department, for that the actual payment is not mandatory. It can be debited to your acc for future payment!!

Its builder's tactics to get payments. But whether any liability  under GST arises or not (in your case), can't be said now.