Gst on flat

i have a query.

         I am planning to buy a new flat. the flat construction is completed and it is ready for the possession. the  construction is completed 1 year back. many people are living in the flat. when i asked the builder for price he said that you will have to pay GST 12% of the value. that comes to around 3 lakh in my  case and the flat is going out of budget. Kindly guide me in this matter. As per the details available on the internet we dont have to pay GST for construction completed before 1st July 2017.  please help me. thanking you all