There is a catch in your query. You said that 95% of the amount was paid by you in Nov-2017. However, the notification of reduction in rate of GST came on 25.01.2018 applicable from the same date.
As such, reduction in GST Rates to amount already paid and billed might not be applicable in the absence of any clear notification by the Govt. in this behalf.
However, there is possibly one solution in your case. Please check that whether the amount paid by you to the builder in Nov 2017 is booked by builder as advance or already billed in your name and GST paid by him. In case he has not yet billed the amount and not paid GST on the same to Govt. till date, he may pass on the reduced rate benefit to you. The date of invoice of the amount paid by you is important in this case.
Discuss with your builder and revert for any further clarifications if you want.