Gst rate changes effect for flat

Hi,

I have booked a flat in Bangalore under RERA in August, 2018 when it was under construction and the builder charged me 12% GST.  The builder has not delivered the flat to me till now. I heard that the GST council has reduced the GST rate to 7% for this segment. Wanted to know from the experts if I am eligible to get the benefits or not. If yes, how can I claim the same ?

Thanks,

Tanmoy