Gst for underconstruction flats

Tax on payment made to builder for underconstruction flats, upto 30/06/2017 comes under VAT & Service Tax (4.5%+1%=5.5%), and Payment made on or after 01/07/2017 is liable for GST(12%).

Is there any official notification or circular or clarification issued by CBEC or MahaVAT Dept reagrding this where it clearly states the above. 

Please share the same if available. 

Thank You.