Greater Noida Authority — gst 12% implication on my remaining 70% amount instead of 4.5% service tax

Address:	Gautam Buddh Nagar, Uttar Pradesh, 201301
Website:	www.acecitynoidaextension.net.in

I booked a flat in Ace city in Aug 2016, at that time they charged 4.5% service tax. Now I get possession letter on which 12% GST is applied for remaining 70% amount. The builder got the CC in Feb 2017 and not charging any tax on new booking. Because of the implication of GST I've to pay 3lac(aprrox) extra amount to the builder. Please let me know how tackle the situation.