Regarding GST rate applicable

Dear Sir/Madam, I had booked a plot of land measuring 1150 sq ft with a builder at Rs 150000/- in August 2019. He was to build a duplex in the said plot of land with built up area 2061 sq ft( Ground Floor- 837sq ft, 1st Floor 837 sq ft and 2 nd Floor 387 sq ft) and sell the duplex along with the piece of land to me. The total consideration amount agreed was Rs 6500000/-, out of which Rs 3500000/- was towards piece of vacant land and Rs 3000000/- was towards construction of duplex. I had applied for loan from bank and as per the terms sale deed for purchase of land was executed on 06/03/2020.I paid Rs 3500000/- to the builder at that time. The construction is going and likely to be completed within 2 months from now. Now the builder is claiming GST at 18% on the whole consideration amount of Rs 6500000/-. But as per CGST norms, GST rate for non affordable residential projects commencing after 01.04.2019 is/ should be 5%. But he is not agreeing on the same. Can you please guide at what % i am liable to pay GST. Thanks and Regards, Sunitha