Gst on residential property

HARDIK (Student) (42 Points)

09 November 2020  
I am going to purchase a flat from builder , in his projects some flats are already occupied and he is saying I have to pay gst @ 6% as he opted under old rates

as per my interpretation when some flats are already occupied then it satisfies first occupancy. and when first occupancy is satisfied then why I have to pay gst?