1. GST paid by you to the promoter is an expense for you since you are unregistered under GST and not doing any business. Please check whether your house is falling under the category of affordable housing scheme of PMAY. If yes then GST rate is 8% instead of 12% otherwise it is 12%.
2. GST on construction of flat before 01.04.2019 is either 8% or 12%. What are other services on which he is asking GST@18%?
Please take phone consultation for detailed discussion.