GST Liability on Landlord under JDA with Builder

Hello Experts

Need guidance on GST applicability if the land is given under GPA to Builder to develop with residential apartment on 50:50 sharing basis. Landlord will retain the residential flats for himself and is not interested in the sale of flats.

The Supplementary Agreement was completed and registered with sub-registrar in April 2017 ie before GST was implemented. The possession of property would be given to landlord in January 2018 with occupancy certificate (currently it's under construction).

Please advise do we (landlord) have to pay GST to the Government for getting these new flats under JDA ( joint development agreement) from Builder.

Builder advised that we need to pay 12% GST on the flat sale price ( equivalent) even if the flats are retained by us (no sale is involved in our share of flats).

Would really appreciate if a proper suggestion is given on the above context on GST to be paid by us (landlord) or not. Thanks.

Regards
Nitin