GST Liability on Landowners in Joint Development Agreement

Need your guidance on GST applicability if the land is given under GPA to Builder to develop with residential apartment on 50:50 sharing basis and there is NO monetary transaction involved in the agreement. Landlord will retain his share of residential flats for himself and is not interested in the sale of flats. The GPA agreement was registered with Sub-registrar office for the development of apartment and later Supplementary Agreement was completed by identifying the flats of Landlord and Builder, and registered with sub-registrar office in April 2017 ie before GST was implemented. The actual possession of flat would be given to landlord in January 2018 with occupancy certificate (currently the apartment is under construction). Please advise do we (landlord) have to pay any GST to the Government for getting these new flats under JDA ( joint development agreement) from Builder for self occupancy. Builder advised that we need to pay 12% GST on the flat sale price (equivalent) even if the flats are retained by us. There is no sale of flats involved in our share of flats. These flats would be retained by us for our self possession. Would really appreciate if a proper suggestion is given on the above context on GST to be paid by us (landlord) or not. Many thanks in advance. Regards Nitin