Is registration required

Hii.. i owned 2 BHK flat in Mumbai,700 sqft & is located on 7th Floor Beside, my Flat there is Open Terrace of 300 Sq ft area where the builder was suppose to construct flat , But due non availablity of FSI that flat was not constructed and it was just kept as open terrace by the builder . At the time of possession ,The builder has instructed me to use this open terrace area and took some amount from me for using the area till the further notice .The builder accordingly mentioned that open terrace area attached to my flat in BMC annexure -A copy for paying the BMC Accessment Tax , So i m paying the Accessment Tax for my flat as well as this open Terrace . Since from possession year 2011 , i am using that open Terrace Area and paying the BMC Assessment Tax . Now from 2017 , the society have been formed and they asking me to vacant the open terrace area , since now the open terrace belongs to Society . When i confirmed with the builder , he said he is ready to give me in writing that the open terrace has been sold to me and he has already taken the amount from me. My question is that if the builder gives in writing on his letter head stating that the open terrace has been sold to me then is it required to Register the same . Coz the Society is taking objection for registering the same saying that its an society property . Kindly Require Legal Opinion on the same. Regards Sandeep Jagtap