Transferring booked apartment after my father passed away

Hello, I booked an apartment in Jan-2014 in name of my father and paid to builder from my bank account. Loan was taken in name of my father and bank has added me and my mother as co-applicants. All the EMI's are going from my bank account. In Aug-2014 my father passed away. I informed builder and bank about the same to transfer the flat to my mother and my name. Since bank had already taken the Legal Heir certificate during loan approval processing so they are not having any issues in removing my fathers name from the the loan. Initially builder agreed for the same and asked for notarized affidavit, notarized family tree and Assurance Indemnity bond. Now builder is asking me to provide relinquishment deed in my mother's name and saying that property can only be registered in my mothers name. They are asking transfer charges for adding my name. I don't want again putting single name in property. Regards