Is it necessary to get succession certificate of a flat

I am hindu by religion. On the subject cited above I had requested whether is it binding for a widow to procure succession certificate when fact of the matter is that a flat was booked in Noida Extn. in the name of wife (Ist name) and husband, the husband died in March 2016. Almost 90% payment made, flat is nearing completion. All payments made from joint account of wife & husband. Now builder called for completion of formalities, and insisting for a succession certificate. The couple had only one daughter who is now in Australia. I maintain that since flat is still under construction and on the death of co-owner, the flat should automatically be in the name of widow, the first name owner. I shall be grateful if an early advice is given. Thanks