Transfer of property

HI, We have property in Delhi the ownership of which was with my late grandfather. After his death ownership of the same was transferred to my grandmother automatically who is still alive. I want to know after my grandmother's death the ownership of the said property will be automatically transferred to my father who is an adopted child (without any legal documentation on adoption by my grandparents) or we have to execute a will for transfer of property to my father. Actually my grandmother’s health is not good & we are planning to have construction work on said property after her death so we do not have to put ourselves in legal complications for obtaining the approval for construction. Pls. advice.