1. Where does the property situate?
2. If it is within the jurisdiction of Kolkata Corporation, then the period of 50 years is not a long time,
3. Has your grandfather constructed the house without any sanctioned plan?
4. If so, regularise the fault,
5. Make a cite plan/blue print of the house with the help of an approved draftsman including the proposed extension,
6. File an application for the said extention alongwith the cite plan being counter signed by a civil engineer duly complying with the formalities & paying the required fee,
7. After the plan is sanctioned, you can go for the said extension.