From the limited facts mentioned by you (and in the absence of having seen your detailed documents), I can say that in the worst scenario, your case may be covered under the concept of adverse possession.
As per the provisions of the Limitation Act, 1963, limitation period for filing a suit for possession of immovable property based on title is 12 years. The longest period of limitation under Limitation Act in respect of an immovable property is 30 years for a mortgager to redeem or recover possession of immovable property mortgaged. That period has also expired in your case since you say that since 1965, you are occupying that property (if at all, such a claim was to be made). It is also laid down in the Limitation Act that on the expiry of the limitation period prescribed for instituting a suit for possession of any property, the right to such property is extinguished.
In these circumstances, it may not be possible for the successors of your grand father’s brother to institute a suit to recover the possession of the 20% area at such late stage, since you claim to be in peaceful and uncontested possession of the same for last about 52 years.
However, keep a proper record and all relevant documents ready with you to show the proof (if necessary, in future) that the building was constructed on the full area in 1965 (including on the extra 20% area) and that you / your family are in peaceful possession thereof since then.
As far as the payment of taxes is concerned, since the building is already in your name / possession, and the full area is also in your possession, I do not think the municipal / revenue authorities (as the case may be) will have any objection if you pay tax for the full area. If necessary, you may consult the concerned authorities.
What is being written here is on the basis of the limited facts mentioned by you. It may be advisable that you show all relevant papers / documents to a lawyer and discuss all relevant details with him for a detailed proper guidance.
Dr. Ashok Dhamija is a New Delhi based Supreme Court Advocate and author of law books. Read more about him by clicking here. List of his Forum Replies. List of his other articles. List of his Quora Answers. List of his YouTube Videos.