Person claims that he has land on our constructed house property

Hi, We have house constructed in Bangalore 1993 and have been living there since then. All the documents are in our favor. But on one day we received a notice from a person who claims that he has the certain piece of land(10*15) which was sold to him by the owner of the land. Owner of the land is deceased now. That person shows the plot on paper but not willing to show it on ground. When we constructed the house, our compound corner was in front of land owner's main door. So the land owner requested to leave certain space for walk say 5 ft. Hence my father told him to give 10 ft at the back of the plot as it was not touching main road and he agreed. But this agreement was not registered and it is only on papers and it is signed by land owner. My question is, the one who is claiming the land, will he get benefit out of it? What are the chances of we winning the case if it run on court ? How can he claim the land when we are staying there from past 20 years and he is arising this issue suddenly after so much gap ? According to the law, is it mentioned anywhere that people staying at certain home for about 15 years or more are legally the owners of the land? Thanks,