• Sale of unregistered house property

My father had bought a 1bhk flat in Kolkata 20 years ago at a price of 3 Lakhs. However, the developer didnot complete the construction and showed dispute and fled. Later, when a police report was filed and the developer was brought before court he showed bankruptcy and told that he didnot get full payments from the parties, also the land owners said they have some amount of money which is pending from developers end. The person was released on bail and the case kept on rolling for more than a year till the flat owners became impatient and decided to complete the remaining part of the construction through self expenses and took the possession. There was no registry or mutation happened as the developer, landowners, flatowners were not able to come to an aggreement. There were meetings in the past 20 years but nobody wanted to spend any more money. And were just happy to live with compromising. Now, our flat was mostly kept locked and were only being used when we visit kolkata for some pupose for the first 10 years 
Then, me and my brother came to kolkata for higher studies and started living there. Soon, we understood that the daily life there is terrible as the flat is ill maintained as nobody cares to give maintainance fee on time, the water supply was never properly completed and for the 24 hours water runs for only 15 minutes a day which was the major problem to live there. However, we somehow managed our college days and soon after getting placed and settled in a job we bought a new proper flat and moved there. Since then we have tried to give the flat on rent but very few wants to stay with all those problem. So, we decided to sale the property on whatever value we get and finally find a buyer willing to buy the property and will pay us 5 lakhs although only documents that we have is the agreement copy, a copy of deed and bank payment documents.
My question is how we should pay the taxes on this 5 lakhs rupees. Can we go ahead and simply show house property section of the income tax or will that create some situations or consequences.