You had purchased a leasehold property and hence it was not your property it was just in your lease and hence you are just selling the rights in this lease and not the house property.
In such case you will be claiming exemption u/s 54F of the Income tax act and in that case to exempt your capital gain you need to invest entire sale consideration in new house.
If you cant do that while filing your return of income you will have to deposit same in capital gain account scheme before filing your return of income.
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