The rate on long term capital gain tax is 20 % . Since your father had bought the bungalow in 1974, you have the option of taking the market value as on 1/4/1981 . You can get valuation done by registered valuer for market price as on 1.4.1981 and then claim indexation .The capital gain indexation for 1981 was 100 . Yes , you can also avail of exemption from LTCG by investing amount in Bonds u/s 54EC of the I T Act.
