From the sale consideration (advance+balance) that will be received by you, the Indexed cost of acquisition of the housing board property has to be deducted. The remaining amount is called Capital Gain. You have to pay 20% tax on the Capital Gain.
There are certain important items to be noted by you.
1. First you have to know (i) the month and year in which the property was purchased from the Housing Board by your grandmother. (ii) the original cost of the property (i.e. the amount paid by your grand mother to the housing board) (iii) the charges if any paid for conversion to freehold (iv) the amount of stamp duty paid. On the basis of the amounts shown in respect of (ii), (iii) and (iv), you have to arrive at the Indexed Cost of Acquisition of the property.
2. First you arrive at the amount of capital gain and the tax payable on it.
3. There are ways in which the capital gains tax can be avoided. One such way is to invest the amount of capital gains (not the entire sale consideration) in the Bonds issued by National High Way Authority of India. The bonds carry 5.25% interest. The interest income is taxable. Currently, i.e. if the Bonds are purchased before 31.3.2018, the amount will be locked for 3 years. However, after 1.4.2018, if NHAI bonds are purchased, then it will be locked for 5 years. Please note, in spite of the interest rate being very low, still at the end of 3 years or 5 years, it will be beneficial THAN paying the Capital gains tax.