Long term capital gains calculation

Suppose if I now buy a plot(with cost 40 lacs after taking a loan from bank) and register it on guidance value( 20 lacs). After 1 year if I build a house on that plot (cost is around 20 lacs). After 10 years, after completing the loan, if I sell that house (with 1 crore) how the long term capital gains will be calculated ? I know that capital gains are calculated on total profit - total cost. In this case the capital gain should be calculated on 40 lacs profit. But how will I prove that time that the construction of the house took 20 lacs. At that point of time, I could say that construction of my house took 40 lacs and so capital gains will be calculated on (1,00,00,000 - (40,00,000 + 40,00,000)) or 20 lacs. How actually the capital gain calculation happens in this scenario ?