The Capital Gains Tax (CGT) is calculated based on the increase in the value of the property from the time it was acquired to the time it is sold. If you bought the plot in 2014 and started constructing a house on it in 2018, the CGT will be calculated on the difference between the sale price of the property and the cost of the land and the cost of construction incurred from 2018 (when construction began) until the time of sale. If you finished constructing the house before selling it, the cost of construction incurred until completion would also be added to the cost basis for calculating CGT.