Is Zakat obligatory on two plots of land owned by an expatriate, a year having passed over them, when he intends to sell one of them to build a house on the other, or to sell both of them together and buy another plot for building upon it? And how is Zakat calculated if it is obligatory?

Zakat is obligatory on land if it was purchased with the intention of trade. It is appraised at the end of the year (upon the completion of its hawl) based on the original capital with which it was bought, and Zakat is paid at a rate of 2.5% of its value. However, if it was not purchased with the intention of trade, then there is no Zakat due on it, even if you intend to sell it later.