How is the Zakat calculated for land purchased at a low price with the intention of selling it later at a high price in order to build a house with its proceeds, knowing that one of the plots of land will be designated for residence, and is Zakat paid once or annually on its estimated value?

Zakat (definition) is not obligatory on land unless it is for trade. Therefore, land designated for construction is not subject to Zakat. As for land intended for trade, its value is estimated at the end of each lunar year (hawl), and its Zakat, amounting to 2.5% of its value, must be paid annually. If it was purchased with gold or currency, its hawl is the hawl of the money with which it was purchased. Zakat must be paid for each year, and paying it once for all years is not sufficient.