Is zakat obligatory on one who buys land with money that has reached the nisab in order to build a secondary residence on it, knowing that he owns sufficient housing, and is zakat paid on this residence annually or only when it is sold?

If you purchased land with the intention of selling it, zakat is obligatory on it and on any house built upon it, as both are considered trade goods. However, if there was no intention of trading with them, then there is no zakat on them. Zakat, however, becomes obligatory on the house's rent if it reaches the threshold (nisab) and a full year (hawl) has passed on it. Zakat on money must be paid for every year that passed on it before it was used to purchase the land.