Land that is bought with the intention of selling it after a while when prices have increased is subject to zakaah, because it comes under the heading of trade goods.
The value of trade goods should be worked out at the end of the year based on the current market price, regardless of the price for which they were bought, and zakaah must be paid. The rate of zakaah is one quarter of one tenth.
See the answer to question no. 38886.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: I have a piece of land and I am waiting for the prices of land to go up so that I can sell it, and it has been several years. Should I pay zakaah on it?
He replied: If a person buys land to make a profit then the land does not sell and becomes cheaper, and he keeps it until the price goes up, then he must pay zakaah on it every year, because it comes under the heading of trade goods. If he does not have anything with which to pay the zakaah, and he cannot find a buyer, then he should work out its price at the time when zakaah becomes obligatory and work out its zakaah, and in the following year he should work out the zakaah on its value, then in the third year and so on. Then whenever he sells it he should pay the total amount of zakaah that he had calculated. End quote.
Majmoo’ al-Fataawa (18/225).
He was also asked about a person who bought some land with the aim of trading in it, but it remained in his possession for a number of years. Does he have to pay zakaah on it?
He replied: If a person buys land for trading, then he has to pay zakaah every year, whether the value increases or decreases, and whether it sells quickly or it does not sell. He should work out its value every year at the current market price, then if he has money he should pay its zakaah from the money that he has, and if he does not have any money he should work out the zakaah for each year, and when he sells it he should pay the zakaah for the past years. End quote from Liqa’ al-Baab al-Shahri (15/12).
And Allaah knows best.