If the property is kept as a merchandise in the market waiting for the increase in value to sell it, then it shall be treated as such, and thus zakah applies to it accordingly. In that case, its value should be estimated every year, and zakah should be paid along with other zakatable assets or savings. If you are not able to pay zakah due to lack of funds, then you may give it when you sell the same. But in that case, you pay for all the years, the property has remained in your possession.

 If, on the other hand, the property is kept for building house or rental facilities, etc., then you need not pay zakah on the property itself; rather you should pay zakah on the income you derive from it, when you derive it. As far as your own residential property is concerned, there is no zakah on it. As far as a rental property is concerned, zakah is not applicable on the property itself; rather it applies only to the rental income -- minus the expenses, debt payments and taxes, wherever applicable. Zakah is applicable at the rate of 2.5%.