I have bought fixed property for investment, that is, to gain rent income.

Answer

If a property is purchased with the intention of gaining income by renting it out, then there is no Zakaat on the property itself. Only if the property is bought with the intention to sell it for profit, will it be Zakaatable. Zakaat will not be payable on the property even if you will sell it at a later date. Zakaat will be payable on the money you obtain for the property.