Dr. Monzer Kahf, a prominent economist and counselor, states: “As for paying Zakah on rental incomes, the majority of classical contemporary jurists argue that the rent one gets in this case doesn’t matter, one just have to see at the end of one’s fiscal Zakah year (i.e., a lunar year from the date one’s income reaches Zakah-payable amount) how much one has as income, and then pay Zakah at 2.5% of this income as long as the amount is above the nisab (Zakah-payable amount).
Concerning the value of the rented real estates, more realistic opinion is that of Sheikh Yusuf Al-Qaradawi, who believes that one does not include the value of rented real estates (obviously your residence is not included) while estimating the nisab.