Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister in Islam, we would like to thank you for showing keenness on knowing the teachings of Islam, and we appreciate the great confidence you have in us. We hope our efforts meet your expectations. We are deeply sorry for the late reply.
With regard to your question, 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. In your case, you pay Zakah at the rate of 10% of the net amount of income from rented real estates (i.e., gross rent minus taxes and maintenance and other expenses)."
You can also read:
Zakah on Rental Property