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.
In this fatwa:
You should pay zakah on the rent you get from the flats and the shop.
Answering your question, Dr. Jasser Auda, Professor and Al-Shatibi Chair of Maqasid Studies at the International Peace College South Africa, states:
You should pay zakah on the rent you get from the rental properties if it reaches the nisab and one full year has passed since acquiring that rent.
You do not have to pay zakah on the property itself.
One quarter of one tenth (2.5%) must be given as zakah.
For example, if the rent is $15.000, zakah can be calculated like this: $15.000*2.5%=$375.
Almighty Allah knows best.