Partial rental income

Dear Experts

Incase where a rented property lies vacant for a Financial year.

How do we take care of this ?

----  Any declaration that needs to be taken from the owner so that the non occupancy can be taken into account when deciding on his / her rental income ?

Eg - Incase rent is Rs.1000 / month and we say that for 6 months it is non accupied , then the rental income should be Rs.6000 for that financial year.

Your advise please.

Regards

Kiran