Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. A cosigner for an apartment takes on the financial responsibility for a lease.Learn how having a cosigner can help someone get approved for an apartment. My dad has agreed to cosign a lease with me but I am afraid my application will just be thrown out since I cannot provide a credit score or any proof of income. Cosigners are necessary for some lease agreements, particularly when an individual is a new or newly reformed renter. When do I need a cosigner? You will need to provide your cosigner with information about the rental property and the lease agreement. What information needs to be included in a cosigner addendum? What information needs to be included in a cosigner addendum? A cosigner guarantees that if the tenant fails to pay the rent, the cosigner will step in to meet the obligation.