Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. However, most cosigners will need to meet income and credit requirements.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. 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. Should cosigner be added to the addendum since the cosigner is responsible for the 17year old' monthly rent. Adding a cosigner is pretty straightforward with these steps: Do your research: Treat the apartment cosigner as an additional tenant. The cosigner is equally responsible throughout the term of the lease. In fact, all signers are fully responsible for the full amount. This guide will provide all the necessary information you need to screen, process, and add a cosigner to a lease.