What Is a Cosigner for a Phoenix Rental Property? Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement.Com or ask your property manager about Leap. We will act as your co-signer to help you qualify for an apartment or rental home. In this article, we will break down what a guarantor is, when you might need one, and what to do if you cannot find someone to act as your guarantor. You might need a cosigner if you're legally an adult, especially if you have no credit history, limited income, or no prior rental history. Any "ground rules" are totally unenforceable. Most of the time the answer will be a very loud NO! A guarantor or cosigner for an apartment could help you fill financial gaps when applying for your rental. Find which is right for you.