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. What Is a Cosigner for a Phoenix Rental Property? When do I need a cosigner? Use our Lease CoSigner Agreement addendum to extend the legal terms of a tenant's lease to a cosigner. A cosigner is someone who signs the lease along with the main tenant of the rental property. Any "ground rules" are totally unenforceable. 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. Never cosign for someone.