A co-signer lease addendum, or “lease guaranty”, allows another person to personally guarantee the lease on behalf of the tenant. This means that if the tenant on the lease is not able to pay rent or damages the premises, that the “co-signer” will pay for any back-rent or other liabilities. A co-signer is common for younger tenants or those with poor credit. The landlord will require a co-signer for tenants that are considered “high risk” after screening through a rental application.