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.Notify the applicant that they need someone to cosign their lease. Cosigning on a lease is essentially guaranteeing that, if another party does not pay under the terms of the lease, the cosigner will pick up the slack. Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. A lease cosigner is any thirdparty individual who agrees to fulfill the financial or legal obligations under the lease if the lessee is unable to do so. Use our Lease CoSigner Agreement addendum to extend the legal terms of a tenant's lease to a cosigner. When do I need a cosigner? A cosigner is someone who signs the lease along with the main tenant of the rental property. If both of you are approved, a co-signer agreement and lease are drawn up.