A lease addendum is as important as the rest of the lease. Title the Document in a clear manner, such as "Lease Addendum" or "Rental Agreement Addendum".An addendum is an attachment to a contract that modifies the terms of the original contract. Read about insurance, lease, and real estate addendums. How Do I Add an Addendum to a Lease Agreement? Before they can move into one of your units, you must screen the tenant, submit four forms, and have the Housing Authority approve the rent amount. How to write an addendum to lease? Utilize our lease agreement addendum to implement alterations or additions to an existing residential lease agreement that is currently in effect. These forms are required for creating a legal lease or rental agreement based on the circumstances. The landlord would like to know if the County plans to renew its lease. Attachments.