Addendums are additional documents that are added to an agreement, such as a listing agreement or a purchase contract. Addendums are required to add language to contracts that are not included within the contracts themselves. 

For a listing agreement, some examples of addendums are the Seller's Disclosure Notice, Addendum for Seller's Disclosure of Information on Lead-Based Paint and Lead-Based-Paint Hazards, and Keybox Authorization by Tenant. For the purchase contract, addendums can be the Third Party Financing Addendum, Non-Realty Items Addendum, and the Addendum for Property Subject to Mandatory Membership in a Property Owners Association.