A contract addendum is an additional document that is added to a pre-existing legal contract to modify, clarify or update the terms of the agreement. It’s important to note that a contract addendum is not a completely new agreement – rather it simply acts in addition to the original contract. Contract addendums are often used when parties to the contract feel that changes are necessary due to changing circumstances or new agreements that have been made since the original contract was written. When created properly, a contract addendum can be just as enforceable as the original contract itself.