[header] How to write an addendum to a lease [title] Understand the purpose of an addendum. [step] An addendum is an addition to an already agreed upon contract. The addendum can be used to add to, subtract from, or compliment the agreement that is already in place. 
The document will modify the terms of the original agreement and will legally control whatever its subject matter is about. [substeps] Unlike an addendum, an amendment to a lease involves changing the original contract itself.