Addendum: Legal Definition and Meaning
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In law, an addendum is a separate document attached to a contract that adds terms or information to it and, once agreed by the parties, becomes part of the agreement. It is typically used to add something, such as extra services, a disclosure or a schedule, while changes to existing clauses are usually made by an amendment.
What an addendum is, when it is binding, how it differs from an amendment, and an example.
Last updated 28 October 2026
Addendum meaning in plain English
"Addendum" is Latin for "something to be added" (plural: addenda). In a contract, an addendum is attached to the main agreement and, once properly agreed, forms part of it. It can be signed at the same time as the contract, such as a home purchase agreement with financing and inspection addenda, or later, when the parties decide to add something.
Addendum vs amendment
| Addendum | Amendment | |
|---|---|---|
| Main purpose | Adds new terms, items or information | Changes, deletes or replaces existing terms |
| Typical timing | At signing or later | After signing |
| Example | Adding a second property to a property management agreement | Changing the monthly fee in clause 4 from $2,000 to $2,400 |
| Usual form | A separate document attached to the contract | A document amending named clauses, or an amended and restated agreement |
The labels are often used loosely, and the title does not decide the legal effect: what matters is what the document says and whether it was validly agreed. If an addendum might conflict with the original contract, it should say which document prevails.
What makes an addendum binding
- Agreement of the parties. An addendum signed by only one party usually does not bind the other.
- The contract's own rules. Many contracts require any change to be in writing and signed by both parties. Follow that clause, often titled "Amendments" or "Entire Agreement".
- Consideration. Under traditional common-law rules, a promise to do or pay more for something already owed may need new consideration to be enforceable. For sales of goods, UCC §2-209(1) says an agreement modifying a contract needs no consideration to be binding, although it must still be made in good faith.
- Required disclosures. Some addenda exist because the law requires the information. For example, US sellers and landlords of most housing built before 1978 must disclose known lead-based paint hazards (42 U.S.C. §4852d), commonly through a lead-based paint disclosure addendum.
Addendum example
ADDENDUM NO. 1 TO SERVICES AGREEMENT. This Addendum is made on [date] between [Provider] ("Provider") and [Client] ("Client") and supplements the Services Agreement between them dated [date] (the "Agreement"). 1. Additional Services. Provider shall provide the monthly reporting services described in Exhibit A to this Addendum for a fee of $750 per month. 2. Effect. Except as set out in this Addendum, the Agreement remains unchanged and in full force and effect. If this Addendum conflicts with the Agreement, this Addendum controls as to the Additional Services.
Addendum in court filings
In litigation, an addendum is material attached to a brief or other filing. In US federal appeals, for example, the relevant parts of statutes or rules a court must study are set out in the brief or in an addendum at its end (Federal Rule of Appellate Procedure 28(f)).
How LexDraft helps
LexDraft can draft an addendum in Microsoft Word from a plain-English description of what you are adding, and its quick checks flag Title-Case phrases used without a definition, unused defined terms and cross-references with no matching target in the document. Start from the free contract addendum template.
Related terms
Frequently Asked Questions
What is the legal definition of an addendum?
An addendum is a document attached to a contract that adds terms or information to it and, once agreed by the parties, becomes part of the contract. It usually supplements the original agreement rather than rewriting its existing clauses.
Is an addendum legally binding?
Yes, if it is validly agreed: the parties must assent to it, it must meet any formalities the original contract or the law requires (often a signed writing), and, outside sales of goods, a change to existing obligations may need new consideration.
What is the difference between an addendum and an amendment?
An addendum usually adds new terms or information to a contract, while an amendment changes, deletes or replaces existing terms. The labels are often used loosely, so the content of the document matters more than its title.
Does an addendum need to be signed by both parties?
Usually yes. An addendum signed by only one party generally does not bind the other, and many contracts expressly require any change to be in writing and signed by all parties.
What does addendum mean in court?
In court filings, an addendum is material attached to a brief or other document, such as the text of statutes or rules the court needs to read. Federal Rule of Appellate Procedure 28(f) allows those provisions to be set out in an addendum at the end of a brief.
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Get Started with LexDraftFurther Reading
- Wex: Addendum (Cornell LII) — Legal encyclopedia entry on addenda.
- UCC §2-209 (Cornell LII) — Modification of contracts for the sale of goods.
- 42 U.S.C. §4852d (Cornell LII) — Lead-based paint disclosure on sale or lease of target housing.
- FRAP Rule 28 (Cornell LII) — Appellate briefs, including addenda of statutes and rules.