Addendum

A supplementary document attached to an existing contract that adds new terms or provisions without altering the original ones.

Example

A retainer agreement gets an addendum adding a new deliverable category (paid social management) for an additional monthly fee, while every other clause in the original contract stays unchanged.

Why it matters for agencies

An addendum is often faster to execute than renegotiating a full contract, because it only needs the client's sign-off on the new material, not a re-read of everything already agreed. That speed matters when an agency wants to formalize a scope change quickly rather than let it slide by informally.