A contract addendum is an agreed-upon addition signed by all parties to the original contract. It details the specific terms, clauses, sections and definitions to be changed in the original contract but otherwise leaves it in full force and effect. Contract addendums are tricky to write, because contract law is very clear that all parties must abide by the contract as it stands. The goal when writing a contract addendum is to only change the parts that all parties want to change while not creating any loopholes or unintended consequences in the agreement as it stands in writing.
Reference the Original Contract
You should make sure that your addendum looks like and is connected to the original contract. It should use the same typeface, margins and font size, and it should be titled, "Addendum to _________ of (date)."
The beginning section should list the parties to the original contract and should specify what certain terms mean, for instance "the Contract" should be specified as the original contract, and "Addendum" should be defined as such. The addendum should specify the date at which it comes into effect.
The most important part of the addendum is to list the terms and parts of the original contract that it modifies. Write things like, "The Contract shall be modified as follows," with the old written portion and the new written portion side by side so that the differences are obvious. You may also use the strikethrough and bold styles to emphasize the added and removed parts, but be sure to identify what each style means. Any deletions from or additions to the original contract should be listed in the body of the addendum as well. Be clear, thorough and concise in your writing.
Finally, you should add a signature line at the end of the addendum so that all parties in the original contract may sign it. The original contract should be affixed to the addendum and filed by all parties.
Getting an Attorney's Help
If the contract is a serious agreement with a lot of money at stake, it is highly recommended that you consult with an attorney to look over your original contract and addendum to make sure that it is airtight and that there are no loopholes created by the new additions. This is especially the case for long contracts where a change in one definition or term may affect the meaning of another.