Contract law typically holds that a written contract, by virtue of existence, represents the entirety of the agreement. This renders inconsequential any terms or conditions that might have been delivered orally prior to the written contract. In order to incorporate any additional aspects of a written contract prior to its execution, it is necessary to produce a separate written addendum that will become incorporated as part of the original agreement.
Reference previous contract. By styling the document as an addendum, the implication is made that a prior contract is being modified. It is helpful, to specify exactly which contract the addendum is intended to be attached to, which is usually done by naming the parties to the contract and the date of execution.
Add new details. An addendum is merely an addition to a written contract that includes additional information about the terms and conditions. It should not contradict any of the terms and conditions of the original agreement.
Execute the document. An addendum should be signed separately by all parties to the contract at the same time the contract is signed. To aid enforceability, an addendum should be witnessed and, if possible, notarised.
An addendum is best used to add further clarity to the terms of the contract, either to the specific work to be done or to the conditions for payment. To modify a contract after it's executed, a contract amendment is the appropriate document.
One of the ways a contract can become void is if an assumed precondition of the contract turns out to be untrue. If this is the factor motivating an addendum, it's safer to start over with a new contract.