To amend a contract, both parties generally have to approve it before the changes take effect, preferably in writing. Unilateral derogations (i.e., where only one party can make an amendment) are only valid in certain circumstances, if they have been the subject of prior agreement. Persistent minor behaviour or offences (i.e., a party has repeatedly violated the treaty) may lead to a tacit change in the contract. This generally applies to work contracts or when a contractor has to meet certain deadlines. If one party does or does not do something that affects the other party to meet the deadlines, an implied clause may be created to extend the period of time. In this case, the party resulting from the modification of the contract must demonstrate that there is a clear pattern of conduct that is inconsistent with the terms of the original contract and that is consistent only with the parties` agreement to change those conditions. In other words, a party will not be able to justify a change in behaviour if the parties had acted or acted exactly as they would have done in the absence of such an agreed amendment. It is therefore often very difficult to find that a contract has been altered by the behaviour, so it is wise for the parties to record the changes in writing in order to avoid disputes over the terms of their relationship. However, until recently, there was some uncertainty as to the binding nature of these clauses.

Despite the clear wording of these variation clauses, they would have led to conflicting decisions of the English court. In one case, the Court of Appeal found that the parties could change their agreement orally orally or by conduct, even though the agreement expressly stipulates that the amendments must be made in writing. In essence, the Court held that, when they agreed orally to amend a substantial part of their agreement, the parties also tacitly agreed that the “written amendment” clause no longer applied. However, following a Supreme Court decision in May 2018, this approach is no longer a right.