Do I need a written instruction to get paid for a change?

Usually the contract says yes, so treat every verbal direction as unpaid until you have confirmed it in writing yourself. An instruction is the formal order to change the work, and most contracts only oblige the other side to pay for changes instructed properly, which often means in writing, by a named person such as the contract administrator or project manager, and sometimes in a set format. A verbal direction, someone on site simply telling you to do something, is not necessarily fatal to your claim. Many contracts include a confirmation procedure letting you write up a verbal instruction and treat it as valid if no one objects within a set time, and even where the contract is silent, a same-day letter or email confirming what you were told, then going unanswered, is powerful evidence the direction was really given. What loses claims is doing the work on someone's say-so and raising the paperwork months later, by which point the other side can simply deny the conversation happened.

Picture an electrical subcontractor on an office fit-out. On a Tuesday walk-round the project manager points at a run of offices and says, off the cuff, move all the sockets to desk height and add two more per room, the paperwork can follow later. The subcontractor could just crack on, but instead sends an email that same afternoon: confirming your instruction this morning to raise the sockets to desk height and add two per office, this will be treated as a variation and priced unless you say otherwise by Friday. No reply comes, so the work goes ahead. Four months on, at the account stage, a new commercial manager refuses the extra, saying there is no signed instruction and no one recalls the conversation. The subcontractor produces the Tuesday email. Because it was sent the same day, described the exact work, and went unanswered by the person running the job, it does what a formal instruction would have done: it fixes what was asked and when. Relying only on the spoken word, the denial would probably have stuck.