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What a standard variation clause requires in writing
Most builders know a variation clause exists somewhere in the contract. Far fewer follow what it requires. Here's what a standard JCT-style clause asks for, in plain terms.
Most standard forms of contract used in UK construction, JCT being the most common for smaller and mid-size jobs, include a variation clause that sets out exactly how changes to the works are meant to be instructed and valued. In practice, almost nobody follows it to the letter, and most of the time that's fine, until the one job where it isn't and a dispute lands.
The core requirement across most JCT-style clauses is that a variation should be instructed in writing by the contract administrator or client, or confirmed in writing within a set period if it was initially given verbally. That confirmation window matters far more than most contractors realise. It typically exists precisely so that a verbal instruction on site can be turned into a written one quickly, closing the exact evidential gap that causes disputes later.
The clause also usually sets out how the variation should be valued: by agreement, by reference to rates already in the contract, or on a fair and reasonable basis if neither applies. Contractors who skip the written confirmation step often end up in a weaker negotiating position on price too, because there's no documented scope to value against, just competing memories of what was agreed and what it should cost.
For domestic and smaller commercial jobs not run on a full JCT contract, the principle still holds even if the formal clause doesn't apply. Any reasonable court or adjudicator looking at a variation dispute is going to ask the same basic questions: was the change instructed, was it confirmed anywhere, and was a price discussed before the work went ahead. Contracts that don't reference JCT explicitly are still generally interpreted against that same backdrop of what's considered standard, reasonable practice in the industry.
The practical fix is treating the confirmation step as non-negotiable, not optional admin. Every variation, however it started, gets written down: what changed, who asked for it, and the cost implication, sent to the client the same day or the next. It satisfies the contract, it protects the relationship, and it's the single habit that keeps variation disputes from turning into anything more serious.