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What is a variation in a building contract?

A variation is an instructed change to the scope, quality or sequence of the works. It should be issued in writing with an agreed price and any time effect before the work is carried out, otherwise disputes over cost follow.

Variations are the main reason self-builds overrun. Each change looks small on its own, but late changes cost more than the same change made at design stage because they disrupt work already done or ordered.

Keep a running variations register showing the instruction, the agreed cost and the revised contract sum, and reconcile it at each stage payment.

Discipline that keeps costs down

  • Freeze the design before going to tender
  • Never instruct verbally on site
  • Price the change before the work is done
  • Track the cumulative effect on the contract sum

Sources

Last reviewed 28 August 2026. General information only — not legal or planning advice.

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