How do I stop extras turning into a bill I did not expect?

Building projects change, and that is normal — walls open up and reveal things, and clients change their minds. What is not normal is finding out what those changes cost at the end. The discipline that prevents it is small, and it has to be established in the first week, before the first informal "while we're at it" conversation happens on site.

Step by step

1. Put the rule in the contract before anything starts.No variation is carried out without a written instruction, and no variation is carried out without an agreed price. A builder who has agreed to this at the start will follow it; one asked to follow it mid-job will not.
2. Use a single running email thread.Every variation on the same thread, numbered, with date, description, price and programme effect. At the end you have the whole history of the job in date order without anyone reconstructing it.
3. Distinguish the three kinds of extra.Genuinely unforeseen — rotten joists behind the plaster. A client change — you moved the door. And an omission — something that should have been in the quote and was not. They are handled differently, and the third is a conversation about the quote, not about an extra.
4. Ask for a price before authorising, every time.Including small ones. "Go ahead and let me know" is how a job with a firm quote ends up with an open-ended final invoice.
5. Ask for the programme effect too.A variation that adds a day to the job is different from one that adds three weeks because a bespoke item has to be ordered. Both facts belong in the same email.
6. Watch provisional sums closely.Provisional sums in the quote get replaced by real prices as the work is defined. Ask for the real price when the item comes up, and compare it with the sum allowed before instructing.
7. Push back on omissions politely but clearly.If something obviously necessary was missing from a fixed-price quote, say so calmly and in writing at the time. It is a much easier conversation on the day than at the final account.
8. Keep a running total.Contract sum, plus agreed variations, equals the current expected final figure. Update it as you go. Nobody is ever surprised by a number they have been watching.
9. Reconcile the final account against the thread.The final invoice should equal the contract sum plus the numbered, priced variations you agreed. Anything else needs an explanation before it is paid.

Tips

• Decide the things that are expensive to change — kitchen layout, socket positions, bathroom position — before first fix. Changes after first fix are the most expensive variations there are.
• A contingency of a sensible percentage, held separately in your own budget, turns most unforeseen items from a crisis into a decision.
• Being unavailable is expensive. Variations wait for your answer, and a job standing still costs both of you.

Common mistakes

• Agreeing extras verbally on site — Nobody writes it down, both parties remember a different figure, and the difference appears on the final invoice when you have no leverage left.
• Authorising work without a price — You have given up the only moment at which the price was negotiable. Once the work is done, the argument is about value rather than about whether to proceed.

If it doesn't work

Questions people ask