How to charge for work that was not quoted
The floor comes up and two joists are rotten. Nobody knew. It was not in the quote because it could not have been, and now it has to be done before anything else can happen.
What happens next decides whether this job makes money. Not the joists: the conversation about the joists, and specifically whether it happens before the work or after the bill.
The version that costs you
You are on site, the customer is at work, the joists need doing today because the tiler comes Thursday. So you do them, mentally note that it is another day and £850 of timber and labour, and carry on.
Three weeks later the invoice goes out at £5,050 against a quote of £4,200. And now you are having the conversation, except now it is a conversation about money already spent, with somebody who has had three weeks to plan around a number you have just changed.
They are not being difficult. From where they sit, a job they had budgeted went up by a fifth without anybody asking them. Every part of that sentence is their reasonable complaint, and none of it is about whether the joists needed doing.
Ninety seconds, at the time
The fix is an email sent from the van, before the timber goes in.
"Lifted the bathroom floor this morning and two joists under the bath are rotten, photos attached. They have to be replaced before the new floor goes down. That is £850 including the extra day, which would take the job to £5,050. Happy to proceed on that basis? Just reply yes and I will carry on."
That is it. Photograph, cause, price, new total, and a request for a yes. Ninety seconds, and the argument has moved from after the bill to before the work, where it is not an argument at all.
Three things in that message are doing the work. The photo, because rotten joists are obvious in a picture and abstract in a sentence. The new total rather than only the extra, because £850 is a number they have to add to something and £5,050 is the number they actually care about. And the explicit ask, because a message that merely informs is a message they can leave unanswered and later say they never agreed to.
What to do when they say no
Sometimes they will. That is a legitimate answer and it is far better than silence.
Usually "no" means "not at that price" or "not right now", and there is a version of the job that works. The joists get done and the tiling waits until next month. Or the scope shrinks somewhere else to pay for it. What you cannot do is proceed anyway because it obviously needs doing, and then bill for it. A variation they declined and you did regardless is a variation you will be arguing about.
Where it is genuinely unsafe or non compliant to carry on, say that plainly and stop. "I cannot certify the installation with that consumer unit in place, so I have to either change it or leave the work incomplete" is a real position and a customer can decide against it. Doing it anyway and adding it to the bill is not.
On commercial work, the email is not enough
A main contractor's system is matching a purchase order to an invoice. If the order says £4,200 and the invoice says £5,050 it does not match, and nothing about a friendly email agreement fixes that, because the person who agreed and the system that pays are not the same thing.
So ask for the order to be amended, in the same message where you ask for the variation. "Can you raise an amended PO for £5,050, or a separate one for the £850, whichever suits your system." Then invoice against what the paperwork says rather than what the site manager said.
Retention applies to variations too, which people forget when the variation is the last thing on the job. Five per cent held on a variation is the same five per cent held on everything else, and it is released at the same time, not when the variation is finished.
Where variations come from in the first place
Some are genuinely unforeseeable. Rotten joists, a buried cable, a wall that turns out not to be where the plan says.
Most are not. Most variations are things the quote did not exclude, and the customer therefore assumed were in. Decorating. Making good. Taking the old suite to the tip. Moving the furniture. None of those are surprises about the building; they are gaps in the document.
Which is why the exclusions on the original estimate are worth more than any variation process. Three lines saying what is not included removes most of the variations you would otherwise have to negotiate, because a customer who read "decorating not included" either arranged a decorator or asked you to price it, back when it was cheap to decide.
The habit worth building
Anything that changes the price gets an email before it happens. Not a text, because texts get deleted and are hard to find in six months. Not a conversation, because conversations are remembered differently by the two people who had them.
One email, one photo, one number, one question. Sent from the van, before the tools come out.