It usually lands on day two or three. The strip-out took a day and a half instead of a morning, the skip’s already full, and you do the sum in your head while you’re loading the van: at this rate you’re working the back half of this job for nothing.

The temptation at that point is to do something daft — go quiet, slow down, start looking for a reason to blame the customer, or send an invoice at the end that doesn’t match the quote and hope nobody notices. All of those cost you more than the money ever would. There’s a cleaner way through it, and it starts with being honest with yourself about which of two very different problems you’ve actually got.

First, work out what actually went wrong

Before you say a word to the customer, separate these:

The scope changed. The job in front of you is not the job you priced. They’ve asked for something extra, or you’ve opened up and found a condition nobody could see at survey — rotten joist ends, no insulation where there should be, a soil pipe exactly where the units go. The work has grown.

You underpriced it. The job is exactly as described, exactly as you surveyed it, and you simply got the number wrong. You allowed two days for three days’ work, forgot the waste, used last year’s material prices, or quoted low because you wanted it.

Only the first of those justifies asking for more money. The second is yours.

That line matters because customers can usually tell which one they’re being sold. Dressing up your own mistake as a “variation” — especially a vague one, raised late — is the quickest way to turn a happy customer into someone who checks every line of your invoice and tells their neighbours to do the same. Whereas a genuine discovery, raised properly, is usually met with a shrug and a yes.

Be strict about the grey area too. “I didn’t realise the loft access was that tight” isn’t a discovery if you stood in the hallway and looked at it. Access, parking, stairs and anything you could have measured are your risk once you’ve been on site.

If the scope genuinely grew: raise it now, in writing

Not at the end of the week. Not on the final invoice. Before you do the work.

Once it’s done, the customer has lost the only real choice they had — to say no, or to say “not at that price”. Even when the extra was needed and the price is fair, presenting it afterwards makes you the person who spent their money without asking.

Stop, photograph it, and send a message from where you’re standing:

Hi Sam — I’ve got the bath out and the joist ends underneath have gone soft where the old waste has been leaking. I can’t tile over that in good conscience. To cut out and sister the two affected joists is about £240 and most of a day, which pushes us to Friday. Say the word and I’ll get on with it. Photo attached.

Four things in one text: what it is, what it costs, permission asked, and the knock-on effect. Their “yes, go ahead” is your authority and your record. If this is a regular occurrence on your jobs, the fix sits further upstream — in how you price the things you can’t see and how tightly you write the scope before you start.

If you simply got it wrong: honour the price

This is the hard one, and the answer is boring. You gave a fixed price, they accepted it, you build it. Finish it properly, to the standard you’d have delivered at the right price, and treat the shortfall as tuition.

It costs less than the alternatives. A customer who feels overcharged doesn’t just withhold the difference — they pay slowly, leave a review you’ll be answering for two years, and never refer you. A job you lost £400 of margin on but finished well still produces the next three jobs.

What honouring it actually looks like:

Don’t let the finish slip. The thing you’ll be judged on in five years is the silicone line, not the price. Cutting corners to claw back half a day converts a quiet loss into a visible one.
Don’t drag it out. Every extra day on a job you’re losing on is a day not earning on the next one. Get in, get it done, get off.
Don’t go cold. Trades under financial pressure stop answering the phone. The customer reads silence as a problem with them, and starts worrying about their money.
Don’t take it out on the extras. Overpricing the shaver socket to recover the shortfall is the same mistake with extra steps.
Don’t tell them. “I’ve underpriced this, you know” sounds like a request for money. Either you’re asking or you’re not — and if you’re not, you’re just making them uncomfortable.

The rare exception

Occasionally it’s not a bruise, it’s a catastrophe — the sort of error that means finishing the job genuinely threatens the business. Then you can ask, but you ask straight, and you ask early:

I need to be upfront with you. I’ve made a mistake in my pricing for this — I’ve missed the [X] entirely and that’s on me, not you. I’m not going to leave you with a half-finished bathroom. What I’d like to do is talk about whether there’s a fair way to share it. If the answer’s no, I’ll honour the price we agreed and finish it.

That last sentence is doing the work. You’ve been honest, you’ve asked, and you’ve told them their answer is safe. Some will meet you halfway; it’s their call, not yours, and their “no” ends the conversation. If they push back on price after that, it’s an ordinary price objection — answer it on value, not on your own cash flow.

Cost it properly before the memory fades

The only thing that makes a loss worth anything is knowing exactly where it went. Do this in the van on the last day, not six months later:

	Quoted	Actual	Difference
Labour (days)	4	6	−2 days
Materials	£X	£X
Waste / skips	£0	£X	Missed entirely
Sundries	£X	£X
Travel / parking	—	£X	Missed entirely

Nine times out of ten the damage isn’t a mystery. It’s one line — the labour estimate, or something you forgot to price at all. Write it down.

Stopping the next one

Survey it properly, every time. The jobs that go wrong are nearly always the ones priced from the doorway or off a few photos on WhatsApp.
Price the invisible work. Loading out, tip runs, protecting floors, isolating services, the half-day of making good, the trip back for the one missing fitting. Build your quote in stages on site so you don’t skip whole categories.
Check what a day actually costs you. If your rate hasn’t moved while materials, fuel and insurance have, you’re underpricing every job, not just this one. Worth rebuilding the number from your real overheads.
Write the exclusions down. “Price assumes sound existing substrate” takes five seconds and is the difference between a conversation and an argument.
Add a contingency on opening-up jobs — and tell the customer it’s there. If it isn’t needed, hand it back. That gesture earns more referrals than it costs.
Don’t quote tired. Pricing at 9pm after a full day on the tools is where the missing day comes from.

One underpriced job a year is the cost of doing business. The same mistake four times a year is a pricing method that needs fixing.

The takeaway

Separate the two problems first: if the job grew, raise it today, in writing, with a photo and a price, before you do the work. If you simply got your own number wrong, honour it, finish it well, and get off site fast. Then cost it line by line while it’s fresh, so the loss buys you something. The trades with the best reputations aren’t the ones who never misprice — they’re the ones whose customers never had to find out.

Queezy lets you build an itemised quote on site, stage by stage, so the waste, the sundries and the making good are priced before you leave the driveway — download it free on iPhone.

What to Do When You've Underpriced a Job