The quote I accepted for my kitchen extension said £45,500 on the front page. By completion, the builder had invoiced £51,007. And that was the well-behaved part of the budget. Another £69,000 of the £120,480 construction cost went to more than a dozen trades and suppliers his quote never mentioned: architect, structural engineer, electrician, plumber, drainage, screed, roofer, glazing, decoration, landscaping.
None of that made him a bad builder. It made me a homeowner who hadn't yet learned to read a quote. The headline total is the least useful number on the page. Working out how to compare builder quotes in the UK comes down to a single discipline: making every quote price the same job before you let the totals argue with each other.
Why Three Quotes for the Same Extension Never Match
Get three quotes for the same extension and you will get three numbers that can sit £10,000 or more apart. Homeowners read that spread as a quality signal. It almost never is. Quotes diverge for four structural reasons:
They bundle different scope. One builder prices the shell and leaves the finishing trades to you. Another prices everything through to decoration. On my build, a second firm quoted £53,400 plus VAT (around £64,080) for a bundled package covering steels, underfloor heating, electrics, plastering, flooring and decoration. The £45,500 quote covered the structural shell only. Both documents said "kitchen extension" on the front. They were pricing different jobs.
They set provisional sums differently. Where the real cost isn't known yet (ground conditions, fittings you haven't chosen), each builder plugs in an allowance. A builder who wants the job sets those allowances low, and the quote looks cheap without being cheap.
They exclude different things. Scaffolding, skip hire, building control fees, structural steel supply. Every exclusion moves cost off the page without removing it from your project.
They sit on different sides of the VAT registration threshold. A small firm under the threshold charges no VAT at all. A larger firm adds 20% to everything. That alone can put £10,000 between two otherwise identical quotes.
Until you correct for all four, comparing totals is comparing noise.
How to Compare Builder Quotes: The Normalisation Method
The fix is boring and it works: take away every builder's ability to define the job for you.
Write one brief. Before anyone quotes, put the scope in writing. Drawings, structural calculations if you have them, named materials and finishes where you've decided, and an explicit list of what the builder should include: scaffolding, waste removal, making good. Every builder quotes against the same document, which means every gap in a quote becomes visible instead of invisible.
Send it to a shortlist of three to five builders. Fewer than three and you have no baseline. More than five and you're wasting everyone's time, including yours. How you build that shortlist matters as much as the comparison itself, and it's covered properly in finding a builder and in how to find a builder you can trust.
Build a line-by-line comparison. One spreadsheet, one row per work package: preliminaries, groundworks, structure, roof, windows and doors, first fix, plastering, second fix, decoration, scaffolding, waste. Put each quote in its own column and force every line to hold a number, "included", "excluded", or "provisional". The step-by-step version of this method, along with the downloadable comparison spreadsheet and quote checklist, lives in getting quotes and comparing them.
Three quotes against one brief stop being three opinions about the job. They become three prices for the same thing, plus a list of questions to ask each builder.
Provisional Sums and PC Sums: Where Cheap Quotes Hide
A provisional sum is an allowance for work that can't be fully specified when the quote is written: ground conditions under the slab, a screed spec that depends on floor build-up, fittings you haven't chosen. A PC sum (prime cost sum) is a close cousin: an allowance for a specific named item, like windows and doors, whose exact supplier price isn't fixed yet. Neither is a price. Both are placeholders that will be replaced by real costs later, and real costs have a habit of arriving higher.
This isn't a textbook abstraction. The £45,500 quote on my build named three PC sums on its face: windows and doors at £15,000 to £20,000, electrics at £50 per point, plumbing at £1,500 to £2,500. Real supplier pricing, plus a roof-window addition, carried the actual glazing spend far past that allowance. Worse, the first glazing supplier went insolvent mid-project with around £10,340 of my money at risk, and the replacement order cost considerably more. A PC sum assumes the named item simply turns up. Sometimes the company holding your deposit doesn't survive long enough to deliver it.
Two things to check on every quote:
Whether the provisional sums are inside the total. Some quotes carry provisional sums outside the headline figure, so the number on page one is not the number you're committing to. Homeowners discover this mid-build, when invoices start exceeding the total they thought they'd agreed.
What proportion of the total is provisional. A £2,000 allowance inside a £60,000 quote is a rounding risk. The same £2,000 inside a £20,000 quote is 10% of the contract riding on a guess. The proportion matters more than the absolute size.
Warning
The Exclusions That Break Comparability
Every quote excludes something. The problem is that different quotes exclude different things, so the totals stop being comparable. The usual suspects:
- Scaffolding: £2,000 to £5,000 for a typical extension, and weekly hire keeps running if the build overruns
- Skip hire and muck-away: soil disposal from foundations is routinely left out
- Building control fees: £300 to £1,200 in England, paid to the council or approved inspector directly, never through the builder
- Party wall costs: surveyor fees if your neighbours dissent
- Structural steel supply: some builders price fitting only, with the steel itself supplied by you
- Kitchen supply and fit: almost always separate, and often the largest single line you'll place
- Decoration: "to plaster finish" means you're painting it
- Service connections: meter moves, new supplies, build-over agreements
£69,000
The point of the exclusions list on a quote isn't to shame the builder into including everything. Shell-only pricing is legitimate. The point is that you price every exclusion yourself and add it to that quote's column, so that a £52,000 quote that excludes roughly £8,000 of work stops beating a £59,000 quote that excludes nothing.
The VAT Trap: £9,600 Between Identical Quotes
Extension work is standard-rated at 20% VAT. There is no reduced rate and no zero rate for extending an existing house; those apply to new dwellings and certain conversions, never to a kitchen extension.
Whether VAT appears on your quote depends on the builder, though. The VAT registration threshold is £90,000 of annual turnover, frozen until April 2027. A small firm under that threshold legitimately charges no VAT. A firm over it must add 20%. So a £48,000 quote from an unregistered builder and a £48,000-plus-VAT quote from a registered one are actually £48,000 versus £57,600. That £9,600 gap says nothing about quality, workmanship or reliability. It's tax arithmetic.
It gets messier: registration status can change mid-project. On my build, the electrician became VAT-registered partway through, so his later invoices carried 20% where the earlier ones carried nothing. When you normalise quotes, convert every figure to its VAT-inclusive value first, then compare.
The Same Extension, Three Quotes, Normalised
Here's how the method plays out. Three plausible quotes for the same single-storey extension, before and after normalisation:
| Line | Quote A (£52,000) | Quote B (£64,000) | Quote C (£57,000) |
|---|---|---|---|
| VAT | None charged (firm under threshold) | Included at 20% | Excluded: add £11,400 |
| Scaffolding | Excluded: add £3,000 | Included | Included |
| Skip hire and muck-away | Excluded: add £1,500 | Included | Excluded: add £1,500 |
| Windows and doors | PC sum £8,000: realistic pricing adds £4,000 | Fixed price, named supplier | PC sum £10,000: realistic pricing adds £2,000 |
| Decoration | Excluded: add £2,500 | Included | Included |
| Normalised total | £63,000 | £64,000 | £71,900 |
On headline totals, Quote A looks £12,000 cheaper than Quote B. Normalised, the gap is roughly £1,000, and Quote C, the comfortable-looking middle option, turns out to be the most expensive of the three. That is the entire argument for doing this exercise: "cheapest" only means something once every column prices the same job.
Red Flags: The Cheap Quote That Costs More
Some patterns show up again and again in quotes that end badly:
A quote at half the price of the others. That isn't a bargain, it's missing scope. Somewhere in that document, work you need has been excluded, under-allowed or simply not understood.
A single lump sum with no breakdown. A builder who can't itemise his own price either hasn't measured the job or doesn't want you to see how he has.
Refusal to itemise when asked. Asking for a breakdown is a normal, professional request. A builder who treats it as an insult is telling you how variations will be handled later.
A large up-front deposit. Deposits and stage payments have well-established norms, and a demand well outside them is a cashflow warning. The mechanics are covered in contracts and payment schedules and in builder deposits and stage payments, so I won't repeat the percentages here.
Cash or "no VAT if you pay cash" offers. Beyond the legality problem, a cash job leaves you with no paper trail, no enforceable scope and usually no insurance backing if it goes wrong.
One more, from experience: watch for substitutions after the quote is agreed. On my build, the builder swapped a specified steel ridge beam for timber without telling anyone. Building control caught it, and the beam had to be replaced with the correct steel section. A quote that names materials only protects you if the named materials are what actually gets installed.
Warning
What a Good Quote Includes
Use this as your acceptance test. A quote worth comparing has:
- Itemised pricing by trade or phase, so you can see where the money sits
- Named materials and finishes, brands and specs rather than "quality flooring"
- Provisional sums and PC sums flagged as such, with each allowance stated and included in the total
- An explicit exclusions list, so nothing moves off the page silently
- Payment terms, tied to milestones rather than calendar dates
- VAT treatment stated plainly: registered or not, included or added
Tip
Quote comparison is one task inside the pre-construction phase, and it leans on decisions made before it (your specification, your shortlist) and feeds decisions after it (your contract, your payment schedule). You can see the whole build mapped out, every phase and task in sequence: browse the kitchen extension tree for free. If you want the full working detail for every task, from quote comparison spreadsheets through to snagging, the Access Pass (£49) opens all of it for the length of your build.
Frequently Asked Questions
Why are my builder quotes so different?
Because they're rarely pricing the same job. Differences in what's included (scaffolding, waste, decoration), how provisional sums are set, what specification each builder assumed for unstated items, and whether the firm is VAT-registered routinely put five figures between quotes for "the same" extension. Normalise all four before reading anything into the spread.
What is a provisional sum in a builder's quote?
A placeholder for work that can't be fully specified at quoting time, such as screed, ground conditions or fittings you haven't chosen. It's a budget allowance, not a fixed price, and it gets replaced by the real cost once the scope is known. A PC sum is the related term for a specific named item, like windows, whose supplier price isn't fixed yet. Check whether these sums sit inside or outside the headline total.
Should I just pick the cheapest builder quote?
Not on the headline number. A quote is often cheapest because it excludes more, sets lower allowances, or misses scope you'll pay for separately anyway. Price the exclusions, correct the VAT, challenge the provisional sums, and then compare. Once every quote covers the same job, the cheapest one is a genuine answer rather than an accounting illusion.
What should a builder's quote include?
Line items broken down by trade or phase, named materials and finishes, provisional and PC sums clearly flagged, a written exclusions list, payment terms tied to milestones, and explicit VAT treatment. A builder who won't provide that on request is showing you what the rest of the project will be like.