How the check works
The check has three steps. Drop in your builder's quote as a PDF or a photo, or paste the text. It reads every line and sorts each one into the 13 categories a full extension usually needs. You then check what it read, fix any line it put in the wrong category, and confirm. Last comes the report, which shows how the quote measures up.
Some of the report is free for every quote, every time. You see the quote total placed against the typical range for your region, size and spec, the cost per m², whether VAT is stated, how many categories the quote says nothing about, how many provisional sums it carries, and whether the price reads as fixed or as an estimate. You also see how many findings the analysis raised.
The full breakdown names the missing items and roughly what each adds, lists every finding against the lines it refers to, and gives you questions to send your builder. Your first full report is free once you sign in. After that, it comes with the Access Pass. If you would rather tick the categories yourself, the manual builder quote checker does the same scope sum without a document.
The check has limits worth knowing. The ranges are indicative, built from typical UK costs rather than a price for your particular site. It can only read the scope that is written down, so anything agreed on the phone is invisible to it. It is a check on the paperwork, and it says nothing about whether the builder is good at the job. A clear, complete quote from a builder with poor workmanship is still a risk, so references and site visits still matter.
Why the headline number misleads
On one documented 55m² kitchen extension, on an early 1980s detached house, the main builder quoted £45,500 for the full build. The true construction cost, with the kitchen itself left out, came to £122,150.
Most of that gap sat in allowances and extras. The quote carried provisional sums for three big items: windows and doors, electrics (priced per point) and plumbing. Each of those trades was paid directly, and each came in well above its allowance. The glazing alone, bifold doors plus roof windows, ended up far beyond the figure the quote had set aside for it. The structural steels, and the changes that followed once the real ground conditions and specification decisions replaced the assumptions, came through as extras on the builder's own invoices.
A few costs sat outside the quote altogether. The design and structural fees, the building control fee, the drainage and service trenches, and the gas and electricity service alterations were all paid to others.
None of that was hidden. The quote stated what it covered. The trouble is that a homeowner comparing totals reads the biggest number on the page as the cost of the extension, and it rarely is. A quote is a price for a scope, and until you know the scope, the price tells you very little.
What a good extension quote should include
A complete quote for an extension covers these 13 categories, or says clearly which ones are excluded and who is doing them instead:
- Foundations and footings
- Walls and blockwork
- Roof structure and roof covering
- Structural steels
- Scaffolding
- Waste removal
- Windows and external doors
- Electrics, first fix and second fix
- Plumbing, first fix and second fix
- Kitchen fitting
- Tiling, floor and walls
- Plastering and decoration
- Garden and external works
A quote can leave some of these out on purpose. Many homeowners buy windows direct or bring in their own electrician, and that is fine as long as you know it and budget for it. The risk is a category the quote simply never mentions. Silence usually means it is not included, and you find out when the builder asks who is doing it.
Look for detail inside each category too. "Electrics" with no number of sockets or lights, or "tiling" with no area or tile allowance, leaves room for the price to move later.
How to compare builder quotes like for like
Quotes only compare fairly when every builder prices the same thing. Send the same drawings and the same written specification to each one, and ask them to price against it line by line. Our guide to finding a builder covers how to shortlist, what to ask and how to check references.
When the quotes come back, lay them out side by side by category. For every category one builder includes and another leaves out, add a fair allowance for it to the quote that is missing it. Convert every figure to the price you will pay including VAT. Only then compare the totals. The difference between the cheapest and the dearest often shrinks sharply once the gaps are filled.
Check the payment terms as well as the price. A deposit in the range of 10–20% is common, and a deposit above 25% is a warning sign. The payment schedule calculator shows how stage payments should track the work. If you prefer to do the comparison on paper, the builder quote comparison spreadsheet sets out the categories for you.
Upload two or more quotes to the same project here and, with the Access Pass, the check does the like-for-like sum for you: each quote's gaps are priced back in, and the totals are set side by side.
Provisional sums, VAT, fixed price and estimate
A provisional sum is an allowance for work the builder cannot price yet, such as steels before the engineer has designed them or drainage before the ground is opened. The final bill uses the real cost, which can be higher or lower. A prime cost sum is similar but covers an item you choose, such as a kitchen or a set of doors, with the builder's fitting priced separately. Both are normal. A quote built mostly from them is closer to an estimate than a fixed price, so ask how each figure was arrived at.
VAT catches people out more than anything else. A VAT-registered builder adds 20% to most extension work, while a builder under the VAT registration threshold of £90,000 does not charge it at all. Two quotes for the same job can look very different for that reason alone. Check whether each figure is "inc VAT", "plus VAT" or silent, and treat silent as a question to ask before you compare.
The difference between a quote and an estimate matters when the costs move. A fixed-price quote commits the builder to the price for the scope described. An estimate is a guide and can change. Whichever you have, put the agreed scope and price in writing, and agree every change in writing before the work is done. Even with a fixed price, keep a contingency of 15-20% for the things nobody could have priced at the start.
Get the whole build in one place
A quote check tells you what a builder has priced. The Access Pass covers everything around it: the full kitchen extension task tree, phase by phase, with the order things need to happen in, what each trade does, what building control will inspect, and real costs from a documented build. It also includes your full quote reports and comparisons. See the Access Pass.