DATA-BACKED GUIDE · UPDATED JULY 2026
How to read a builder's quote and spot the padding
Reading a builder's quote properly means checking three things: what is actually included, which figures are only estimates rather than fixed prices, and where vague wording lets extra cost creep in later. A quote is not just a total at the bottom of the page. It is a set of promises about materials, labour and who carries the risk if something goes wrong, and the wording tells you as much as the number does. Homeowners planning a home extension often compare quotes on price alone, then find out during the build that half the job was never actually priced in. The real prices on this page show just how wide that gap can get once the extras, the provisional sums and the vague labour lines are stripped out.
The quick version
- Provisional sums are placeholder figures for work that has not been fully specified yet, so the final bill can land well above the quoted amount.
- Prime cost (PC) sums cover items like sanitaryware or kitchen units at a nominal allowance, and upgrading them almost always adds to the total.
- Vague labour lines such as 'labour as required' or 'making good' hide how many days are actually being charged for.
- A quote that excludes scaffolding, skips, building control fees or waste disposal will always look cheaper than one that includes them.
- Payment schedules tied to vague milestones, rather than completed stages of work, give a builder more room to ask for money early.
- The two cheapest and two most expensive quotes for the same job rarely differ on quality. They differ on what has been left out or guessed at.
What people actually paid
Real prices, in people's own words
- £45,000“Single storey 6.5m wide x 4m long (2022) peak inflation material costs. Included 4 keylite windows and 8ft bi fold. Gable roof. £45k”
- £51,816“Total cost £51,816 (give or take a couple of quid)”
- £80,000“We just finished a single storey. It was £2.5k per square meter plus VAT. Cost 80k in total”
- £95,000“Approx 95k, 5x3m. Included a new kitchen, downstairs WC, exterior power, knocking together upstairs bathroom & WC to make 1 room and unexpected new electrical works to most of the downstairs.”
- £95,000“Total cost £95k, of which build cost was £65k. Kitchen & utility £15k, flooring £3k, architect and planning £2k”
- £100,000“Our recent 2 storey side extension was upwards of £100k building costs.”
Genuine amounts posted publicly. We publish the price and the quote, never the person.
Why the price varies so much
Quotes for the same extension can look wildly different because builders are pricing different levels of detail, not necessarily different levels of quality. A firm that has done a proper site visit, checked the ground and priced the steel will usually come in higher than one that has guessed from a floor plan, even though the second quote looks like better value on paper. The gap often lives in the small print: provisional sums for groundworks, a prime cost allowance for the kitchen units, and a line that just says 'electrics' with no breakdown of sockets, circuits or a new consumer unit (fuse board) if the supply needs upgrading. Who is actually running the job matters too. A sole trader pricing their own labour and a handful of trusted subcontractors will often quote differently to a larger firm carrying office overheads, project managers and a wider supply chain, and neither is automatically the cheaper or the more reliable option. Ownership changes what gets priced in as well. A landlord renovating a rental property usually wants a fixed price with every line specified, because there is no one living on site day to day to make decisions as issues appear, and delay costs rent. Someone extending the home they live in might happily accept a wider set of provisional sums in exchange for room to change their mind on finishes as the work goes on, and a homeowner planning to sell soon after the build tends to quote-shop on price alone rather than on how well the job is documented. Region plays its part as well: a builder working somewhere good tradespeople are in short supply and travel time between jobs eats into the day will price the same extension differently to one working somewhere labour is easier to book and materials are closer to hand, and access in a terraced street or conservation area can mean scaffolding, parking permits and waste removal are quoted very differently to the same job on a detached house with its own driveway. The same padding shows up across other trades too. A kitchen fit-out quoted as one lump sum can bury the tiling and plumbing separately, a bathroom addition might not say whether the waste run needs extending, and a rewire mentioned in passing on an extension quote rarely specifies how many new circuits are involved. Reading past the total and into the line items, and asking who is actually going to be on site doing the work, is the only way to compare quotes fairly.
How to pay less
- Ask every builder to itemise provisional sums and prime cost allowances separately, so you can see exactly what is still a guess.
- Get groundworks and structural steel priced from a site visit, not a desk estimate, before you compare quotes side by side.
- Push back on vague labour lines and ask for a rough day count, since a builder who cannot estimate days is probably padding the figure.
- Confirm scaffolding, skips, building control fees and waste disposal are all included in the total, not added later as extras.
- Agree a payment schedule tied to finished stages of work, such as roof on or first fix complete, rather than round-number instalments.
- Get at least three quotes on the same written specification, not three verbal walk-rounds, so you are comparing like with like rather than three different guesses at the job.
Common questions
What is a provisional sum in a builder's quote?
A provisional sum is a placeholder figure for work that cannot be priced precisely until it starts, such as unknown ground conditions or hidden pipework. It is an estimate, not a fixed price, so the final bill can end up higher or lower once the work is actually done. Always ask what assumptions the provisional sum is based on.
What does 'prime cost' mean on a quote?
A prime cost, or PC, sum is a nominal allowance for an item you have not chosen yet, like kitchen units or bathroom sanitaryware. If you pick something above that allowance, the difference gets added to your final bill. It is worth asking for the PC figure in writing so you know exactly what it assumes.
Why do builder's quotes vary so much for the same extension?
Quotes vary because builders price different levels of detail, not just different profit margins. One firm might include scaffolding, building control and a proper steel allowance, while another leaves them as extras. Comparing the line items, not just the totals, is the only fair way to judge which quote actually represents the job.
Should I accept the cheapest quote for my extension?
Not automatically. A low quote often means something has been left out, priced as a provisional sum, or assumed at the cheapest possible spec. Check what is actually included before assuming it is the better deal.
What should a fair payment schedule look like?
A fair schedule ties payments to completed stages of work, such as foundations poured, walls up, or roof on, rather than a deposit plus round monthly amounts. This protects you if the builder falls behind, and it means you can hold back the next payment until each stage is properly finished and checked.
Does it matter if I'm renovating to rent the property out rather than to live in it?
Yes. A landlord usually wants a fixed price with every item specified in writing, since there is no one living on site to sign off decisions as they come up and any delay affects rental income. Owner-occupiers sometimes accept more provisional sums for the freedom to decide on finishes as the job progresses, so it is worth telling the builder upfront which situation applies and asking the quote to reflect it.