You cannot tell whether a builder’s quote is fair by looking at the total. Three quotes for the same extension can differ by 30% and all three be honest prices — because they are almost never pricing the same job. What tells you whether a quote is fair is what sits behind the number.
This is how to compare quotes properly, what a good one shows you, and the checks worth making before you sign anything.
Why comparing three totals tells you almost nothing
When you ask three builders to quote, you are not running a like-for-like comparison unless someone has written the scope down for them. Without that, each builder decides for themselves what is included, what is assumed, and what they have left out because they were not sure.
So one quote allows for decorating and another does not. One assumes the existing drainage can be reused and another has priced a diversion. One has included the kitchen and another has left it as a rough allowance. The totals are different because the jobs are different, and nothing on the page tells you that.
The cheapest quote is very often the one with the most missing from it. That is not usually dishonesty. It is what happens when someone prices quickly from drawings that do not say enough.
Worked example: the same extension, three quotes
A single-storey rear extension, 25 m². Three builders quote:
- Builder A — £48,000
- Builder B — £57,000
- Builder C — £61,500
A to C is a gap of £13,500, about 28%. On the face of it, Builder C looks expensive and Builder A looks like the obvious choice.
Now read what each one actually covers. Builder A excludes the kitchen fit-out, the patio and external works, and all decoration, and has assumed the existing drainage can be reused. Builder C includes all of it and has allowed for a drainage diversion.
Bring Builder A up to the same scope:
- Builder A as quoted: £48,000
- Kitchen fit-out: £6,500
- Drainage diversion: £2,200
- Decoration: £1,800
- Patio and external works: £2,400
- Adjusted total: £60,900
Against Builder C’s £61,500, the difference is £600 — about 1%.
A 28% gap became a 1% gap, and nothing changed except the scope being made the same. Builder A was never cheaper. The extra £12,900 was always going to be spent; it just was not going to be spent through Builder A, and you would have found that out one item at a time after work started.
Figures here are illustrative, but the pattern is the ordinary case rather than the exception.
The five things a fair quote shows you
- A breakdown, not just a total. Trade by trade or stage by stage. A single number with no working is impossible to check and impossible to compare.
- What is excluded, in writing. The exclusions list is the most useful page in any quote. A quote with no exclusions has not been thought about.
- The assumptions behind it. Ground conditions, existing drainage, what happens if something unexpected is found under the floor.
- Whether VAT is in or out. This alone can explain an apparent 20% difference between two quotes.
- A programme. How many weeks, and what happens to the price if it runs longer.
On VAT specifically, the treatment depends on the type of work rather than being a flat addition to everything. New builds, conversions and alterations are treated differently, and the current position is set out on gov.uk. If one quote shows VAT and another does not say, you are not comparing two prices. You are comparing a price and a riddle.
What a “provisional sum” on your quote means
If you see this phrase, read it carefully, because it is not a fixed price.
A provisional sum is an allowance for something not yet decided or not yet designed — often the kitchen, the bathroom, the flooring or the landscaping. It is a placeholder. When the real choice is made and the real cost is known, the allowance is replaced by that cost, up or down.
So a quote showing a £6,000 provisional sum for a kitchen is not promising you a £6,000 kitchen. It is saying: we have put £6,000 in the total for now. If you then choose an £11,000 kitchen, the quote goes up by £5,000 and the builder has done nothing wrong.
Two quotes can look far apart purely because one carried generous provisional sums and the other carried thin ones. Check them before you compare totals, and where you can, decide the item and get it priced properly instead.
What a big price gap usually means
A wide spread is not automatically a sign that someone is trying it on. In order of how often we see it:
- Different scope — by a distance the most common reason, and the one above
- Different risk appetite — one builder has priced for finding a problem under the floor and another has assumed there isn’t one
- Different programme — a job run over ten weeks costs more to run than the same job over eight, because site costs accrue weekly
- Genuinely different overheads — a firm with a site manager and proper insurance costs more than a two-person outfit, and sometimes that is what you want
- They don’t want the job — a deliberately high number is a polite no, and it is more common than people realise
The one to be genuinely wary of is a quote well below the others with no breakdown and no exclusions. That is not a bargain. It is an incomplete price, and the missing parts arrive later as extras when you have no leverage and no other builder on site.
Questions to ask before you sign
- Can I have this broken down by trade or by stage?
- What is specifically excluded?
- Is VAT included?
- Which items are provisional sums, and what happens when the real cost is known?
- What have you assumed about the ground, the drainage and the existing structure?
- How long will it take, and what happens to the price if it overruns?
- How will changes be priced once work has started?
- What are the payment stages, and what is the retention?
That last-but-one question matters more than most people realise. Changes are normal on building work, and the time to agree how they get priced is before anyone starts, not the morning a wall comes down. We have written about how changes get valued from the contractor’s side, and the same mechanics apply to you.
Do the basics before you compare anything
Two checks worth five minutes each. Confirm the work has the building control approval it needs — gov.uk sets out when approval is required, and a quote that has not allowed for it is short of a real cost. And check the builder’s public liability insurance is current and covers the value of your job.
When to get an independent estimate
An independent estimate is a priced breakdown of your job prepared by someone with nothing to gain from who builds it. It is worth getting when the quotes are far apart and you cannot work out why, when you only managed to get one quote, when the job is large enough that a 10% error is a lot of money, or when you simply want a number to hold the conversation against.
It is not about catching anyone out. Most builders quote honestly. It is about knowing what the work is worth before you commit, so that a conversation about price is one you can actually take part in.
If you want to understand where the underlying numbers come from, build cost per square metre explains how those figures are put together and what they leave out.
Who this is for
- Homeowners
- Buyers pricing work before an offer
- Landlords
- Self-builders
FAQ
How do I know if a builder’s quote is too high? Compare the breakdown rather than the total, and make sure every quote covers the same scope with the same exclusions. Most apparent overpricing turns out to be a difference in what is included. If the scopes genuinely match and one is well above the rest, ask what it has allowed for that the others have not.
Why are my three quotes so different? Almost always because they are pricing different jobs. Without a written scope, each builder decides what to include and what to assume. Differences in VAT treatment, provisional sums and programme length account for most of the rest.
Should I always take the cheapest quote? No. The cheapest quote is frequently the least complete one, and the gap gets recovered later as extras once work has started and you have no alternative. Compare adjusted totals, not headline ones.
What is a provisional sum on a builder’s quote? An allowance for work not yet decided or designed, such as a kitchen or flooring. It is replaced by the actual cost once that is known, so the final figure can be higher or lower than the allowance shown.
Is it worth paying for an independent estimate? On a job of any size, usually yes. It costs a small fraction of the build and tells you what the work is worth before you commit, which is the point at which you still have room to negotiate.
Should the quote include VAT? It should say clearly either way. VAT treatment depends on the type of work, so check the current rules rather than assuming a flat percentage applies to everything.
Where to start
If you have quotes in front of you and cannot work out why they differ, or you would rather know what the job is worth before you ask anyone, we can prepare an independent estimate from your drawings. Send them over along with any quotes you have already received, and we will come back with a fixed fee and a turnaround date before starting.