Builder quotes: how to read and compare them fairly

Written quote, estimate or tender?

The word on the top of the page changes what you can hold the builder to.

Estimate: an informed guess at the likely cost. It is useful for budgeting but is not a fixed price, and the final bill can differ.
Quote (quotation): an offer to carry out defined work for a stated price. Once you accept it, a contract exists for that work at that price, even if the acceptance was only spoken.
Tender: a quote submitted against a full package of drawings and specification that you or your designer sent to several builders at once.

Ask for every price in writing, and ask each builder to confirm that it is a fixed price for the work described, how long it stays open (30 to 90 days is normal) and what could change it. A figure given on the doorstep, or headed "estimate", is not something you can compare with a firm quote.

The sections of a good builder quote

A quote broken into sections is far easier to check than one figure and a paragraph. These are the ten sections our quote comparison tool uses, with the share of a single-storey extension price each often takes. Glazing-heavy or high-specification schemes will differ.

Typical sections of a single-storey rear extension quote
Section	What it covers	Often around
Preliminaries	Set-up, welfare, skips and waste removal, scaffold, insurance, supervision, protection	8% to 15%
Demolition and strip-out	Taking down walls, old structures and fittings	2% to 5%
Groundworks and drainage	Excavation, foundations, slab, insulation, drains	12% to 20%
Structure and walls	Brick and block, cavity insulation, steel beams, lintels	15% to 25%
Roof	Structure, deck, covering, rooflights, gutters	8% to 15%
Windows and doors	External glazing, bifolds or sliders	8% to 18%
Mechanical and electrical	Wiring, lighting, heating, plumbing, ventilation	8% to 14%
Finishes	Plaster, screed, flooring, decoration, joinery	10% to 16%
Provisional sums	Allowances for work not yet defined	Ideally under 10%
Contingency	A buffer for unknowns, if the builder includes one	0% to 10%

Preliminaries are the cost of running the site rather than building anything: scaffold hire, skips, the site toilet, protection, insurance and the supervisor's time. Very low prelims usually mean they are hidden elsewhere or missing. Most are charged by the week, so a longer programme costs more.

Three printed builder quotes on a pine kitchen table with rows marked in pink highlighter and red pen, next to a calculator, pencil and mug of tea — Mark up every quote against the same list of sections before you look at a single total.

Building regulations and building control costs in a quote

Almost every extension needs building regulations approval, and someone has to pay the building control body to check the plans and inspect the work. Quotes treat this in three different ways, which is one of the commonest reasons totals do not line up:

Excluded: the quote says "building control fees by client". You pay the council or registered approver directly. For a single-storey extension under about 40 m² this is often in the range of £800 to £1,500 including VAT in 2026, set locally.
Included as a sum: the builder makes the application and adds the charge, sometimes with a handling margin.
Silent: nothing said. Treat that as excluded until the builder confirms otherwise in writing.

Look for the related items too: structural engineer's calculations, party wall surveyor's fees, consent from the local sewerage undertaker if a public sewer runs under the footprint, and an electrician who can self-certify their own work. Each is either in the price or yours to pay. The chapter on building control and party walls explains each approval.

Health and safety: what CDM 2015 means for your quote

The Construction (Design and Management) Regulations 2015 apply to domestic projects as well as commercial ones. As a homeowner you are a domestic client, and your client duties pass automatically to the builder: to the contractor if there is only one, or to the principal contractor if there is more than one. If your architect agrees in writing to take on the client duties as principal designer, they pass to the architect instead.

In practice this means a competent builder's quote should allow for a construction phase plan, site welfare (a toilet and somewhere to wash), safe scaffold and edge protection, and a secure site. These usually sit in the preliminaries. A quote with no prelims and no mention of scaffold or welfare is one to question, because those costs either reappear later or are not being spent.

Ten things to check on every quote

Which drawings and revision were priced? Revision B against revision D is not a fair comparison.
VAT. Plus VAT, including VAT, or not registered? A £70,000 plus VAT quote costs you £84,000.
Provisional sums. Add them up and divide by the total. Above about 15%, the "fixed price" is largely a guess.
Exclusions. Look for "excluded" or "by client": building control fees, party wall costs, skip permits, kitchen supply, flooring, decoration, garden reinstatement.
Prime cost (PC) items. "Bathroom suite PC £1,500" means you choose it and the price is adjusted to actual cost plus an agreed handling percentage.
Who supplies what. Kitchen, sanitaryware, flooring and light fittings. Supply-and-fit versus fit-only moves totals by thousands.
Programme. Weeks on site and earliest start. A start eight months away may come with a price rise.
Payment terms. Deposit, stages and retention. See retention and stage payments.
Insurance. Public liability and contractor's all risks, with policy details.
Validity. How long the price holds and whether material increases can be passed on.

How to compare builder quotes like for like

Builders lay out quotes differently, so you need one structure for all three. The tool does the arithmetic, but the method works on paper.

Send everyone the same package. Same drawings, same specification, same list of who supplies what. Comparisons fail most often at this step.
Re-sort every line into the same ten sections. If a builder gives one figure for "shell", ask them to split it.
Put every price on the same VAT basis. Compare what you will actually pay.
Flag blanks. A blank section is included elsewhere, excluded or forgotten. Ask which, in writing.
Fill gaps for comparison only. Where one builder left out a section others priced, add the highest of the other prices. It is a cautious check, not a real price.
Measure provisional sums as a share. Two quotes with the same total can carry very different risk.
Then compare totals, and look hard at anything more than about 15% from the middle quote.

Worked example: the cheapest quote that was not

Three builders priced the same rear extension:

Builder A: £74,000 plus VAT, provisional sums £4,000. You pay £88,800. Provisional share 5.4%.
Builder B: £58,200 plus VAT, no windows and doors line, provisional sums £11,500. You pay £69,840. Provisional share 19.8%.
Builder C: £93,100 including VAT, with a £4,800 contingency already priced. You pay £93,100. Provisional share 3.9%.

Builder B looks £19,000 cheaper. Fill the missing glazing at Builder C's £13,900 and B's adjusted total becomes £83,740. Nearly a fifth of B's price is allowances; if they run 50% over, which is common for a kitchen or flooring left as a lump sum, B overtakes A. The real choice is between A and C: C costs £4,300 more but has priced a contingency and carries the fewest allowances.

Load this exact example in the quote comparison tool and change the figures.

Why the lowest quote often costs more

A low quote is not automatically bad, but find out why it is low. The usual reasons are genuine efficiency, a gap in the builder's diary, or a price that misread the drawings, left out sections or pushed risk into provisional sums. The last three return as variations and claims during the build, when competition can no longer protect you.

Ask the low bidder to walk you through the price section by section against the drawings. A builder confident in the number will do it willingly.

How to ask for clarifications

Send the same written questions to every builder and keep the answers with the quote:

"Please confirm your price is based on drawings [numbers and revisions] and the specification dated [date]."
"Please list all provisional and PC sums with amounts."
"Please list anything shown on the drawings that your price excludes, including building control fees."
"Please confirm whether VAT is included, and give your VAT number."
"Please confirm the programme in weeks and your earliest start."

The printable questions to ask a builder sheet has the full list, including insurance, references and payment.

A note on VAT

Most extension and renovation work is standard-rated at 20%. Some work can be zero or reduced-rated, such as certain energy-saving materials, conversions that change the number of dwellings and some work on long-empty homes; the rules are in HMRC VAT Notice 708. A builder may leave VAT off only if they are not VAT registered, which is compulsory once taxable turnover passes £90,000 in a rolling 12 months (the threshold in November 2026). A firm pricing a whole extension may well be over it, so ask.

Changing your mind after accepting

If you agreed the work at your home, by phone or online rather than at the builder's premises, the Consumer Contracts Regulations 2013 normally give you 14 days to cancel from the day after you agree. The builder must give you cancellation information; if they do not, the period is extended, up to a maximum of a year. If you ask them to start within the 14 days and then cancel, you pay for work already done. Citizens Advice explains the exceptions.

Common questions

How many builder quotes should I get?

Three is usual. Fewer gives little comparison; more uses builders' time and some will decline. Send all of them the same drawings and specification.

Do builders charge for quotes?

Most quote free for domestic work. Some charge for detailed pricing, especially design and build schemes where pricing involves design work, or for small jobs with a long trip. Agree any fee before the visit and ask whether it is credited if you go ahead.

How long does a builder take to give a quote?

For an extension with full drawings, two to four weeks is normal; busy builders may take longer. Give a return date when you send the package, and treat a quote that arrives in two days without questions with some caution.

Can a builder change a quote after I accept it?

Only as the agreement allows: variations you ask for, provisional sums adjusted to actual cost and any price adjustment clause you agreed. A written contract sets this out. See JCT Minor Works.

What is the difference between a builder and a building contractor?

In everyday use there is little difference. "Building contractor" usually means a firm that takes on the whole job under a contract, runs the site and employs or manages subcontractors such as electricians and roofers. A sole-trader builder may do much of the work personally. For an extension, what matters is that one party takes responsibility for the whole job in writing.

Is a price per square metre useful?

As a sense check only. Single-storey extensions in England in 2026 often fall around £2,000 to £3,500 per m² of floor area including VAT, with London and heavy glazing above that. Kitchens, large openings and drainage diversions move the figure a lot.

Sources

VAT Notice 708: buildings and construction, HMRC
CDM 2015: domestic clients, HSE
Consumer Contracts Regulations 2013, Part 3, legislation.gov.uk