House Extension Quotes UK (2026): How to Compare Builders

Estimate vs quote: what is the difference?

Citizens Advice puts it simply: a quote (sometimes called a quotation or fixed estimate) “is a promise to do work at an agreed price”, while an estimate “is the trader’s best guess as to how much the work will cost”.

	Estimate	Quote (quotation)
What it is	An informal, approximate price	An agreed price for a defined scope of work
Can the price rise?	Yes, but the final bill should be reasonable, and you can dispute one that is a lot higher	Only for a good reason, such as extra work you agreed to pay for
Rising material costs	Can be a factor in a higher final bill	Not a valid reason to charge more
Based on	A site visit or rough description	Drawings, structural calculations and a written specification

Citizens Advice adds that if a trader made a mistake in the quote, you have a right to the work at the quoted price. That only protects you if the quote describes the work clearly; “build extension as discussed” gives you little to hold a builder to.

How many quotes should I get for an extension?

The Federation of Master Builders (FMB), the HomeOwners Alliance and BestBuilders all recommend at least three. The FMB adds that asking for more than three or four can be counterproductive, because builders may be wary of a tender with more than four firms. The HomeOwners Alliance suggests starting with a shortlist of about five, as some will not reply, and allowing two to four weeks for builders to price.

Give every builder the same information: the drawings, the structural engineer’s calculations, a written specification and your preferred start date. Share any answer to one builder’s question with all of them, so they keep pricing the same job.

What a good quotation for a house extension includes

A detailed written quote should set out:

• the builder’s full business name, address, VAT number if registered, and the drawings and revision numbers priced;
• an itemised breakdown by stage or trade, not a single total;
• which items are fixed prices, which are prime cost (PC) sums and which are provisional sums;
• whether VAT is included, and at what rate;
• a clear list of exclusions;
• start date, expected duration and working hours;
• the payment schedule, deposit and any retention;
• how variations (changes) will be priced and agreed;
• insurance held, guarantees offered and who arranges building control;
• how long the price is valid.

House extension quotation sample: 20m² single storey

A modelled illustration, not a real builder’s price: a 4m × 5m (20m²) single storey rear extension on a detached house in the Midlands at a standard finish, with matching brick, a warm flat roof and lantern, aluminium sliding doors, underfloor heating and a steel beam. The £47,600 ex VAT total is £2,380 per m², mid-range for our calculator’s regional standard rate.

Item	Type	Price (ex VAT)
Preliminaries: site set-up, scaffolding, skips, welfare, protection to existing house	Fixed	£2,800
Excavation, spoil removal and concrete foundations (to 1m depth)	Fixed	£5,200
Drainage: new runs and inspection chamber, connect to existing	Provisional sum	£2,000
Ground floor slab, insulation and screed	Fixed	£3,800
External cavity walls to match, insulation, lintels, cavity closers	Fixed	£9,200
Steel beam, padstones and temporary support to open rear wall	Fixed	£3,000
Warm flat roof: joists, deck, insulation, EPDM, fascia and guttering	Fixed	£4,200
Roof lantern (supply)	PC sum	£2,000
Aluminium sliding doors, 3.6m (supply)	PC sum	£4,500
Side window, supplied and fitted	Fixed	£700
Electrics: first and second fix, lighting, sockets, certificate	Fixed	£2,400
Heating: underfloor heating and manifold, alter existing radiator	Fixed	£2,200
Plasterboard, plastering and making good to existing kitchen	Fixed	£2,000
Second fix carpentry: skirting, architrave, window boards	Fixed	£900
Decoration: mist coat and two coats to walls and ceiling	Fixed	£900
Floor finish (client to choose)	Provisional sum	£1,500
Final clean and waste removal	Fixed	£300
Builder’s quote, ex VAT		£47,600
VAT at 20%		£9,520
Builder’s quote, inc VAT		£57,120

Exclusions on this sample: kitchen supply and fitting, architect and engineer fees, building control fees, the lawful development certificate, party wall surveyors, drainage diversions beyond the provisional sum, and landscaping.

Costs that sit outside the builder’s quote

Professional fees are usually paid directly. Here they add about £5,300:

• Architectural design and building regulations drawings (5%): £2,380 plus VAT
• Structural engineer: £1,000 plus VAT
• Building control: £800 plus VAT
• Lawful development certificate: £274 (no VAT)

That brings the all-in cost to about £62,400, inside our calculator’s £55,500–£72,500 range for a standard 20m² extension in the Midlands. Keep a 10% contingency on the build (about £5,700 including VAT) on top. A new kitchen would add roughly £8,000–£15,000 at budget level; see our kitchen extension cost guide and single storey extension cost guide for other sizes.

Provisional sums and prime cost sums explained

• Provisional sum: an allowance for work that cannot be priced exactly yet, such as drainage that will only be known once the ground is opened, or extra foundation depth. The HomeOwners Alliance advises listing provisional sums in the contract to cover unforeseen work. The final cost is whatever the work actually costs.
• Prime cost (PC) sum: an allowance for buying a specific item you have not chosen yet, such as the doors, roof lantern, sanitaryware or tiles. The builder’s fitting labour is usually priced separately. If you choose a more expensive item, you pay the difference.

Check the allowances are realistic: a £2,000 PC sum for doors that will really cost £5,000 makes a quote look cheaper than it is. Aluminium bi-folds cost about £3,150–£7,200 fitted (MyJobQuote, September 2026). Set the allowances yourself in the tender so every builder uses the same figures, and ask whether they add a margin on PC items.

VAT on extension quotes

Building an extension to an existing house is normally charged at the standard 20% rate. Make sure every quote states whether VAT is included: a quote of £47,600 “plus VAT” is £57,120. A sole trader whose turnover is below the £90,000 registration threshold will not charge VAT, which can make a quote look 20% cheaper. That is legitimate, but if a VAT-registered builder offers a “no VAT” price for cash, walk away.

Exclusions to look for

The items most often left out, and most likely to return as extras:

• skips, scaffolding and taking spoil away;
• drainage alterations, build-over agreements and moving manholes;
• foundations deeper than standard, or tree root and clay problems;
• making good, plastering and decorating in the existing rooms you open into;
• electrical certification and any consumer unit upgrade;
• floor finishes, kitchen fitting and appliance plumbing;
• building control fees, party wall costs and reinstating the garden.

Payment schedules, deposits and retention

A fair quote asks for a small deposit, then stage payments as work is completed, with a retention at the end. BestBuilders (August 2026) says no deposit should exceed 10%, and to pay by bank transfer to the business account against checkable stages, never in cash. The HomeOwners Alliance suggests holding back about 5% until the end to cover snagging. Paying a deposit by credit card can give Section 75 protection on purchases of £100–£30,000.

Stage	Share	Amount
Deposit on signing, towards materials	5%	£2,856
Foundations and drainage complete, passed by building control	15%	£8,568
Slab and walls to roof height	20%	£11,424
Roof on and watertight	20%	£11,424
Doors and windows in, first fix complete	15%	£8,568
Plastering complete	10%	£5,712
Practical completion	10%	£5,712
Retention, released after snagging	5%	£2,856

Pay for each stage once it is finished. Record every change on a written, priced variation agreed before the work is done. If you need to borrow, our guide on how to finance an extension explains how to match funding to stage payments.

Contingency: why the quote is not your budget

A fixed price covers only what is described. Hidden drains, poor ground, changes of mind and provisional sums that come in high all add cost. Keep at least 10% of the build cost aside and do not tell the builder it exists. On the sample above that is about £5,700 including VAT.

Insurance, guarantees and memberships

Ask for insurance certificates and check them with the insurer or broker directly. BestBuilders recommends:

• Public liability insurance of at least £2m, which the HomeOwners Alliance also says to ask for with any tender.
• Employers’ liability insurance of at least £5m, a legal requirement if the builder employs anyone.
• Contract works (contractor’s all risks) cover for the value of the build.

Memberships help, but check them on the scheme’s own website rather than trusting a logo:

• TrustMark is the Government Endorsed Quality Scheme for work in or around the home; registered businesses are vetted to meet required standards.
• FMB members are vetted before joining (trading history, credit checks and proof of public liability insurance), have their work inspected, must provide written quotes and contracts, and their clients can use a free, government-approved dispute resolution service.
• Warranties: BestBuilders says FMB members can offer an optional two-year Build Assure warranty, and suggests a 6–10 year structural warranty (about £400–£1,200) if you may sell within a decade. Ask what any guarantee covers and whether it survives the builder going out of business.

Gas work needs a Gas Safe engineer, and electrics are best done by a scheme-registered electrician such as NICEIC, who can self-certify.

How to compare builder quotes like for like

1. Put every quote into one spreadsheet using the same headings as the sample above.
2. Add back anything excluded, using the other builders’ prices for that item.
3. Level the PC sums to the same allowance, so you compare the builders, not their guesses about your doors.
4. Check provisional sums: a quote with lots of them carries more risk of extras.
5. Put all prices on the same VAT basis.
6. Compare programme and payment terms, not just price.
7. Shortlist two and meet them, as the HomeOwners Alliance suggests, and visit recent jobs.

Why the cheapest quote is not always the cheapest

The HomeOwners Alliance warns against picking the cheapest quote on price alone, and the FMB says suspiciously low quotes usually are too good to be true. Build It describes the “loss leader”: a low price to win the job, then extras for work not included. A very low quote can also mean the builder has misunderstood the job or plans to cut corners you will pay to put right.

If you are working to a tight budget, our guide to cheap house extensions shows where you can genuinely save before you go to tender.

Red flags in builder quotes

• A verbal price, or a quote by text message with no breakdown.
• Pressure to start immediately or sign today.
• A large deposit up front, or payment requested in cash or to a personal account.
• A “no VAT” price for cash from a VAT-registered business.
• No fixed business address, no insurance certificates, or reluctance to sign a written contract.

Contracts and cancellation rights

Turn the chosen quote into a written contract before work starts. BestBuilders names the JCT Homeowner contract and the FMB Domestic Building Contract as common options. The HomeOwners Alliance suggests agreeing a start date and weekly liquidated damages for overruns; it says £250–£500 a week is the norm for extensions. If you agree the contract at home, online or by phone, Citizens Advice says you normally have a 14-day cooling-off period. If you ask the builder to start within it, you pay for work done before you cancel. Get approvals sorted first: see our guides to extension planning permission and party wall agreements.

House extension quote checklist

Print this list and work through it for every quote.

Before you ask for quotes

• Drawings and structural calculations complete, with revision numbers
• Written specification, including doors, roof, heating and finishes
• Planning permission, prior approval or lawful development certificate in hand or applied for
• Drain survey done and party wall notices planned
• Shortlist of about five builders; same pack sent to each

When each quote arrives

• Written, on headed paper, with business address and VAT number
• Itemised by stage or trade, matching the drawings priced
• PC sums and provisional sums listed, with realistic allowances
• VAT shown clearly
• Exclusions listed
• Start date, duration and validity period stated
• Deposit no more than 10%, stage payments tied to completed work, retention offered

Before you sign

• Public liability, employers’ liability and contract works insurance certificates checked
• Memberships verified on the TrustMark or FMB website
• Warranty or guarantee terms in writing
• At least two references called and a recent job visited
• Written contract with dates, variations procedure and liquidated damages
• 10% contingency set aside

House extension quotes FAQs

How many quotes should I get for an extension?

At least three, according to the FMB, the HomeOwners Alliance and BestBuilders. Start with a shortlist of about five, but avoid asking more than four builders to price in full, as good builders may not bother.

What deposit should a builder ask for an extension?

A small one tied to materials. BestBuilders advises no more than 10% and refusing 30% or more. Pay the rest in stages as work is completed.

How much should a quote be for a 20m² extension?

For a standard single storey extension in the Midlands, a builder’s quote is typically about £51,500–£63,500 including VAT, and the all-in cost with fees about £55,500–£72,500. Add roughly 25% in London.

Can I get house extension quotes online?

You can get a ballpark online, from our calculator or a builder, but a real quote needs a site visit and drawings. Treat any price given without either as an estimate.

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