What Should a House Extension Quote Include? Line-by-Line Guide

Last updated: June 2026 · Figures checked against live North London quotes and current government fee schedules.

What should a house extension quote include?

Quick answer: A credible extension quote itemises at least nine cost sections — preliminaries, demolition, groundworks, structure and steels, roof, glazing, first fix, second fix and finishes — plus a written exclusions list. If an £80,000 job arrives as a single figure on one page, ask for the full breakdown before signing.

Homeowners comparing builders across Barnet, Enfield and Haringey are routinely handed three documents that look nothing alike: a two-line WhatsApp price, a one-page letter, and a four-page priced schedule. Only the last one can actually be scrutinised. This guide walks through every line a proper quote should carry, decodes the PC sums and provisional sums buried inside them, and sets out the exclusions you should insist are confirmed in writing.

Why does the structure of a quote matter so much?

Because the structure is where disputes are born. When a price is one number, every later disagreement — who pays for the sixth skip, whether the steels were an "extra", whether VAT sits on top — becomes your word against the builder's. Renovation forums are littered with threads where three quotes on identical drawings landed £40,000 apart, and in nearly every case the cheapest document was also the thinnest. A detailed schedule protects both parties: the builder is paid for precisely what was priced, and you can see whether £8,800 of glazing or £4,000 of glazing sits inside the number. On a typical 14-week North London build, an itemised quote also makes stage payments auditable, because each instalment can be matched to completed lines.

What are the nine sections every quote should itemise?

1. Preliminaries. Site setup and running costs: scaffolding (£1,200–£2,500 on a standard rear extension), welfare facilities, skips (£250–£350 each, and a typical project fills 6–10 of them), temporary protection and site supervision. Prelims normally absorb 8–12% of the contract value. A quote with no prelims line has not deleted those costs — it has concealed them.

2. Demolition and strip-out. Taking down the rear wall, removing an old conservatory or outrigger, and carting the rubble away. Budget £2,500–£4,500 including disposal on most North London semis.

3. Groundworks and foundations. Excavation, concrete, underpinning risk and drainage alterations. London clay generally pushes foundations to 1m deep, and 2.5m or more where trees stand nearby, so this line carries the most genuine uncertainty. Expect £9,000–£14,000 for a 15–20m² footprint.

4. Structure and steels. Blockwork, brickwork, lintels and the steel beams that carry the back of your house once the wall comes out. Beams cost £400–£900 each supplied, but fabrication, padstones and installation push a two-beam opening to £4,000–£7,000.

5. Roof. Joists, insulation (new flat roofs must hit a U-value of 0.15 W/m²K), covering and rooflight upstands — usually £6,000–£9,000 on a single-storey addition.

6. Glazing. The widest-swinging line on any schedule. uPVC French doors can be £1,800 fitted; a 4m aluminium slider runs £9,000–£12,000; a roof lantern adds £2,500–£6,000. This is where PC sums do their quiet damage — more below.

7. First fix. Wiring, pipework, soil runs and structural carpentry before the walls close up — typically £6,500–£9,000 for a kitchen-led extension with underfloor heating.

8. Second fix. Sockets, switches, radiators or manifolds, internal doors and sanitaryware connections, commonly £5,500–£7,500.

9. Finishes. Plastering, screed, flooring, tiling and decoration. Floor finishes alone span £40–£120 per m² depending on specification.

What does a sample £80,000 rear extension breakdown look like?

Here is a representative line-item schedule for a 20m² rear extension in outer North London, priced at build cost excluding VAT, kitchen units and professional fees:

Line item	Cost	Share
Preliminaries (scaffold, skips, welfare, supervision)	£8,000	10%
Demolition and strip-out	£3,200	4%
Groundworks and foundations	£9,600	12%
Drainage alterations	£2,400	3%
Structure, steels and masonry	£14,400	18%
Roof structure and covering	£7,200	9%
Glazing (doors, windows, rooflights)	£8,800	11%
First fix (electrics, plumbing, carpentry)	£7,600	9.5%
Second fix (sockets, doors, sanitaryware)	£6,400	8%
Plastering and screed	£4,800	6%
Finishes (flooring, tiling, decoration)	£5,600	7%
Contingency / provisional sums	£2,000	2.5%
Total (ex VAT)	£80,000	100%

Your percentages will wobble a few points either way, but if one line sits wildly outside these proportions — prelims at 2%, say, or glazing at 25% with no lantern specified — that is your cue to ask questions.

What are PC sums and provisional sums, and how do they distort quotes?

A PC (prime cost) sum is an allowance for an item you have not yet chosen: tiles at £35 per m², sanitaryware at £600, a glazing allowance of £4,000. A provisional sum covers work that cannot be measured yet, such as £1,500 for possible drainage diversion. Both are legitimate tools — and both are routinely gamed. A builder who wants to look cheap inserts a £3,000 glazing PC sum on a design that clearly needs £9,000 of sliding doors, then invoices the £6,000 difference mid-build when you have nowhere to go. Two defences: insist every PC and provisional sum is listed separately with its assumed figure, and treat any quote where such sums exceed 15% of the total as an estimate wearing a quote's clothing.

Which exclusions should you demand in writing?

The exclusions list is the most-read paragraph of a good quote and the most-missing paragraph of a bad one. Get explicit written answers on each of these: Is VAT included? At the standard 20% rate, an ex-VAT £60,000 price becomes £72,000 by completion. Are kitchen units and appliances in or out — they almost never are. Are professional fees covered: design and drawings run £1,200–£2,500, a structural engineer £500–£1,500, building control £500–£1,500, and a party wall surveyor £700–£2,500 per affected neighbour. Who pays the council? A householder planning application costs £548 since 1 April 2026, and the Planning Portal adds a £91.02 processing charge unless you pay the council directly. Are skips, scaffold strikes and protection included? And is "making good" — replastering and redecorating the rooms the extension punches into — inside the price or left for you to discover?

Frequently asked questions

Is a one-page quote ever acceptable?

For a £2,000 repair, perhaps. For a five-figure extension, no. One page cannot itemise nine cost sections, state PC sums or list exclusions, which means it cannot be checked, compared or enforced when something changes on site.

What is the difference between a quote and an estimate?

A quote is a fixed offer capable of acceptance; an estimate is an informed guess that may move 10–20% in either direction. Many documents headed "quotation" are estimates in disguise because their PC and provisional sums leave the real price open.

How long should a quote remain valid?

Most builders hold prices for 30–90 days. Materials inflation has settled compared with 2022–2023, but steel and glazing prices still shift quarterly, so confirm the validity window in writing if your start date is months away.

Who should pay for skips and scaffolding?

The builder, through the preliminaries line. If skips and scaffold appear as exclusions, add roughly £3,500–£5,500 to your comparison figure — and ask why a contractor priced a job they apparently did not intend to access or clear.

Should I share my budget before receiving the quote?

Share a range, not a ceiling. A specification-first conversation keeps the pricing honest, whereas naming a single figure invites a quote that lands suspiciously close to it regardless of what the work genuinely costs.

Prefer all of this rolled into one fixed figure with nothing hidden? Scope your project in minutes with the Extension Builder, or book a free site visit — The Extension Company prices every job line by line from our Cockfosters base, on 020 3051 9430.