How to Price a House Extension: Labour, Materials and Margin Guide

Quick Answer: A typical UK single-storey rear extension priced by a builder in 2026 ranges from £1,800–£2,800 per m² for a standard finish, £2,800–£3,800 per m² for high specification, and £3,500–£5,000+ per m² for London and the south-east or for premium kitchens and bi-fold glazing. Labour typically accounts for 40–55% of the price, materials 35–45%, and net margin 15–25%. Build-up the price line-item by line-item from the trade-rates and material schedule rather than assuming a global rate; rate-based estimates produce inconsistent margins and lose money on complex projects.

Summary

Pricing a house extension is the single most common large-job estimating challenge for a UK domestic builder. Get the price too high and the homeowner walks; too low and the build runs at a loss for six months. The mistake most builders make is using a global per-m² rate inherited from peers or trade press — these rates were correct for someone else's project, with someone else's overheads, in someone else's region, two years ago.

The right method is to build the price up from a structured estimate: groundworks, masonry, structural, weather envelope, internal fit-out, mechanical and electrical, finishes, externals, contingency, overhead, profit. Each line is priced from current local rates and a material take-off that matches the drawings. Done properly, the build-up takes 6–12 hours of estimating time per job, and it produces a price the builder can defend to the homeowner and stand behind for the duration of the build.

This guide covers the price build-up for a single-storey rear extension in the 15–40 m² range — the most common project size. Larger extensions (40 m²+) and double-storey extensions follow the same logic but with more parallel trade activity and additional structural complexity. See the price build-up for two-storey extensions for the larger comparison.

Key Facts

Quick Reference Table

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Element % of total price (typical 25 m² extension) Typical £/m² (mid-spec)
Groundworks and foundations 12–18% £350–£500
Structural (walls, beams, lintels) 15–22% £400–£650
Roofing 8–12% £200–£350
Windows and external doors (incl. bi-folds) 7–14% £200–£400
Internal fit-out (plaster, joinery, decorating) 12–18% £350–£500
Mechanical (plumbing, heating, drainage) 6–10% £180–£280
Electrical (first and second fix) 4–8% £120–£220
Kitchen / bathroom fit-out (if in scope) 8–18% £250–£500
Externals (paving, drainage, landscaping) 4–8% £100–£220
Overhead and profit 15–25% (sub-total above) n/a
Contingency 7.5–10% n/a

Detailed Guidance

Setting Up the Estimate Document

A defendable extension price comes from a structured spreadsheet. Three columns of pricing data are essential per line: quantity (from take-off), unit rate (from current rates), and line value. A fourth column for notes captures assumptions (e.g. "assumes M3 strip foundations to 1.0m, no underpinning").

Group lines into categories matching the build sequence: enabling and groundworks, masonry, structural, weather envelope, MEP first fix, internal fit-out, MEP second fix, decoration, externals, and prelims. Each category sums to a sub-total; the full estimate sums to the trade total, then overhead and margin are added.

The discipline of line-by-line estimating makes the price defensible. If the homeowner asks why the price is higher than a competitor's, the answer is in the spreadsheet — not "because we use better quality" or "because we price to win". Detailed line-item estimates also make scope changes during the build easier to price; a £1,200 change on a single line is straightforward where the original price had that line at £4,500.

Groundworks and Foundations

Strip foundations are standard for most UK extensions: 1.0 m deep, 600 mm wide, mass-fill C25 concrete. Trench excavation is approximately £18–£32 per linear metre on standard ground, plus £100–£140 per cubic metre of concrete supply and place. A 25 m² extension with 18 metres of foundation runs typically costs £4,000–£6,500 in groundworks.

Variations: clay shrinkable soils may require deeper foundations (1.2–1.5 m); near trees, foundations may need to be 1.5–2.5 m deep with heave precautions (Classic NHBC Chapter 4.2 detail or BR 380 standard); contaminated land or restricted access doubles the cost easily.

Add: site strip and clearance (£300–£800), spoil removal (£20–£40 per cubic metre), service relocation if applicable, and oversite preparation (DPM, blinding, slab) at typically £40–£70 per m².

Masonry and Structural

A typical cavity wall (100 mm block + 100 mm cavity + 100 mm block, fully filled with insulation) costs £140–£200 per m² of wall area for materials and labour. Block sizes (100 mm or 215 mm), insulation choice (PIR, mineral wool, EPS bead), and brick choice (facing brick £450–£900 per 1000, handmade £1,200+) drive the variation.

Structural openings need calculation by a structural engineer. Steel beams typically cost £85–£140 per linear metre supply, plus £200–£400 to lift and install per beam, plus padstone provisions. A typical extension with one large rear opening (4–5 m clear) and two side window openings (1.2 m each) needs three structural beams totalling £1,800–£3,500 in steel and installation.

Floor structure: ground floor typically a beam-and-block or oversite concrete slab (£40–£80 per m² supply and lay); upper floors on double-storey extensions use TJI or solid joists (£35–£55 per m² supply and lay). See beam and block floor specification for floor detail.

Roof and Weather Envelope

Flat roof with EPDM or GRP costs £100–£180 per m² with insulation, deck, membrane, and trims. Pitched roof with concrete tile is £85–£140 per m² with felt, batten, tile, hip and ridge. Pitched roof with handmade clay tile or slate is £160–£280 per m².

Glazing is the high-cost item. Velux centre-pivot rooflights are £450–£800 per unit fitted. Bi-fold doors (4 m clear) are £4,000–£8,500 supply and fit; sliding doors of equivalent span are typically 30–50% cheaper. Roof lanterns are £4,000–£8,000 fitted depending on size — see the cost build-up for roof lanterns.

Internal Fit-Out

Plastering and decoration are £35–£60 per m² of wall and ceiling area (skim coat) plus £20–£40 per m² for two-coat decoration. Joinery (skirting, architrave, internal doors) is £30–£60 per m² of floor area for standard mid-spec.

Kitchen fit-out is the biggest single variable: a £4,000 budget kitchen and a £25,000 high-end kitchen both fit in the same 4 × 4 m space. Quote either based on the homeowner's spec (with builder's margin on the supply) or as a Provisional Sum (PC sum) with the homeowner contracting separately for the kitchen. PC sums simplify the contractor's risk and the homeowner's choice.

Mechanical, Electrical, Plumbing

First-fix plumbing (waste runs, hot/cold supplies, gas, central heating piping) is £30–£60 per m² of new floor area. Second-fix (radiators, sanitaryware connections, valves) adds £20–£40 per m². New radiators are £80–£160 each fitted; underfloor heating is £45–£75 per m² fitted.

Electrical first fix (lighting, sockets, fused spurs) is £25–£45 per m². Second fix (fittings, tests, certification) is £15–£30 per m². Approved Document Part P notification adds £200–£400 if going through a competent person scheme self-cert. Notifiable work (kitchens are special locations) requires either competent person scheme membership or full Building Control notification.

Overhead and Margin

Overhead recovery on a builder's contract typically runs 8–15% of contract value, covering yard, transport, insurance, vehicles, office, and unallocated supervision time. Net margin is the additional 10–20% that compensates for the contractor's risk and capital tied up in the project.

A practical rule: total mark-up from trade cost to client price is typically 22–35% (overhead + margin combined). A trade cost of £80,000 (labour + materials + sub-contractors) with 25% mark-up produces a client price of £100,000.

Contingency

Add 7.5–10% contingency on a fully-detailed extension price to cover unforeseen ground conditions, services, structural surprises, and design clarifications during the build. On a £100,000 trade cost, contingency is £7,500–£10,000.

Contingency is not margin. It is held against specific risks; if the risks do not occur, the contingency is either returned to the homeowner (transparent contracting) or absorbed as additional margin (less transparent but more common). The contractual treatment should be agreed in advance — Provisional Sum or Provisional Quantity items are the JCT mechanism.

Frequently Asked Questions

How do I price a house extension as a builder?

Build the price up from a structured line-item estimate matching the drawings, using current local trade rates. Add overhead recovery (8–15%), margin (10–20%), and contingency (7.5–10%). Quote on a fixed-price basis with provisional sums for items the homeowner will choose later (kitchen, sanitaryware, finishes). Avoid global per-m² rates as the only basis — they hide errors and produce inconsistent margins.

What is a fair price per square metre for an extension?

UK averages in 2026 are £1,800–£2,800 per m² for standard, £2,800–£3,800 per m² for high spec, and £3,500–£5,000+ per m² for London / south-east or premium specifications. The right price for a specific project depends on access, ground conditions, kitchen and bathroom inclusion, glazing specification, finishes choice, and the contractor's overhead structure. Per-m² rates are useful for sense-checking a build-up, not for setting the price.

Why are extension prices so different between builders?

Three main reasons. First, scope differences — one builder may include the kitchen, drainage, and party wall costs while another excludes them as PC sums or owner-supply. Second, specification differences — handmade brick versus standard facing brick triples the brick cost. Third, overhead and margin — a single-trader on low overhead can price 20–30% below a larger contractor with multiple management layers, but the larger contractor brings more redundancy and project management capability. See the difference between a quote and an estimate for the cost difference between formal quotes and informal estimates.

How long does an extension take to build?

A 25 m² single-storey rear extension typically takes 12–20 weeks site time from groundwork start to handover, including 1 week for snagging. A 40 m² extension typically takes 16–28 weeks. A double-storey extension of 50–60 m² typically takes 20–32 weeks. Programmes extend in winter (groundworks weather), where party wall agreements delay start, or where supply chain issues affect specific items (bi-folds, structural steel).

Can I get a fixed price for an extension?

Yes, but only against a fully detailed specification — drawings to Building Regulations level, written specification of finishes and fittings, structural engineer's calculations. Quotes on outline drawings are estimates, not fixed prices. The fixed-price contract works because both parties understand exactly what is being built; if specification gaps exist, the price either changes during the build (to the homeowner's surprise) or the contractor absorbs the gap (at their loss).

Regulations & Standards