How to Price a Single Storey Extension: Builder's Rates, Materials and Markup

Quick Answer: A single-storey rear extension in the UK in 2026 typically prices £1,800–£2,800 per m² for a standard finish, £2,800–£3,800 per m² for a high specification, with London and the south-east 20–40% above national rates. For a typical 25 m² rear extension, expect £55,000–£80,000 standard, £80,000–£105,000 high spec. Within that price, builder's day rate runs £200–£280 (standard areas) to £350–£480 (London), labourer £140–£200, with materials at £450–£900 per m² depending on glazing and finish choices. Build the price up line by line from the trade rates and material schedule rather than using a single global per-m² rate.

Summary

The single-storey rear extension is the most common UK domestic project that requires builder pricing. Roughly 50,000–70,000 are completed each year across the UK, and they account for the bulk of the small-builder market. The pricing problem is that "how much does an extension cost?" admits no general answer — the variation between the cheapest and the most expensive 25 m² extension is roughly 3:1, driven by glazing, kitchen specification, finish quality, ground conditions, access, and overhead structure of the contractor.

The right pricing method for a builder is to build the price from the trade-day-rate plus material take-off, line by line, against the architect's drawings. This produces a defendable price that the contractor can stand behind for 16–24 weeks of build, with margin protected against unexpected site conditions through a contingency line. Per-m² rates are useful for sense-checking a build-up, but they are not a sound primary method.

This guide focuses on the price build-up at the level a small builder needs: hourly and day rates for each trade, material take-off categories with current prices, and the overhead and margin structure that produces a sustainable business. The approach is the same as in the general extension pricing guide but with deeper trade-rate detail.

Key Facts

Quick Reference Table

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Trade Day rate (standard) Day rate (London) Typical activity per day
Builder (general) £200–£280 £350–£480 Multi-discipline, supervisory
Labourer £140–£200 £220–£320 Mixing, lifting, clearing
Bricklayer £220–£300 £350–£500 500–800 brick / day on cavity
Carpenter (1st fix) £230–£310 £350–£500 Joists, studwork, OSB
Carpenter (2nd fix) £230–£310 £350–£500 Skirting, doors, joinery
Plumber £260–£380 £400–£550 First/second fix, gas
Electrician £260–£380 £400–£550 First/second fix, P certification
Plasterer £230–£320 £350–£500 30–60 m² scratch, 20–30 m² skim
Tiler £220–£310 £350–£500 7–12 m² floor, 8–14 m² wall
Decorator £180–£260 £280–£400 25–40 m² emulsion 2 coats
Roofer (felt/tiles) £230–£320 £350–£500 8–14 m² flat roof, 15–25 m² tile

Detailed Guidance

Building the Estimate

Start with the architect's drawings and structural engineer's calculations. Mark up the drawings with a take-off of:

Each take-off line generates a labour estimate (in trade-days) and a material requirement. Multiply by current rates to get the line price. Sum to a sub-total, then add overhead, margin, and contingency.

A typical 25 m² rear extension take-off produces roughly 60–90 line items across the build sequence. The estimate should align with the build sequence so that progress can be tracked against estimate during construction — useful for the contractor's cost control and for issuing interim payment applications.

Groundworks Detail

Strip foundation: assume 1.0 m deep × 600 mm wide as standard. For a 25 m² extension with two side walls (5 m each), one rear wall (5 m), and a tie into the existing house (5 m of trim), the foundation runs are 20 m total.

Excavation: 1.0 × 0.6 × 20 m = 12 m³ of spoil. At a typical productivity of 5–7 m³ per day with a 1.5–3 tonne mini digger, the excavation takes 2–3 days. Day rate for digger + operator £150–£250 + day rate for one labourer £140–£200 = £290–£450 per day. Three days = £870–£1,350 excavation cost.

Spoil removal: 12 m³ + 25% bulking = 15 m³ to remove. One 8-yard skip holds about 6 m³, so 2.5 skips at £280–£420 = £700–£1,050.

Concrete: 12 m³ × £130–£170/m³ = £1,560–£2,040. Place and finish: 1 day labour for two operatives = £280–£400.

Total groundworks for foundations: £3,400–£4,800 typical.

Add: oversite slab (DPM, blinding, slab) at £40–£70 per m² × 25 m² = £1,000–£1,750.

Total groundworks line: £4,400–£6,550 for a typical 25 m² extension.

Masonry Detail

Cavity wall (100 mm block + 100 mm cavity + 102 mm brick outer leaf):

For three new external walls of 5 m × 2.4 m each = 36 m² wall area. Subtract 8 m² for door and window openings = 28 m² net wall.

Bricks: 28 m² × 60 bricks/m² (102 mm wall) = 1,680 bricks. Allow 5% wastage = 1,765 bricks. At £550 per 1000 = £970.

Blocks: 28 m² × 10 blocks/m² (100 mm wall, 440 × 215 mm) = 280 blocks. Allow 5% wastage = 295 blocks. At £1.50 each = £445.

Cavity insulation (PIR partial fill, 75 mm): 28 m² × £30 = £840.

Wall ties: 280 ties at £0.45 each = £125.

Mortar: 28 m² of 102 mm wall plus 28 m² of 100 mm block = mortar volume approximately 1.2 m³. 25 bags of cement and 5 tonnes of sand = approximately £400.

Materials total for walls: ~£2,800.

Bricklayer labour: 28 m² of cavity wall at productivity of 6–8 m² per day per gang of bricklayer + labourer = 4 days. £220–£300 (BL) + £140–£200 (LB) = £360–£500 per gang day. Total £1,440–£2,000 labour.

Total masonry line for typical 25 m² extension: £4,200–£5,800.

Structural Steel Detail

A typical 25 m² rear extension has one large rear opening (typically 4–5 m clear span) plus existing wall opening to the kitchen.

Rear opening steel (4.5 m clear, UC 152 × 152 × 30): material cost £350–£500. Padstones: £80–£150. Steel installation labour (2 trades for half a day, plus crane or forklift hire): £600–£1,000.

Existing wall opening (3 m clear, UC 152 × 152 × 23): material cost £200–£300. Padstones: £80. Installation labour: £400–£600.

Total structural steel: £1,710–£2,630 for a typical 25 m² extension. Add structural engineer fee £700–£1,400 (separate from build cost, usually paid direct by homeowner).

Roof Detail

Flat roof construction (warm deck — see warm flat roof construction details):

For 25 m² flat roof: deck (18 mm OSB) £25–£35 per sheet × 9 sheets = £270. Insulation (140 mm PIR) £45–£65/m² × 25 m² = £1,250. Vapour barrier and breather: £6–£10/m² × 25 m² = £200. Membrane (EPDM single-ply or GRP) £25–£45/m² × 25 m² = £875. Trims, drip edges, parapet: £600–£900.

Roofer labour (full strip, deck, insulate, membrane): 4–6 days at £230–£320 standard = £900–£1,900. Plus labourer 4–6 days at £140–£200 = £560–£1,200.

Total flat roof: £4,000–£6,800 typical for 25 m².

Pitched roof construction (concrete tile): deck and felt £15–£25/m² + battens £4–£8/m² + tiles (concrete plain) £14–£20/m² + ridge tiles £25–£40 each + flashing £10–£15/m² + insulation in roof £25–£45/m². Total typically £100–£170 per m² of slope area.

Glazing Detail

Bi-fold doors are typically the highest-cost single item. A 4 m × 2.1 m bi-fold:

Aluminium 4-pane bi-fold (mid-range, e.g. Origin OB-72 or Smart Visofold): supply £4,500–£6,800 for the unit. Installation £400–£600. Total typically £5,000–£7,500 fitted.

Triple-glazed bi-fold: supply £6,000–£9,000. Installation as above.

Sliding patio doors (alternative): typically 30–40% cheaper than bi-fold of equivalent span, with thinner profile but no full opening.

Velux rooflights: 1.14 × 1.18 m typical size, £450–£800 each fitted.

Roof lantern: see the cost build-up for roof lanterns for detailed pricing.

Standard window (1.2 × 1.2 m PVC casement double-glazed): £450–£750 fitted. Aluminium equivalent: £900–£1,400 fitted.

Internal Fit-Out

Plastering: scratch and skim of new walls. Wall area for 25 m² extension is approximately 50 m² (back, two sides, tied wall). Plus ceiling 25 m². Total 75 m² of scratch and skim.

Two-coat plastering: 75 m² × £35–£60/m² = £2,625–£4,500.

Decoration (mist coat + 2 top coats): 75 m² × £20–£40/m² = £1,500–£3,000.

Joinery (skirting, architrave, internal door): typically £1,200–£2,500 for a typical 25 m² extension.

Floor finish: variable from £20/m² laminate to £100+/m² engineered hardwood. Typical mid-spec: £45–£70/m² fitted = £1,125–£1,750.

Mechanical and Electrical

Plumbing first fix (drainage runs, water supplies, gas extension if applicable): £700–£1,500 for typical 25 m² extension without bathroom. Plus £1,500–£3,000 for new bathroom or en-suite if in scope.

Electrical first fix (lighting circuit, ring/radial sockets, fused spurs): £600–£1,200 for typical 25 m² extension. Plus £200–£400 for Part P certification.

Heating extension (radiators on existing system): £80–£160 per radiator fitted. Typical extension needs 2 radiators = £160–£320. Underfloor heating: £45–£75/m² fitted = £1,125–£1,875 for 25 m².

Kitchen fit-out: highly variable. Budget kitchen £4,000–£8,000 fitted; mid-range £8,000–£18,000; high-end £18,000–£40,000+. Often handled as a Provisional Sum to allow homeowner to make their own choice.

Overhead, Margin, Contingency

Sub-total of all trade line items for a typical 25 m² extension: £40,000–£55,000 in trade cost.

Overhead recovery 8–15%: £3,200–£8,250.

Net margin 12–22%: £4,800–£12,100.

Contingency 7.5–10%: £3,000–£5,500.

Final price typically £55,000–£80,000 for standard finish, £80,000–£105,000 for high spec.

VAT at 20% if the contractor is VAT-registered (most are at this turnover level). Some homeowners are surprised by VAT — clarify in the quote whether the price is gross or net of VAT.

Frequently Asked Questions

How do I work out a fair day rate as a builder?

Start with what you need to take home weekly (after tax, NIC, insurance, business expenses). Add overhead (yard, vehicle, tools, insurance, accountancy) typically 25–35% of gross. Divide by working days per year (typically 220–230 actual chargeable days). Add margin for risk and pension. The result is your minimum sustainable day rate.

A self-employed sole-trader builder targeting £45,000 take-home typically needs to charge £200–£260 per day. A small builder running a team of 3–4 typically needs to charge their own time at £280–£380 per day to cover team supervision and yard overhead. London and south-east premium reflects the higher cost of living and overhead.

Why is the same extension priced so differently by different builders?

Three main reasons. First, scope — one builder includes the kitchen, drainage, and party wall costs while another excludes them as PC sums or owner-supply. Second, specification — handmade brick versus standard facing brick, triple-glazed bi-fold versus double-glazed, premium kitchen versus mid-range — these change material costs by 50–200% on the same line item. Third, overhead and margin structure — a single-trader on low overhead can price 20–30% below a larger contractor, but the larger contractor brings more redundancy and better project management. See the difference between a quote and an estimate for the formal versus informal cost difference.

What's a sensible builder margin?

For domestic single-storey extensions, net margin of 12–22% on contract value is the working range for a sustainable business. Below 10%, the business is loss-prone after the inevitable delays and snagging eat into margin. Above 25%, the business is uncompetitive against most rivals on like-for-like work. Combine with overhead recovery 8–15% for total mark-up of 22–37% on trade cost.

How long does a single-storey rear extension take to build?

A 25 m² 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. Programmes extend in winter (groundworks weather), where party wall agreements delay start, where supply chain issues affect specific items (bi-folds, structural steel, kitchens), or where homeowner indecision delays choice of finishes during the build.

Should I do my own kitchen as a homeowner to save money?

Yes, if you have time and project management skills. A kitchen supplied by the homeowner directly (Howdens, Wickes, IKEA) and fitted by the builder during the extension typically saves 8–15% versus the builder buying and supplying. The trade-off is the homeowner becomes responsible for kitchen quality, delivery timing, and any disputes. Most homeowners find the saving is worth the effort on a £15,000+ kitchen.

Regulations & Standards