How to Price a Double Storey Extension: Structure, Labour and Materials Breakdown

Quick Answer: A typical UK double-storey rear extension in 2026 prices at £1,950–£2,950 per m² of usable floor area for standard finish, £2,950–£4,000 per m² for high specification, with London and the south-east 25–45% above national rates. For a typical 35 m² double-storey extension (17.5 m² ground + 17.5 m² first floor), expect £75,000–£115,000 standard, £115,000–£155,000 high spec. Double-storey extensions cost 90–110% more than single-storey of equivalent footprint, not 200%, because foundations and roof are shared between the two floors. The first-floor scope (bedroom plus en-suite) typically adds the highest cost-per-m² uplift due to plumbing fit-out density.

Summary

A double-storey extension is the highest-value domestic job in the small-builder market — an £80,000–£155,000 project that sits at the upper end of what a single small builder can manage without a multi-team operation. Pricing them properly matters because the variations between a profitable and a loss-making double-storey are larger than for any other domestic project size. Mistakes in foundation pricing, in steel fabrication for the first floor, or in scope of bathroom or kitchen included can swing margin by £15,000–£25,000 either way.

The pricing logic differs from single-storey in three ways. First, the cost split changes: double-storey has more wall area per m² of floor (because two floors share the same footprint) so masonry is a higher proportion. Second, the structural steel demand is higher, both for the first-floor structure and for opening up the existing dwelling to the new floor area. Third, the first-floor element typically includes a bedroom and en-suite, pushing plumbing and bathroom fit-out cost. Per-m² rates for double-storey are slightly higher than single-storey when normalised for finish quality.

This guide focuses on the build-up approach for a typical double-storey rear extension in the 30–50 m² range. Larger extensions, side-and-rear wraparound double-storey extensions, and L-shape variants follow the same logic with larger scopes. The Party Wall Act, planning permission considerations, and Building Regulations are typically more demanding for double-storey because the impact on neighbours is greater.

Key Facts

Quick Reference Table

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Element % of total price (typical 35 m² double-storey) Typical £/m² of floor area
Groundworks and foundations 7–11% £150–£250
Structural (walls, beams, lintels) 18–24% £450–£700
First-floor structure 6–10% £150–£280
Roofing 7–11% £180–£300
Windows and external doors 10–15% £250–£450
Internal fit-out 12–18% £300–£500
Mechanical and electrical 8–12% £180–£280
First-floor en-suite (if in scope) 5–9% £100–£200
Kitchen / ground-floor fit-out 6–14% £150–£400
Externals 3–5% £80–£150
Overhead and profit 18–25% n/a
Contingency 7.5–10% n/a

Detailed Guidance

Why Double-Storey Is Cheaper Per m² Than Single-Storey

A 35 m² double-storey extension (17.5 m² ground + 17.5 m² first floor) typically costs 90–110% more than a 17.5 m² single-storey extension. The savings come from shared elements:

The cost increases come from:

The net result: per-m² cost is typically 5–15% lower for double-storey than for single-storey of equivalent specification. This is the financial argument for going double-storey when planning permits — the marginal cost of the upper floor is lower than the value it adds.

Foundations and Ground Floor

Foundation cost is broadly similar to single-storey of equivalent footprint, with marginal increase for the additional load: 1.0–1.2 m deep × 600 mm wide strip foundation typical, with C25 mass-fill. For a 17.5 m² ground-floor footprint with 16 m foundation perimeter, this is approximately 9.6–11.5 m³ of foundation concrete — £1,250–£1,950 in concrete plus £900–£1,400 in excavation and labour.

Ground floor slab: same as single-storey, typically £40–£70 per m² for DPM, blinding, slab finish.

Where double-storey adds load to existing foundations (when the new structure ties into and bears on the existing house), the existing foundations may need underpinning or augmentation. This is one of the largest cost variables — most existing strip foundations are adequate for the new load, but verification by structural engineer is essential before quoting.

Cavity Wall Construction

Cavity wall area for 35 m² double-storey is approximately 80–95 m² (two storeys of 4–5 m wide × 5 m deep extension, with fewer subtractions for openings on the upper floor than the lower).

Per m² wall cost: brick (£15–£20/m² material), block (£15–£20/m² material), insulation (£20–£35/m² material for partial fill PIR), wall ties and ancillaries (£3–£5/m²), bricklayer + labourer (£60–£90/m² standard areas).

Total cavity wall: £113–£170/m² × 85 m² = £9,600–£14,500 typical for 35 m² double-storey.

Bricklayer productivity is slower on double-storey because of scaffolding access and more cuts at openings — typically 5–7 m² per gang day versus 6–8 m² for single-storey. Allow 12–17 gang-days for a typical double-storey wall build.

First-Floor Structure

First-floor joists span between perimeter walls and any internal load-bearing walls. For a typical 5 m × 3.5 m first-floor, the joists span 3.5 m (across the shorter direction) and bear on side walls and a central support if needed.

Standard joist size for 3.5 m span: 200 × 50 mm C24 timber joists at 400 mm centres. Approximately 9 joists for 3.5 m wide first-floor at 400 mm spacing. Each joist 5 m long. Total: 45 linear metres of joist at £8–£12/m = £360–£540 in material.

Add: trimmer (header) joist at the existing wall connection, typically doubled, plus joist hangers (£3–£6 each, 18 units), strapping, and noggins.

Floor decking: 22 mm tongue-and-groove chipboard or moisture-resistant chipboard, typically £14–£18/m² supply and fit. For 17.5 m² first-floor: £245–£315.

Acoustic insulation between floors per Approved Document E: 100 mm mineral wool quilt £8–£14/m² + resilient bars £3–£5/m². Total £11–£19/m² × 17.5 m² = £190–£330.

Typical first-floor structure including timber, decking, insulation, and labour: £80–£140 per m² of first-floor area = £1,400–£2,450 for 17.5 m² first-floor. Plus the steel beams that support the first-floor (see structural section below).

Structural Steel for Double-Storey

Two main steel requirements:

Internal opening beam (ground floor) — supports the load of the existing first-floor structure when the existing wall between the house and extension is removed. Typically a UC 152 × 152 × 30 or similar, depending on span. Cost: £400–£800 per beam fitted.

First-floor support beam(s) — support the new first-floor joists where they need a longer span than economic in timber. Typically one or two beams across the new first-floor area. Cost: £200–£500 per beam fitted.

External rear opening beam — supports the rear wall above any large rear opening (typically bi-fold or sliding doors). Cost: £400–£800 per beam fitted.

Total structural steel for typical double-storey: £1,400–£3,200, plus £700–£1,800 in structural engineer's fees. Heavier or more complex designs (large openings, knock-through with existing house, lateral stability calculations) push up to £4,500–£7,000+.

Roof — Pitched or Flat

Pitched roof typically continues the existing house roof at matching pitch and tile. For a typical rear double-storey, the new roof slopes back from the new gable wall to match the existing roof.

Pitched roof construction: rafters + collar + ridge + felt + battens + tiles + insulation. Cost £100–£170 per m² of slope area (which for a typical pitch is roughly 1.2–1.4× footprint).

Flat roof: see warm flat roof construction details for warm deck construction. Typical £100–£180 per m².

For a typical 17.5 m² ground-floor footprint with pitched roof: roof slope area ~ 24 m². At £130/m² = £3,120.

First-Floor Fit-Out

A first-floor bedroom typically includes:

The en-suite is the highest-cost line item by far. A typical 4 m² en-suite at £4,500–£10,000+ is £1,125–£2,500+ per m² of en-suite area. The cost driver is sanitaryware (£1,500–£4,500), tiling (£1,000–£2,500), plumbing (£800–£1,500), and finishings (£1,000–£2,000).

The first-floor circulation (landing, stair from existing) is often included in the new build cost. The new stair to the first floor of the extension is typically £1,500–£3,500 for a domestic timber stair; if a new stair is needed in the existing house structure to access the new floor, planning permission and structural alterations become significant.

Heat Loss and Heating Capacity

The double-storey extension adds significant heat demand to the existing dwelling. Typical 35 m² double-storey adds 4–7 kW heat demand at design conditions. Check the existing boiler capacity:

Adding heat demand to an underspec boiler causes underheating in winter, which is a common post-construction complaint. Always assess the existing heating system before quoting; a boiler upgrade £1,800–£3,500 may be needed, and is the homeowner's responsibility but should be flagged in the quote.

Frequently Asked Questions

Do I need planning permission for a double-storey extension?

Almost always, yes. Double-storey rear extensions are not normally permitted development; they require a planning application. Some specific permitted development cases exist (small terrace extensions, some side extensions on detached houses), but the general rule is full planning permission. Allow 8–12 weeks for planning consent before contract signing.

What's the cost of a double-storey extension in the UK?

For typical 35 m² double-storey rear extension in 2026: £75,000–£115,000 standard finish, £115,000–£155,000 high spec. London and south-east premium 25–45% above national. The biggest variables are the kitchen specification (£8,000–£40,000 range), the en-suite specification (£4,500–£10,000+), the glazing specification (£5,000–£15,000+ for the rear doors), and any structural complexity in the existing house (knock-throughs, beam alterations).

How long does a double-storey extension take?

Site time: 20–32 weeks for typical 35 m² double-storey. Add 8–12 weeks for planning permission, 4–10 weeks for party wall procedures, and 2–4 weeks for Building Regulations approval. Total elapsed time from instructing an architect to completion: 10–14 months typical.

Why is per-m² cost lower for double-storey than single-storey?

Foundations, roof, site setup, and mobilisation are shared between two floors. The marginal cost of the upper floor is lower than the average cost of the ground floor because the foundation cost is fully covered by the ground floor and the upper floor only adds wall, structure, and fit-out cost. See the per-m² rates for single-storey for comparison for the comparison.

Can I do a double-storey on a single-storey extension's foundations?

Sometimes. If the original foundations were sized for a future double-storey (some forward-thinking original builders did this), they may be adequate. More commonly, the original foundations are sized for single-storey and need to be either underpinned or augmented before the double-storey is built. Structural engineer assessment of existing foundations is essential before pricing this scenario; typical underpinning cost is £1,200–£2,500 per linear metre of foundation augmented.

Regulations & Standards