Double Storey Extension Cost UK: 2024 Pricing Guide

Quick Answer: A typical UK double storey side or rear extension prices at £1,500-£2,200/m² in the North, £1,800-£2,500/m² in the Midlands, and £2,200-£3,000/m² in the South East and London — lower per m² than a single storey because the roof and foundations are spread over double the floor area. A 40m² double-storey (20m² ground + 20m² first floor) typically totals £60,000-£120,000 depending on region and spec. Add £8,000-£15,000 for a first-floor bathroom.

Summary

A double storey extension is the most cost-efficient way to add space to a UK home on a £/m² basis. The expensive items — foundations, roof structure, scaffolding, site setup, professional fees — are essentially the same cost as for a single storey, but you get twice the floor area to spread them across. The pricing rule of thumb is that the second storey adds roughly 60-70% of the first storey cost, not 100%, because you save on foundations and roof.

This is also where pricing gets dangerous. A builder used to single-storey extension pricing who simply doubles the £/m² rate for a double storey will overprice and lose the job. Conversely, applying a single-storey £/m² to the double-storey floor area underprices it because the first floor adds real costs the single storey doesn't have: first-floor joists or beam-and-block, internal staircase, first-floor bathroom (often), scaffold for a longer programme, party wall implications on both levels, and structural engineer time on a more complex calc.

The right approach is to price the elements separately: foundations and ground floor priced as you would for a single storey, then a first-floor cost added as a separate block, then a roof spread over the full footprint. This guide gives you the cost breakdown in that structure so you can build the quote in a way that holds up under customer scrutiny.

Key Facts

Quick Reference Table

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Stage Cost Range (40m² double — 20m² × 2 floors) £/m² (over 40m²)
Site setup, demolition, scaffold erect £4,500-£7,500 £110-£190
Foundations £4,800-£8,500 £120-£215
Ground floor slab (insulation, DPM, screed) £2,200-£3,500 £55-£90
Ground floor cavity walls £8,500-£12,500 £215-£315
First floor structure (joists + decking) £2,500-£4,500 £65-£115
First floor cavity walls £8,000-£11,500 £200-£290
Steels (all beams + padstones + fitting) £8,000-£18,000 n/a
Roof (pitched, tiled to match) £7,500-£12,500 £190-£315
Windows + doors (UPVC, both floors) £4,500-£8,500 £115-£215
First floor bathroom (if fitted) £8,000-£15,000 n/a — separate
Internal staircase £1,500-£3,500 n/a
Electrics first + second fix £4,500-£7,500 £115-£190
Plumbing (heating extension + bathroom rough) £3,500-£6,500 £90-£165
Plasterboard + plaster £5,500-£9,500 £140-£240
Floor finishes (both floors) £3,500-£6,500 £90-£165
Decoration £1,800-£3,200 £45-£80
Building Regs + structural calcs + drawings £3,000-£5,500 n/a

Detailed Guidance

Foundations — Don't Skimp Because You're Going Up Twice

The temptation on a double-storey is to assume the foundations need to be twice as strong. They don't, generally — strip foundations sized for a single-storey extension on a clear site are usually adequate for a double-storey too, because the bearing pressure on the strip is well within the capacity of the underlying soil. But the structural engineer will check this. On shrinkable clay (London clay, Oxford clay, Gault clay) you may need deeper foundations or pile-and-beam regardless, and on a double storey the extra load makes a borderline case more often need the pile-and-beam solution.

Cost terms: a 40m² double-storey with conventional strip foundations on a clear site is £4,800-£8,500 in foundations costs. If pile-and-beam is required, that jumps to £12,000-£25,000+ depending on pile depth and count. Always get the structural engineer's recommendation before quoting.

First Floor Structure

Two routes: timber joists (TJI, solid C24, or engineered I-joists) or beam-and-block (precast). Timber is faster and cheaper (£35-£55/m² supplied and fixed) but has more bounce and transmits sound. Beam-and-block (£60-£90/m²) is heavier, slower, but quieter and stiffer. Most domestic doubles use timber, beam-and-block only when there's a specific reason (very long span, sound transmission concerns, fire compartmentation).

The joist depth is sized to span and load — typical 4m span with 1.5kN/m² imposed load needs 220mm engineered I-joists at 400mm centres, with a 22mm chipboard or P5 deck above. The bearing detail at each end (wall plate, joist hangers or built into the inner leaf) is where Building Control inspects carefully — joist hangers must be the correct gauge and fixed with 4 nails per side as a minimum.

Steels — Always More Than You Think

A double-storey almost always needs more steels than a single-storey. Where the single storey has one beam for the ground-floor knock-through, the double will commonly have:

  1. Ground floor opening beam — the knock-through into the existing kitchen, same as a single storey
  2. First floor opening beam — if you're opening up the first floor through the existing wall (e.g. extending the master bedroom)
  3. Stair head beam — supporting the upper floor over the stair opening
  4. Eaves beam — if the roof is complex or if the side wall above the first floor opening is non-loadbearing-on-its-own

Each beam needs structural calcs, padstones, fire protection and skilled installation. Steel package for a 40m² double is realistically £8,000-£18,000 — much more than the £4,000-£8,000 for a single storey knock-through. Don't underprice this.

Roof — The Efficiency Win

The roof is where double-storey wins on £/m². The footprint is the same as the first floor — say 20m² — and the cost of the roof is essentially the cost of covering that 20m². On a single-storey extension you paid £4,500-£7,500 for a 25m² flat roof or £6,500-£10,500 for a pitched. On a double-storey of the same 25m² footprint, you pay roughly the same — but you've got 50m² of floor area underneath it, so the £/m² of roof spread over floor halves.

Tying the new roof into the existing house roof is the technical challenge. A side-extension double storey often goes hipped or gabled to match; a rear extension goes gable, mono-pitch or flat warm roof. Each option has implications for the existing roof — flashings, valley gutters, and possibly a soakaway redesign for the rainwater that's now landing in a different place.

The First-Floor Bathroom Premium

If the brief includes an upstairs bathroom (very common — en-suite to a new master bedroom), add £8,000-£15,000 to the budget. The cost driver isn't the sanitaryware; it's the plumbing rough-in (running hot, cold and waste to the right places, often through a structural wall or floor zone), the tiling (£60-£100/m² fully fitted), the underfloor heating if specified (£35-£70/m² fully wired and installed), and the extract ventilation (Part F requires a humidity-controlled extract or PIV for an internal bathroom).

The waste run is the critical item. A first-floor bathroom needs a 110mm soil pipe with sufficient fall (at least 1:40 for foul, 1:80 minimum for waste — Part H) connected to the existing stack or via a new external stack. If the existing stack is too far away, you may need a new SVP up the wall, which is a visible architectural item. Sort this on plan, not on site.

Worked Example — 40m² Double Storey, Midlands

A typical brief: 5m × 4m double-storey rear extension on a 1970s semi in the West Midlands, kitchen-diner extension on ground floor, master bedroom + en-suite on first floor. Mass concrete strip foundations 1.0m deep. Cavity construction. Tiled pitched roof tied into existing. Three UPVC windows + French doors. Existing kitchen rebuilt as part of the works.

Item Cost
Site setup, demo, scaffold £6,500
Foundations £6,000
Ground floor slab £2,800
Ground floor walls £10,500
First floor structure £3,500
First floor walls £9,500
Steels (4 beams) £12,500
Roof £9,500
Windows + French doors £5,500
Bathroom (fittings + tiling + UFH) £11,000
Stairs £2,200
Electrics £5,500
Plumbing £4,500
Plaster £7,000
Flooring (both floors LVT) £4,500
Decoration £2,500
Drawings + Reg + structural £4,500
Subtotal trade cost £107,500
Preliminaries (12%) £12,900
Contingency (7%) £7,525
Subtotal before margin £127,925
Margin (20%) £25,585
Total ex VAT £153,510
VAT (20%) £30,702
Total inc VAT £184,212

That's £4,605/m² inc VAT — but the headline £/m² without VAT and including a high-spec bathroom is £3,838/m². If the bathroom and the kitchen weren't included, the figure would drop to roughly £2,400-£2,600/m² — which sits inside the Midlands benchmark band. Always clarify what's included before comparing.

Frequently Asked Questions

Is a double-storey extension really cheaper per m² than a single?

Yes, but only on the shell. By the time you add the first-floor bathroom and the staircase, the saving is smaller than the £/m² benchmark suggests. A like-for-like comparison (no bathroom, no kitchen, just shell-and-fit) sees the double-storey come in 15-25% cheaper per m². With a bathroom included the saving compresses to 5-12%.

Will I need planning permission?

Almost always for a double-storey side or rear extension. The permitted development rights for double-storey are tight: max 3m projection, 7m to rear boundary, materials matching, single pitch roof rather than complex. Most double-storey extensions exceed at least one of these and require a full planning application. Budget £400-£600 for the application fee and £1,500-£3,500 for an architect's drawings if not already done.

Do I need a Party Wall award?

If the extension shares a wall with a neighbour (semi-detached, terraced) or excavates within 3m of their building below their foundation depth, yes. Notice must be served 2 months in advance for line-of-junction work, 1 month for adjacent excavation, under the Party Wall etc. Act 1996. If the neighbour dissents, an award is needed — £750-£2,500 per side for an agreed surveyor, more if separate surveyors are appointed.

How long does a double storey take?

18-26 weeks for a 40m² extension, weather-dependent. The longer programme vs. single storey is driven by scaffold time, sequential floor builds (you can't roof until first floor is up, can't fit first floor until ground floor walls are up), and the stacking of trades. Allow 4-6 weeks for foundations and ground floor, 4-6 weeks for first floor and roof, 6-10 weeks for fit-out and finishes.

Should I match the existing brick?

Yes — both planning and customer expectation. A brick blend can usually be sourced via a brick-matching specialist (Imperial Bricks, Northcot, or local merchants who match) for £600-£1,500 per 1000 bricks against a £400-£700 standard. Budget the brick match in the quote.

Regulations & Standards