UK Renovation Cost Index 2026 · Build-ups

Ground Floor Extension: Materials & Labour Build-Up

The classic rear extension: knock through the back wall, pour a new slab, build cavity walls to a flat or pitched roof, and open the back of the house onto the garden. The shell is where most money goes; the kitchen and glazing decide whether you land at the bottom or top of the range.

BasisSingle-storey rear extension, 20 m² (5 m × 4 m), open-plan kitchen-diner with bifolds

Typical all-in £36,000–£56,000

Per m²£1,800–£2,800 per m²

Duration10–14 weeks on site

Average split32% materials · 53% labour · 15% fees & prelimsLabour is ≈62% of materials + labour, the ratio trades quote on. City uplift lands mostly on labour (materials get ~30% of it, fees ~80%), so the split shifts toward labour in bigger cities.

Where is the project? Prices scale with the city, London carries the full +25%.

Line-by-line build-up

Materials = supply cost; labour = what the trade charges for that element; fees & prelims = design, Building Control, party wall, scaffold, skips and site costs. Ranges are typical 2026 UK figures outside London. Every line links to the guide that explains it; labour figures assume the day rates in the Cost Index.

Element	Materials	Labour	Fees & prelims	Notes
Design, structural calcs & Building Control index ↗	–	–	£3,000–£5,300	Drawings, planning/PD check, engineer, Building Control £900–£1,200
Party wall (if on/near boundary)index ↗	–	–	£550–£1,750
Excavation, foundations & drainage index ↗	£1,850–£2,550	£3,400–£4,750	–	Trench-fill to ~1 m, new drains & build-over if needed
Floor slab, insulation & screed index ↗	£1,100–£1,550	£1,550–£2,150	–	Concrete slab, DPM, PIR, sand-cement or liquid screed
Cavity walls (brick/block) & lintels	£2,500–£3,400	£4,650–£6,250	–	Facing brick outer, block inner, full-fill insulation
Steel beam to open the back wall index ↗	£650–£1,100	£1,100–£1,800	–	Supply, install, padstones, making good
Roof (flat GRP/EPDM or pitched)index ↗	£1,450–£2,200	£2,350–£3,650	–	Incl. insulation, fascia, guttering
Bifold/sliding doors & rooflights index ↗	£2,000–£3,550	£1,250–£2,200	–
Electrics index ↗	£350–£550	£1,250–£1,900	–
Plumbing, heating & UFH index ↗	£650–£1,100	£1,400–£2,400	–	Wet UFH at £60–£120/m² installed
Plastering index ↗	£400–£550	£1,500–£2,000	–
Flooring, decoration & second-fix joinery index ↗	£900–£1,500	£1,400–£2,300	–
Skips, scaffold & site costs index ↗	–	–	£900–£1,450
Sub-totals (32% / 53% / 15%)	£11,850–£18,050	£19,850–£29,400	£4,450–£8,500
Build-up total	£36,150–£55,950			vs published guide range £36,000–£56,000

The published guide for this job quotes £36,000–£56,000, which is the figure the build-up above is reconciled against.

Common additions & variations

New kitchen (mid-range, fitted), £12,000–£25,000
Kitchen fitting labour (fit, plumbing, electrics, tiling), £4,000–£7,000 (higher in London)
London uplift, +15–25%
Wraparound (side return + rear) instead, see guide

How to use this

Treat the build-up as a sanity check on quotes, not a price list: a builder's quote that lands inside the total range with a similar materials/labour split is priced normally; one far below it is usually missing scope. Get three itemised quotes, compare them line by line against this table, and ask about anything that's absent. Figures exclude VAT where the trade is VAT-registered (most are above ~£90k turnover), and exclude fees you pay directly such as planning application fees.

Other build-ups

Loft conversion, £45,000–£65,000
Double storey extension, £80,000–£120,000
Internal renovation, £62,000–£122,000