UK Renovation Cost Index 2026 · Build-ups

Loft Conversion: Materials & Labour Build-Up

A rear dormer is the most common London loft conversion: it adds full-height space at the back, takes a staircase over the existing one and usually gets an en-suite in. This build-up follows the job from scaffold to second fix, with each line linked to the guide that explains the figure.

BasisRear dormer conversion on a 3-bed semi, one bedroom + en-suite, ~25 m² of floor

Typical all-in £45,000–£65,000

Per m²£1,800–£2,600 per m²

Duration8–12 weeks on site

Average split31% materials · 54% labour · 15% fees & prelimsLabour is ≈64% 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 ↗	–	–	£3,750–£5,700	Architect/designer drawings, engineer’s calculations, Building Control £1,000–£1,500
Party wall surveyor (semi/terrace)index ↗	–	–	£750–£2,300	Per surveyor; two-surveyor disputes cost more
Scaffolding index ↗	–	–	£1,300–£1,900	8–12 week hire incl. erect/dismantle
Steel beams (ridge + floor) & padstones index ↗	£1,100–£1,750	£1,800–£2,850	–	Supply, fabrication, crane/handball and install
New floor joists & flooring deck	£1,300–£1,650	£2,800–£3,500	–	C24 timber joists, 18 mm chipboard
Dormer structure, roof & cladding index ↗	£3,600–£4,150	£7,650–£8,850	–	Timber frame, flat roof (GRP/EPDM), tile-hung or cladding sides
Roof windows & dormer window index ↗	£1,400–£2,250	£850–£1,350	–	2 rooflights + dormer window, fire-egress compliant
Insulation (roof, dormer, floor)index ↗	£850–£1,200	£1,300–£1,850	–	PIR between/over rafters to meet Part L
Staircase (supply & fit)index ↗	£950–£1,900	£1,200–£2,350	–	Softwood straight/winder flight, Part K compliant
Electrics (first & second fix)index ↗	£500–£700	£1,750–£2,500	–	New circuits, sockets, lighting, smoke alarms, Part P
Plumbing & heating index ↗	£600–£1,000	£1,350–£2,050	–	Radiators, extension of heating, en-suite services
Plasterboard & plastering index ↗	£700–£900	£2,400–£3,200	–	Boarding, skim, fire-rated board where required
En-suite bathroom index ↗	£1,700–£2,750	£2,050–£3,350	–	Shower, WC, basin, tiling
Fire doors, joinery & decoration index ↗	£1,050–£1,600	£1,600–£2,400	–	FD30 doors to the escape route, skirting, architrave, paint
Skips & waste index ↗	–	–	£650–£900	2–4 skips over the build
Sub-totals (31% / 54% / 15%)	£13,750–£19,850	£24,750–£34,250	£6,450–£10,800
Build-up total	£44,950–£64,900			vs published guide range £45,000–£65,000

The published guide for this job quotes £45,000–£65,000, which is the figure the build-up above is reconciled against.

Common additions & variations

Velux-only conversion instead (no dormer), £22,000–£35,000 all-in
Hip-to-gable + dormer, £55,000–£75,000
Full mansard, £65,000–£90,000+
London uplift, +10–15%

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

Ground floor extension, £36,000–£56,000
Double storey extension, £80,000–£120,000
Internal renovation, £62,000–£122,000