For trades · Estimating tool · September 2026

Price an extension the way a builder does

This tool prices a house extension element by element, the way a builder's estimator or quantity surveyor does: measured quantities for foundations, walls, roof, openings, finishes and services, each at a sourced unit rate, then preliminaries, overheads and profit, contingency and VAT. Every rate is editable and saved in your browser. Rates are dated September 2026 and exclude VAT until the VAT line.

By Estiquote · Rates reviewed 25 September 2026, next review December 2026 · 50 elements from 88 published UK sources · Rate book and sources

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How the quantities are worked out

The extension is treated as a rectangle sharing its long side with the house, so three new external walls are built. Enter the length along the house and the projection out from it, measured to the outside of the new walls. The rules below drive every quantity in the table; the tool shows the computed figures under Derived quantities and in the Show assumptions block.

External walls. Length + 2 × projection, times the wall height and the number of storeys, less openings: the bifold width × 2.1 m and 1.2 m² per window. Facing brick, render on block and timber cladding are priced as complete cavity walls with insulation, ties, DPC and lintels.
Foundations and groundworks. Trench-fill 600 × 1,000 mm along the external wall length plus 1 lm per return. The sourced trench-fill rate includes the trench dig, so the separate excavation line is the oversite dig (floor area × 0.3 m). Muck-away is the trench and oversite spoil bulked by 25%.
Floor. Ground-bearing slab build-up (hardcore, blinding, DPM, PIR, concrete) and a sand and cement screed over the plan area. Two-storey extensions add a joisted and decked first floor.
Roof. Plan area × 1.05 for a flat EPDM warm roof (joists, deck, insulation and membrane) or × 1.25 for a pitched tiled lean-to measured on the slope. Fascias, soffits and gutters are not priced.
Opening into the house. One steel beam sized to the opening, the engineer's calculations and a day, two days or three days of a bricklayer and labourer to needle, prop, take down and make good.
Finishes. Plasterboard and skim to the inside face of the new walls (less openings) and to the old back wall where it is opened up; ceilings at floor area × storeys; decorating to walls and ceilings; skirting round the internal perimeter less the doorways; your chosen floor finish over the floor area.
Services. Electrical points default to one per 2 m² with a minimum of eight, plus a consumer unit if you tick it. Heating is radiators (with pipework from the existing system) or wet underfloor heating in the screed, plus an allowance to extend the pipework. Drainage alterations are a single sourced allowance.
Site. Skips = floor area ÷ 12 rounded up, plus one. Scaffold is one job for a pitched roof, two storeys or walls over 3 m, otherwise half a job. Welfare runs for the programme, which defaults to 8 weeks plus one per 10 m².
Money. Preliminaries are a percentage of the net cost; overheads and profit are applied to net plus preliminaries to give the tender price; contingency is applied to the tender; VAT at 20% is added if the builder is registered. The labour and materials split uses a labour share for each element. Regional factors from the Estiquote model are applied to the labour share of each default rate only, so a bifold set moves far less than a day of bricklaying.

Rates and sources

Defaults are the typical figures. Low and high are the sourced band, shown under each element in the tool; a rate you type outside the band is flagged in amber. Where no source prices an element directly the note says how the figure was derived, and allowances with no direct source are labelled Estiquote allowance. All figures exclude VAT and are for the UK average outside London.

Estiquote element rate book: supply and fit unless stated, excluding VAT, UK average outside London (September 2026)
Element	Unit	Low	Typical	High	Labour share	Sources
Substructure
Trench-fill foundation 600 x 1,000 mm: machine dig, C20 concreteIncludes the trench dig and about 0.7 m of concrete. Muck-away is priced separately. Hand dig or poor access runs £220 to £280 per lm.	lm	£120	£145	£180	40%	Checkatrade, TradesmenCosts, ConcreteMath
Oversite excavation: strip and reduce levels 300 mm under the slabMachine dig, measured net. Typical is the midpoint of Mainline £25 to £50 and ConcreteMath £28 to £55. The trench dig is already inside the foundation rate.	m³	£25	£40	£55	75%	Mainline Groundworks, ConcreteMath
Muck-away: load and remove spoil, bulked 25%Derived: £150 to £450 per grab load divided by about 6 m³ bulked per 10 t grab. Trade rate for inert soil is £28 to £45 per tonne.	m³	£25	£42	£75	40%	MaterialCalculator, Armstrongs Group, ConcreteMath
Ground-bearing floor slab: hardcore, blinding, DPM, 100 mm PIR, 100 to 150 mm concreteDerived from components: slab £65 to £120, hardcore £18 to £35, PIR £7 to £20, membrane £5 to £15. Concrete Choice prices the full build-up at £150 to £300, which sets the high figure.	m²	£95	£140	£300	40%	Concrete Choice, MyBuilder, ConcreteMath, MyJobQuote
Sand and cement screed 65 to 75 mmCheckatrade averages £33 all-in (materials £18 to £20, labour £12 to £16); MyBuilder £20 to £33. Liquid screed is £25 to £40.	m²	£20	£30	£36	45%	Checkatrade, MyBuilder
Drainage alterations: new foul and surface water run, inspection chamber, connectionDerived: chamber £280 to £480 plus foul drain £40 to £90 per m; MyJobQuote allows £250 to £1,550 for drain work on an extension. Build-over agreements and diverting a public sewer are extra.	item	£500	£1,000	£1,550	55%	Squote, Mainline Groundworks, MyJobQuote
Superstructure
External cavity wall: facing brick, 100 mm cavity insulation, block inner leaf, ties, DPC, lintelsCheckatrade £230 insulated; Build It £150 finished or £180 with builder's profit; BookaBuilderUK £180 to £300. Measured net of openings.	m²	£150	£215	£300	50%	Checkatrade, Build It, BookaBuilderUK
External wall: rendered block outer leaf, insulated cavity, block inner leafDerived: no source prices a rendered cavity wall per m². Block outer leaf £70 to £120 plus render £30 to £80 replaces a facing-brick leaf at £100 to £180, so it costs about the same as brick.	m²	£180	£230	£320	55%	Checkatrade, Checkatrade, BookaBuilderUK, Checkatrade
External wall: timber cladding on battens over block outer leaf, insulated cavity, block inner leafDerived: block outer leaf £70 to £120 plus larch or cedar cladding £45 to £105 (average £65) in place of the facing-brick leaf. Cladding rate covers boards, battens and fixing only.	m²	£190	£235	£340	50%	MyJobQuote, BookaBuilderUK, Checkatrade, Checkatrade
First-floor structure: 47 x 200 C24 joists, hangers, strapping, 22 mm chipboard deckEstiquote allowance, derived: timber joist structure £25 to £45 per m² (MyJobQuote, roof structure) plus a fixed timber deck £25 to £45 per m² (Trade2Base). Joists cost £4.17 to £7.28 per m supplied.	m²	£50	£70	£90	45%	MyJobQuote, Trade2Base, Bradfords Building Supplies, W Burton and Sons
Steel beam to opening up to 2 m: supply, install, padstones, propsMyBuilder £800 to £1,800 for a 2 to 3 m opening; typical is the midpoint of MyJobQuote's 3 m beam at £800 to £1,100.	item	£800	£950	£1,800	65%	MyBuilder, MyJobQuote
Steel beam to 3 to 4 m opening: supply, install, padstones, propsMyJobQuote 3 m £800 to £1,100 and 4 m £900 to £1,280 plus props and padstones £100 to £200; MyBuilder 4 to 5 m £1,500 to £2,500. Making good is priced separately.	item	£900	£1,250	£2,500	65%	MyJobQuote, MyBuilder, Checkatrade
Steel beam to opening over 4 m: supply, install, padstones, propsMyBuilder £1,500 to £2,500 for a 4 to 5 m span; typical is the midpoint. Very wide or heavy beams may need a crane or a goalpost frame, which is extra.	item	£1,500	£2,000	£2,500	60%	MyBuilder, Checkatrade
Structural engineer: beam design and calculationsCheckatrade and MyJobQuote £250 for calcs alone; MyBuilder and MyJobQuote £300 to £500 with a site visit. Structural design for a whole extension is £1,500 to £2,500.	item	£250	£400	£500	90%	Checkatrade, MyJobQuote, MyBuilder, Simplify Structures
Form opening in existing wall: needle and prop, take down, cart to skip, make good revealsEstiquote allowance: one bricklayer (£240 to £320 per day) and one labourer (£110 to £240) per day. Allow 1 day for an opening up to 2 m, 2 days for 3 to 4 m and 3 days for wider.	day	£350	£450	£560	100%	Checkatrade, Green's Brickwork, Checkatrade, RenoCalc
Roof
Flat roof: joists, firrings, OSB deck, vapour control, PIR warm roof, EPDM membrane and trimsDerived: EPDM £80 to £120 plus deck £25 to £45 plus warm-roof insulation £30 to £60 (Trade2Base, ex VAT) plus joists £25 to £45 (MyJobQuote). GRP is about £10 per m² more. Fascias and gutters are extra.	m²	£160	£200	£270	45%	Trade2Base, Trade2Base, MyJobQuote, Checkatrade
Pitched lean-to roof: rafters or trusses, membrane, battens, concrete tiles, insulationSources disagree: Checkatrade gives tiles at £120 to £230 per m²; built from components (trusses £25 to £32.50, concrete tiles fitted £58 to £83 ex VAT, insulation £18 to £30) the total is about £100 to £145. Measured on the slope.	m²	£120	£175	£260	50%	Checkatrade, Checkatrade, Fix My Roof, Build It
Openings
Aluminium bifold door set, about 3 m (3 panels), supply and fitCheckatrade £3,000 to £4,500; MyJobQuote £3,150 to £4,650 (supply £2,550 to £4,050 plus £600 fitting). Lintel over the opening is in the wall rate.	set	£3,000	£3,900	£4,650	15%	Checkatrade, MyJobQuote
Aluminium bifold door set, about 4 m (4 panels), supply and fitCheckatrade £4,500 to £5,500; MyJobQuote £4,300 to £5,700 (supply £3,500 to £4,900 plus £800 fitting).	set	£4,300	£5,000	£5,700	15%	Checkatrade, MyJobQuote
Aluminium bifold doors wider than 4 m, per linear metre, supply and fitDerived from the 4 m set: about £1,000 to £1,550 per linear metre. Sliding doors run about £720 per m² of door (Checkatrade).	lm	£1,000	£1,250	£1,550	15%	Checkatrade, MyJobQuote
uPVC casement window about 1.2 x 1.2 m, supply and fitCheckatrade new-windows guide: £300 to £400 (600 x 900) and £450 to £550 (1,200 x 1,200); its double-glazing guide gives £500 to £1,250. Fitting labour alone is about £80.	item	£300	£500	£1,250	20%	Checkatrade, Checkatrade, Checkatrade
Rooflight or roof window about 0.8 x 1.2 m, supply and fit including upstand or flashingCheckatrade £750 to £1,750 (average £1,250); MyJobQuote £755 to £995 for Velux units. The sources price pitched-roof windows; fixed flat-roof glass rooflights cost about the same at this size.	item	£750	£1,100	£1,750	35%	Checkatrade, MyJobQuote
Aluminium roof lantern about 1.5 x 2 m, supply and fit including kerbMyJobQuote £1,900 to £2,300 medium and £2,500 to £3,800 large, average £2,250 installed; MyBuilder £800 to £1,500 for smaller units.	item	£1,800	£2,250	£3,800	25%	MyJobQuote, MyBuilder
Internal finishes
Plasterboard on dabs and two-coat skim to wallsTrade2Base and Kent Plasterers both give £18 to £30 per m² for board and skim. Plasterboard is about £3.50 per m² of the rate.	m²	£18	£24	£30	70%	Trade2Base, Kent Plasterers
Plasterboard and skim to ceilingsMyBuilder £18 to £39 per m² for boarding ceilings including skim. Checkatrade prices a skim at £24 per m².	m²	£18	£28	£39	70%	MyBuilder, Checkatrade
LVT flooring: subfloor preparation, supply and fitDerived from Checkatrade: supply £25 to £40, fitting £10 to £60 (average £45), preparation £2 to £13.	m²	£40	£80	£115	50%	Checkatrade
Engineered wood flooring, supply and fitCheckatrade £50 supply plus £45 fitting; MyBuilder £40 to £50 plus £20 to £45. No source gives a premium figure, so the high end matches typical.	m²	£60	£95	£95	45%	Checkatrade, MyBuilder
Porcelain floor tiles, supply and fit including adhesive and groutCheckatrade £60 to £130 per m² (labour £30 to £60, mid-range 9 mm tiles about £35).	m²	£60	£90	£130	55%	Checkatrade
MDF skirting, supply and fitCheckatrade £18 to £26 for 75 mm boards and £20 to £40 in its carpentry guide. Architrave to the opening is not priced (no supply-and-fit source).	lm	£18	£22	£40	60%	Checkatrade, Checkatrade, MyJobQuote
Decorating: mist coat and two coats of emulsion to walls and ceilingsCheckatrade £10 to £20 (average £15); MyBuilder £8 to £25. Gloss to skirting is included in this allowance.	m²	£8	£15	£25	80%	Checkatrade, MyBuilder
Services
Electrical point (socket, switch, light, spur): first and second fix, testedElec-Mate, Midlands: £85 to £110 per point on a new build, £100 to £130 in an existing property. Labour alone is £40 to £75 per point. Light fittings are supplied by the client.	point	£85	£105	£130	55%	Elec-Mate, Checkatrade
Consumer unit replacement or upgrade, tested and certifiedElec-Mate £450 to £850 if the existing board has no spare ways or does not meet current regulations; typical is the midpoint.	item	£450	£650	£850	50%	Elec-Mate
Wet underfloor heating: pipe, manifold, clips, controls, commissioning (screed separate)Checkatrade new build £85 to £100; MyJobQuote £100 to £140; Squote £80 to £140; MyJobQuote's kitchen extension guide £50 to £100. Manifold alone is £250 to £500 supplied.	m²	£50	£95	£140	50%	Checkatrade, MyJobQuote, Squote, MyJobQuote
Radiator in a new position, supplied and fitted with pipeworkBookaBuilderUK £300 to £700 for a new radiator on new pipework. A like-for-like swap is only £180 to £300.	item	£300	£450	£700	60%	BookaBuilderUK, Checkatrade, MyJobQuote
Extend heating pipework from the existing system to the extensionBookaBuilderUK £100 to £300 on top of the per-radiator rate; typical is the midpoint. A boiler that cannot carry the extra load is not allowed for.	item	£100	£200	£300	70%	BookaBuilderUK, Squote
Preliminaries and site
8-yard skip, delivered, collected and tippedCheckatrade £250 to £320 in the North and £290 to £360 in the South (London £330 to £450); Mick George £230.10 ex VAT. A road permit adds £15 to £60.	skip	£240	£300	£360	30%	Checkatrade, Mick George
Scaffold to one elevation, 6 to 8 weeks hireMyJobQuote £450 to £800 for one elevation over 6 to 8 weeks, then £15 to £30 per week. A flat-roofed single storey often needs only a half lift or tower, hence 0.5 of a job.	job	£450	£625	£800	60%	MyJobQuote, Checkatrade
Site welfare: portable toilet with weekly serviceGigLoo ex VAT: portable toilet £15 to £30 per week, welfare cabin £60 to £150 per week. Typical assumes a serviced toilet; delivery is extra.	week	£15	£30	£150	20%	GigLoo
Percentages
Preliminaries: supervision, insurance, small plant, protection, cleaningLynch Brother Homes 5 to 15%; Rise Design Studio 12 to 18% in London; MyBuildAlly £40 per m², about 3% of direct trade costs. Typical is the midpoint. Applied to the net cost.	%	5%	10%	15%	80%	Lynch Brother Homes, Rise Design Studio, MyBuildAlly
Overheads and profitCheckatrade 15 to 25% for managing the job; MyBuildAlly main-contractor addition 20 to 35%; VoxTrade renovation net margin 20 to 30%; Access Group overheads 10 to 20% plus profit 5 to 10%. Applied to net cost plus preliminaries.	%	15%	20%	30%	0%	Checkatrade, MyBuildAlly, VoxTrade, The Access Group
ContingencyFMB 10%, or 15% and more for structural work and older homes; Homebuilding & Renovating 10% plus 5% for inflation; Rise 10 to 15%. A client-side allowance, not part of the tender.	%	5%	10%	15%	0%	Federation of Master Builders, Homebuilding & Renovating, Rise Design Studio
Reference: materials
Ready-mix concrete C20/25, deliveredReference only: already inside the foundation and slab rates. Loads under 4 m³ add £30 to £60 per m³; some suppliers charge delivery at £100 to £200.	m³	£95	£115	£160	10%	Checkatrade, MyBuilder, MyJobQuote
Facing bricks, per 1,000, supply onlyReference only, inside the wall rate. Selco £0.66 to £1.37 per brick; BuildingMaterials.co.uk £504.40 per 500. About 60 bricks per m² of facing.	item	£660	£1,000	£1,370	0%	Selco Builders Warehouse, BuildingMaterials.co.uk
Reference: trade day rates
BricklayerCheckatrade £240 to £320; Green's Brickwork £250 to £320; TradeDayRates typical £250. London £360 to £576.	day	£240	£280	£320	100%	Checkatrade, Green's Brickwork, TradeDayRates
LabourerCheckatrade £112 to £360 (average £240) sits above RenoCalc £112 to £144 and MyJobQuote £90 to £125; typical is a midpoint. London agency rate £150 to £250 ex VAT.	day	£110	£170	£240	100%	Checkatrade, RenoCalc, MyJobQuote
Carpenter or joinerCheckatrade £240 to £360 and £300; TradeDayRates £285. London up to £500.	day	£240	£285	£360	100%	Checkatrade, Checkatrade, TradeDayRates
PlastererKent Plasterers £170 to £210 in the Midlands; TradeDayRates £250; Checkatrade £300. London £250 to £300.	day	£170	£250	£300	100%	Kent Plasterers, TradeDayRates, Checkatrade
ElectricianElec-Mate £220 to £300; TradeDayRates £325; Checkatrade £300 to £500. London £280 to £380.	day	£250	£325	£400	100%	Elec-Mate, TradeDayRates, Checkatrade
PlumberSquote £240 to £360 regional (Gas Safe £320 to £480); TradeDayRates £315; Checkatrade £320 to £480. London £300 to £440.	day	£240	£315	£400	100%	Squote, TradeDayRates, Checkatrade
GroundworkerSleepless Tradesman £180 to £320 (Midlands £180 to £260); ConcreteMath £200 to £280 for self-employed CIS groundworkers. London £280 to £380.	day	£180	£240	£300	100%	Sleepless Tradesman, ConcreteMath

Scroll the table sideways to see every column.

Limits

This is an estimate, not a bill of quantities and not a quote. It assumes normal ground with a 1 m trench-fill foundation, clear access for a machine and skips, no asbestos, no drains under the footprint and no work elsewhere in the house. It does not price design fees, building control, planning, party wall matters, fascias and gutters, external steps and paving, landscaping, or a boiler upgrade. Two-storey estimates cover the shell, first floor, roof and open-plan finishes; they do not include upstairs partitions, doors, a bathroom or the extra scaffold lifts a two-storey job needs, so expect them to sit at the bottom of the planning range or below it. There is no site survey behind it: soil, trees, access and the condition of the existing house can move a real tender well outside the range. Check the result against real quotes and your own rates before you rely on it. Read the Estiquote method for how the planning ranges it compares against are built.

Questions builders ask

Is this a bill of quantities?

No. It is an elemental estimate: the quantities come from simple rules (three new walls, a trench-fill foundation, a roof at plan area times a pitch factor, and so on) rather than from drawings. Use it to sanity-check a tender or to build a first price before you measure the job properly.

Why does the total differ from the Estiquote planning calculator?

The planning calculator applies a benchmark price per square metre from published cost guides. This tool builds the price up from measured elements and unit rates, then adds preliminaries, overheads and profit, contingency and VAT. The two should land in the same region for a typical job; the tool shows the planning range and says whether your tender sits within it.

Can I use my own rates?

Yes. Every rate in the table is editable and the totals update as you type. Your rates are saved in your browser (nothing is sent to us), Reset rates puts the defaults back, and Copy link shares the whole estimate, including your rates, with a colleague.

Sources

Every default rate traces to one or more of the published pages below, read in September 2026. Where a page quotes a range, typical is its stated average or the midpoint; where pages disagree the note in the rate book says so.