House Extension Cost Calculator: UK Labour & Materials 2026

Quick Answer: A typical UK single-storey rear extension costs £1,800–£2,800 per m² fully finished (2026 prices), with London and the South East at £2,400–£3,500/m² and the North at £1,600–£2,200/m². Build the price from first principles: labour by trade and duration, materials by phase (substructure, superstructure, roof, fit-out), then add preliminaries (10–15%), contingency (5–10%) and margin (15–20%). VAT is standard 20%, but the 5% reduced rate applies to renovations of dwellings empty for 2+ years.

Summary

Pricing an extension is the single most important commercial skill in residential building. Get it right and the job is profitable, the client is satisfied, and you can repeat the formula on the next one. Get it wrong and you lose money silently for six months, then struggle to find out why. The most common pricing mistakes are (1) underestimating duration, (2) forgetting preliminaries, (3) skipping contingency, and (4) treating margin as a tip on top rather than a planned profit. This guide breaks the pricing process down into the components a competent builder uses on a £40k–£200k extension.

The right structure for an extension quote is a build-up, not a top-down "£X per square metre" figure. Per-square-metre rates are useful for sense-checking and for early conversations with clients, but they are not a substitute for a real cost build-up. A £40k single-storey extension to a Victorian terrace with party walls, deep foundations and a steel beam through the kitchen is a completely different beast from a £40k extension to a 1990s semi with standard strip footings and a simple lean-to roof. Same square-metre figure, very different jobs.

A common misconception is that the way to compete is to drop margin. This is wrong on two counts. First, margin is what keeps the business solvent during the inevitable bad job — strip it out and the next overrun closes the business. Second, clients buying on price alone are almost always the worst clients to work for. Better-quoted, properly-margined jobs come with better clients, fewer disputes and easier cash flow.

Key Facts

• Typical UK range (2026) — £1,800–£2,800/m² fully finished for a single-storey rear extension. Two-storey works out slightly cheaper per m² (shared foundations and roof) at typically £1,600–£2,400/m².
• Regional variation — London and South East +20–30%, South West and East roughly UK average, Wales and Midlands typically -5–10%, North England and Scotland -10–20% versus UK average.
• Standard builder day rate — £200–£300/day (lead builder), £150–£200/day (labourer). Self-employed sole traders often £180–£280/day.
• Trade-specific labour rates — Electrician £35–£50/hr (£280–£400/day), Plumber £40–£60/hr (£320–£480/day), Plasterer £180–£250/day, Carpenter (1st/2nd fix) £180–£280/day, Tiler £180–£250/day, Roofer £200–£280/day, Bricklayer (gang of 2) £400–£550/day combined.
• Preliminaries — 10–15% of construction cost. Covers site setup, skip hire, scaffolding, welfare, supervision, plant hire, temporary services.
• Contingency — 5–10% of construction cost. Reserve for unknowns (rotten timber, soft ground, asbestos discovery, structural variation).
• Margin (profit) — 15–20% of total cost (a 20% margin = 25% mark-up on cost). Below 15%, the business cannot absorb a bad week.
• VAT — Standard 20%. Reduced 5% for dwellings empty 2+ years (VAT Notice 708 section 8). New-build extensions to listed buildings: no longer zero-rated since 2012 (removed by Finance Act 2012).
• CIS — Construction Industry Scheme deductions (20% standard, 30% unregistered) for subbies. Only applies if the contractor is a sole trader/company contracting to a non-domestic client, OR contractor is registered as a "contractor" under CIS rules. Domestic homeowner jobs are typically outside CIS.
• Building Regs — Building Notice or Full Plans application required. Typical local authority fees £400–£900 depending on extension size and council.
• Planning — Many single-storey rear extensions fall under Permitted Development (up to 4m on detached, 3m on semi/terrace, larger via prior approval up to 8m/6m until further notice). Confirm before quoting.
• Party Wall Act 1996 — If digging foundations within 3m of a neighbour's wall (or 6m if foundations are deeper), Party Wall Notice is required. Surveyor cost £700–£2,000 typically.
• Foundation cost — Strip foundations: £150–£250 per linear metre dug, including muck-away. Trench-fill: £200–£350/m. Piled foundations (clay shrinkage, tree roots, soft ground): £8,000–£20,000+ depending on engineer's design.
• Steel beam typical — Supply and install of an RSJ for a typical knock-through: £2,500–£5,000 including padstones, supports, and structural engineer's calculations (~£400–£800).
• Bifold doors — £2,500–£5,000 per linear metre for aluminium bifolds, supply and fit. Sliding doors slightly more for large spans.
• Flat roof typical — EPDM single-ply £80–£120/m², GRP fibreglass £100–£140/m², warm-deck construction.

Quick Reference Table

Detailed Guidance

Step 1 — Define the scope precisely

Before pricing anything, define the scope in writing. A typical single-storey rear extension scope might be:

• 4m × 6m rear extension (24 m² external footprint)
• Trench-fill foundations 1m deep, mass concrete
• 100mm block inner leaf, 100mm cavity (PIR insulated), 100mm facing brick outer
• Steel beam in opening between existing kitchen and new room
• Warm-deck flat roof, EPDM finish, 3 rooflights
• 4m bifold door, 1 standard window, 1 internal door
• Underfloor heating wet system, screed finish
• Re-route waste and supply pipes, new radiator circuit
• Re-wire affected area, new consumer unit if upgrade needed
• Skim plaster all new walls and ceilings
• Decorate emulsion only (kitchen fitting by client's chosen fitter)

Use the scope to drive the quote. Every line item in the build-up should map to a piece of scope. If something is not in the scope, it should be flagged as "by client" or "excluded". Vague scope = scope creep = lost margin.

Step 2 — Build up cost from first principles

SUBSTRUCTURE
  Setting out, excavation, muck-away (incl. plant hire)
  Concrete foundations
  Drainage diversions / new connections
  Damp proof course

SUPERSTRUCTURE
  Bricklaying labour
  Bricks (facing)
  Blocks (inner leaf)
  Wall ties, DPC, insulation, cavity closers
  Lintels, padstones
  Steel beam supply
  Steel beam install (typically 1 day for 2 builders + scaffolder)

ROOF
  Roof joists / firrings
  Decking (OSB or ply)
  Insulation (warm deck PIR typically 150mm)
  Vapour control layer
  EPDM membrane (or GRP)
  Skirtings, flashings, drips
  Rooflights (supply + install)
  Guttering / downpipe

EXTERNAL FINISHES
  Bifold or sliding doors
  Windows
  Soffit / fascia
  Render or brick pointing

INTERNAL FIRST FIX
  Stud walls (if internal divisions)
  Electrical first fix (chases, back boxes, cabling)
  Plumbing first fix (radiator pipework, hot/cold runs)
  Underfloor heating pipework + manifold
  Insulation to floor

INTERNAL SECOND FIX
  Plasterboard + skim
  Electrical second fix (sockets, switches, lights)
  Plumbing second fix (radiators, taps)
  Internal doors, skirting, architrave
  Flooring substrate / screed

FINISHES
  Painting & decorating
  Floor finish (or by client)
  Final clean

PRELIMS
  Skip hire (typically 3–5 × £350 per skip)
  Scaffolding (4 weeks typical, £1,200–£2,500)
  Welfare hire / portaloo (£35–£60/week)
  Site supervision (lead builder day count)
  Tool hire (mixer, breaker, etc.)
  Insurance (allocated portion)

CONTINGENCY
  5–10% of construction cost

MARGIN
  15–20% on total

Step 3 — Worked example (4m × 6m single-storey rear extension)

For a 24 m² single-storey rear extension in the Midlands, standard finish:

That works out at £4,123/m² of internal floor area — but per-m² of external footprint (often how clients think about it) it is roughly £4,125/m² × 0.85 effective rate ≈ £2,800/m². Within UK norms for a standard-finish Midlands extension.

Step 4 — Where pricing goes wrong

• Underestimating duration. A 24m² extension is rarely an "8-week job". Foundations to handover is typically 10–14 weeks. Every day of duration adds prelims and labour.
• Forgetting the steel. A steel install is more than the supply price. Calcs, padstones, lifting, propping, sometimes scaffolding alterations.
• Forgetting drainage. New rainwater run-off, foul connections, kitchen waste diversion. Easily £1,500–£3,500 of work that gets missed.
• Forgetting building control. Notice fee, inspections, certificates. £400–£900.
• Forgetting Party Wall costs. If a neighbour invokes their right to a surveyor, the client pays the neighbour's surveyor too. Flag it.
• Forgetting making good the existing house. Re-decorating the original kitchen where the wall came down, retiling, flooring transitions. Often forgotten and absorbed by the builder.
• Materials inflation between quote and start. Insulation and timber prices have been volatile since 2020. Either lock prices with a supplier, or include a materials inflation clause for jobs starting >8 weeks out.

Step 5 — Margin discipline

Margin is the planned profit on the job. It is not negotiable downwards without removing scope or quality. If a client asks to negotiate the price, the answer is "what would you like to remove?" not "I'll take a haircut on margin". Builders who give up margin to win jobs are subsidising the client's project from their own retirement.

A useful sanity check: at 18% margin on a £58,730 cost base, you have roughly £10.5k of profit to absorb the bad stuff. The wettest week of the year that loses you 4 days, the structural engineer who wants a re-design, the bifold supplier who's a month late — that's where the margin goes. If margin is 10%, the first bad event eats the lot.

VAT — the 5% reduced rate

Under VAT Notice 708, the 5% reduced rate applies to:

• Renovations of dwellings empty for 2 years or more before work starts (proven by Council Tax records or letter from local authority)
• Conversions changing the number of dwellings (e.g. one house to two flats)
• Conversion of non-residential to residential (e.g. barn to house)

A typical extension to an occupied family home is standard-rated at 20%. Always state VAT treatment explicitly in the quote. Mistakes are costly — over-charging VAT means refunding the client; under-charging means paying it from margin.

Frequently Asked Questions

How do I quote when I don't yet have full drawings?

Quote a "Stage 1" estimate based on the scope and known unknowns, with explicit assumptions. State that a firm price requires structural engineer's drawings, building regs detail, and a final spec on doors, windows and fit-out. Many builders use a Pre-Construction Services Agreement (PCSA) — a small paid agreement to do the pricing work properly.

Should I quote fixed price or cost-plus?

For most domestic extensions, fixed price (lump sum) is what clients expect and is what gets you the job. Use cost-plus only when scope is genuinely uncertain (heritage refurb, unknown ground conditions) and the client is sophisticated. Always carry a generous contingency in fixed-price quotes.

How do I handle variations?

Issue a written variation notice for every change before doing the work. State the cost change (plus or minus), the time impact, and have the client sign or email confirmation. Never start a variation on a verbal "go ahead". This is the single biggest source of payment disputes.

What about CIL and Section 106?

Community Infrastructure Levy may apply to larger extensions (typically >100m² of new floor area, though thresholds vary by council). Check with the local planning authority before quoting — CIL can run to thousands of pounds and is the client's liability but you need to flag it.

Should I charge for the quote?

For a one-page estimate, no. For a detailed cost build-up with drawings reviewed, structural input, schedule of works and a fixed price — yes, charge for it. £500–£2,000 is normal for a serious pricing exercise. Many builders refund the fee if the job goes ahead.

Regulations & Standards

• Building Regulations 2010 (England) — Notably Parts A (structure), B (fire safety), C (moisture), F (ventilation), J (combustion appliances), K (protection from falling), L1B (energy in existing dwellings), M (access), N (glazing safety).

• The Building Safety Act 2022 — Higher-risk buildings provisions; mostly affects developments >18m or 7+ storeys, but introduces wider duty-holder concepts.

• Town and Country Planning (General Permitted Development) Order 2015 — Permitted Development rights for householder extensions.

• Party Wall etc. Act 1996 — Notices, surveyors, and dispute resolution for work affecting party walls or close to neighbours' foundations.

• CDM 2015 — Construction (Design and Management) Regulations 2015. Applies to virtually all domestic extensions; client duties transfer to the contractor by default on single-contractor jobs.

• VAT Notice 708 — Buildings and construction. The reduced-rate and zero-rated provisions for housing.

• HGCRA 1996 (as amended 2009) — Construction Act. Payment and adjudication framework, except for residential occupier contracts.

• Consumer Rights Act 2015 — Applies to domestic contracts. Quality and service standards.

• LABC Cost Guide — Local Authority Building Control reference

• RICS Building Cost Information Service — Industry-standard cost benchmarks

• Planning Portal — Permitted Development rules and application processes

• HMRC VAT Notice 708 — Buildings and construction VAT rules

• FMB Cost Guide — Federation of Master Builders pricing surveys

• BCIS — Building Cost Information Service — Granular cost data for construction professionals

• loft conversion pricing guide — Loft conversion pricing in the same format

• retention payment guide — Handling retention in extension contracts

• site induction checklist — CDM 2015 induction requirements on extension projects