House Extension Cost UK – 2026 Price Guide
Planning a house extension and trying to work out the real cost? This guide uses live UK pricing data to give you realistic figures for single-storey, double-storey and side extensions — with regional breakdowns, worked examples and a free quote tool.
Do you need planning permission for this?
Most single-storey rear extensions are Permitted Development, but two-storey extensions, side extensions, conservation areas, and Article 4 zones often need full planning. Always check before signing contracts.
Read the house extension planning rules →House Extension Cost at a Glance
Real cost ranges drawn from BestBuilders data across 519 UK towns. Use these as your starting benchmark — then get 3 local quotes for your exact project.
Most popular type. Creates kitchen-diner or family room at ground level. Typically 3–5m deep, 10–14 weeks on site.
Get quotes →Adds ground-floor room plus bedroom above. Strong ROI. Requires full planning permission in most cases.
Get quotes →Fills narrow side alley on terraced homes. Transforms cramped kitchen into full-width space. Often under PD rights.
Get quotes →Costs depend on size, spec and location. The only accurate price is from a builder who's seen your property.
Get free quotes →Extension Cost Per m² (UK 2026)
| Specification | Cost per m² | What's included |
|---|---|---|
| Basic build | £1,800 – £2,400 | Standard materials, basic finishes, minimal structural changes |
| Mid-range | £2,400 – £3,000 | Better glazing, higher finish standard, more design input |
| High-end | £3,000 – £3,800+ | Premium materials, large glazing, complex structure, bespoke details |
Cost per m² covers the main structure and basic finishes. Fitted kitchens, bathrooms, professional fees and landscaping are additional.
House Extension Cost by Size (2026)
Looking up costs for a specific size? The tables below give 2026 UK price ranges for the most-searched single-storey and two-storey extension footprints, from a small 3m × 3m extension through to a 6m × 5m two-storey extension. Mid-range build cost only — kitchen, bathroom, professional fees and contingency are on top.
Jump to: Single-storey by size · Two-storey by size · 2nd-floor add-on · By bedroom count
Single-Storey Extension Cost by Size (3×3m to 8×5m)
| Extension size | Floor area | Build cost (mid-range) | Full project budget |
|---|---|---|---|
| 3m × 3m extension | 9m² | £18,000 – £25,000 | £28,000 – £42,000 |
| 4m × 3m extension | 12m² | £24,000 – £34,000 | £36,000 – £52,000 |
| 5m × 3m extension | 15m² | £30,000 – £43,000 | £44,000 – £63,000 |
| 4m × 4m extension | 16m² | £32,000 – £46,000 | £47,000 – £67,000 |
| 6m × 3m extension | 18m² | £36,000 – £52,000 | £52,000 – £75,000 |
| 5m × 4m extension | 20m² | £40,000 – £58,000 | £58,000 – £83,000 |
| 6m × 4m extension | 24m² | £48,000 – £69,000 | £68,000 – £98,000 |
| 5m × 5m extension | 25m² | £50,000 – £72,000 | £70,000 – £100,000 |
| 6m × 5m extension | 30m² | £60,000 – £86,000 | £82,000 – £118,000 |
| 8m × 4m extension | 32m² | £64,000 – £92,000 | £87,000 – £125,000 |
| 8m × 5m extension | 40m² | £80,000 – £115,000 | £108,000 – £155,000 |
Build cost based on mid-range £2,000–£2,900/m² national average. Full project budget includes professional fees, planning, kitchen/bathroom fitting and 12% contingency. London & the South East add 25–40%; Scotland, Wales and Northern Ireland sit 10–15% below.
Two-Storey Extension Cost by Size (4×3m to 6×5m)
A two-storey (or "second floor") extension stacks a new bedroom or bathroom above the ground-floor extension. Because foundations and roof are shared, the upper floor typically costs 50–70% of the ground floor per m² — making double-storey extensions strong value per pound spent.
| Two-storey size | Total floor area | Build cost (mid-range) | Full project budget |
|---|---|---|---|
| 4m × 3m two-storey | 24m² (12m² × 2) | £40,000 – £60,000 | £58,000 – £85,000 |
| 5m × 3m two-storey | 30m² (15m² × 2) | £50,000 – £75,000 | £72,000 – £105,000 |
| 5m × 4m two-storey | 40m² (20m² × 2) | £65,000 – £95,000 | £92,000 – £132,000 |
| 6m × 4m two-storey | 48m² (24m² × 2) | £78,000 – £115,000 | £108,000 – £158,000 |
| 6m × 5m two-storey | 60m² (30m² × 2) | £95,000 – £140,000 | £130,000 – £190,000 |
2nd Floor Extension Cost UK: Adding a Second Floor to an Existing Single-Storey Extension
If you already have a single-storey rear extension and want to add a bedroom above it, expect £1,400–£2,200 per m² for the new upper floor — provided your existing foundations and walls are structurally sufficient. A structural engineer's survey is essential before pricing this work, as foundation strengthening can add £8,000–£20,000 if the original build was not designed for a second storey load.
Typical 2nd floor add-on: 12–20m² → £20,000–£40,000 build cost (plus enabling works). Full planning permission almost always required.
House Extension Cost by Type (2026)
Single-Storey Extension
A single-storey extension is the most popular way to add space — usually at the rear or side of the property. Ideal for a larger kitchen-diner, family room or home office. Adding rooflights or bi-fold doors increases cost but dramatically improves light and usability.
Double-Storey Extension
A double-storey extension adds floor area on two levels — typically new bedrooms above and enlarged living space below. Because foundations and roof are already being built for the ground floor, the second storey costs less per m² than the first. Requires full planning permission.
Side-Return Extension
Victorian and Edwardian terraces often have an unused side alley. A side-return extension fills this in, transforming a narrow kitchen into a bright, full-width open-plan space. Access, drainage and party wall considerations make this more complex than a simple rear extension.
Wrap-Around Extension
A wrap-around combines a rear and side-return extension, creating an L-shaped space that can transform the entire ground floor. Best for creating large open-plan kitchen-living areas. More complex structurally than a straightforward rear extension.
House Extension Cost by Bedroom Count (2026)
Most homeowners think in bedrooms, not square metres. As a 2026 UK rule of thumb, allow about 12-18m² per additional bedroom, plus circulation space.
| Extension scope | Typical floor area | Build cost (mid-range) | Full project budget |
|---|---|---|---|
| 1 bedroom extension cost | 12-18m² | £24,000-£52,000 | £36,000-£75,000 |
| 2 bedroom extension cost | 24-36m² | £48,000-£105,000 | £68,000-£150,000 |
| 3 bedroom extension cost | 36-54m² | £72,000-£156,000 | £100,000-£220,000 |
A 2-bedroom extension is usually delivered as a two-storey rear or wraparound extension. A 1-bedroom extension can be a single-storey side return with an ensuite, or a loft conversion at a similar price point.
Worked Cost Examples (2026)
These examples show full project budgets — not just the build cost. Always include professional fees and a contingency.
Example 1: 20m² Single-Storey Rear Extension (Mid-Range)
Mid-range finish. London/SE add 25–40%.
Example 2: Double-Storey Side Extension (40m² total)
Mid-range finish. London/SE add 25–40%.
What Affects House Extension Cost?
Your final cost depends on far more than just m². These are the main variables that move extension budgets up or down.
📍 Location
London and the South East are typically 25–40% above the national average. Labour costs are the primary driver — materials cost is more consistent across the UK. Always use local quotes rather than national averages for your budgeting.
🏗 Design complexity
Simple rectangular plans with standard roofs are cheapest. Vaulted ceilings, complex roof forms, large spans, curved walls and bespoke structural details all add significant cost. Every deviation from a simple box increases engineering and labour time.
🪟 Glazing specification
Standard windows and doors are budget-friendly. Bi-fold doors add £3,000–£8,000 per set; large roof lanterns £2,000–£6,000; frameless glazed roofs significantly more. Glazing is often the biggest variable in an otherwise similar extension budget.
🔧 Structural alterations
Removing load-bearing walls and inserting steel beams adds £1,500–£5,000 per opening, but can transform how the extended space connects to your existing home. Often worth the cost for open-plan layouts.
🌍 Site conditions
Restricted access, sloping sites, poor ground conditions (clay, made ground, high water table) and the need for deep foundations all increase cost. A structural engineer's assessment early identifies risks before they become expensive surprises on site.
🛁 Fittings & finishes
The builder's quote covers structure and basic finishes — kitchen, bathroom, flooring, lighting and decoration are on top. A mid-range kitchen adds £10,000–£25,000; a bespoke kitchen £25,000–£60,000+. Budget for these separately before comparing builder quotes.
House Extension Cost Breakdown by Trade (2026)
For a typical mid-spec single-storey rear extension (around 20m²), here's how the build cost typically splits across the trades that go into it. Use this to sanity-check itemised quotes from builders.
| Trade / Stage | % of build cost | Typical cost (20m²) | Notes |
|---|---|---|---|
| Groundworks & foundations | 15–18% | £7,000 – £8,500 | Excavation, concrete, reinforcement, drains. Higher on poor ground or sloping sites. |
| Structural steel & bricklaying | 22–28% | £10,500 – £13,000 | Steel beams, blockwork, brickwork, lintels. Each major opening adds £1,500–£3,000. |
| Roof structure & covering | 10–14% | £4,800 – £6,500 | Trusses or rafters, insulation, membrane, tiles or flat roof. Roof lanterns add £2k–£6k. |
| Windows & external doors | 8–14% | £3,800 – £6,500 | Standard double-glazed: £3k–£5k. Bi-folds: +£3k–£8k. Triple-glazing: +20–30%. |
| First-fix electrics & plumbing | 7–10% | £3,300 – £4,500 | Cabling, sockets, lighting circuits, hot/cold runs, waste. Excludes consumer-unit upgrade if needed. |
| Plastering & second-fix | 8–11% | £3,800 – £5,200 | Plaster, skirtings, internal doors, sockets, switches, sanitaryware fitting. |
| Decoration & flooring | 5–8% | £2,300 – £3,800 | Paint, basic flooring. Engineered wood or premium tile add £30–£80/m². |
| Site setup, scaffolding, skips | 4–6% | £1,800 – £2,800 | Welfare, scaffolding hire, skips, deliveries, plant. Often missed in lowball quotes. |
| Builder margin & overheads | 10–15% | £4,500 – £7,000 | Project management, insurance, profit. Reputable builders carry £1m+ public liability. |
Percentages and figures based on a £45,000–£50,000 build for a 20m² mid-spec single-storey rear extension. Use these as a sanity-check against itemised quotes — if a trade is dramatically below the typical %, ask the builder how they're achieving it. Lowball quotes usually shortchange foundations, roof structure or scaffolding.
House Extension Hidden Costs (2026)
Most builder quotes price the build only — there's a layer of project costs that catches first-time extenders out. Here's what to budget on top of the headline figure.
VAT (often quoted +VAT)
Most builders' quotes are stated +VAT at 20%. A £50,000 quote becomes £60,000 after VAT. Always ask whether quotes are inclusive or exclusive — this single line item catches more first-time extenders than any other.
Architect / designer fees
Drawings for planning and building regs typically cost 7–15% of build cost. Lower end (7–10%) is a designer or architectural technologist; full architect-led service is closer to 12–15%.
Structural engineer
Required for steel-beam calculations and any work affecting the existing structure. Even small extensions usually need at least one engineer's calc set for building control.
Planning & building regs fees
Householder planning application: £258 (England 2026). Building regulations submission and inspection fees: £400–£1,200 depending on council and project size. Both are paid even if your project is permitted development (building regs only).
Party wall agreements
If you're building within 3m of a neighbour's structure, you need party wall notices. If neighbours dissent (about 1 in 3 do), each side appoints a surveyor — costs typically fall to you. Skipping this can stop construction and lead to legal action.
Kitchen / bathroom fit-out
Builder quotes usually exclude the kitchen or bathroom. A mid-range fitted kitchen costs £10,000–£25,000 supplied and installed; bespoke £25,000–£60,000+. Bathroom refits £4,000–£15,000.
Utility upgrades
Moving drains £1,500–£3,500. Consumer unit upgrade £600–£1,500. Gas pipework re-route £400–£1,200. Often discovered mid-build and not in original quote.
Decoration & finishing
Hard flooring upgrades, carpets, blinds, paint specification, light fittings, furniture. Most builders include a basic spec — anything bespoke is on top.
External works
Patio re-laying, drainage modifications, garden re-instatement, replacement fencing, render or rebuilding to match. Easy to underestimate — and unavoidable on most extensions.
Contingency (always include)
Even with detailed plans, surprises happen — buried obstacles, hidden rot, off-spec ground conditions, mid-project changes. Industry standard is 10–15% of total project budget, held back until completion.
House Extension Cost by Region (2026)
| Region | Relative cost | Typical cost per m² | Single-storey guide |
|---|---|---|---|
| London | Highest | £2,400 – £3,800+ | £40,000 – £85,000+ |
| South East & Home Counties | High | £2,200 – £3,400 | £35,000 – £75,000 |
| South West & Midlands | Medium | £2,000 – £3,000 | £28,000 – £60,000 |
| North of England | Medium-low | £1,800 – £2,700 | £22,000 – £55,000 |
| Scotland, Wales & NI | Medium-low | £1,800 – £2,600 | £22,000 – £52,000 |
Guide ranges for a mid-spec 20m² single-storey extension, structure and basic finishes only. Get local quotes for your exact postcode.
Don't Just Budget for the Build
Many homeowners focus on builder quotes and forget the other costs that make up the full project budget. Here's what to include:
How to reduce cost without cutting corners
Rectangular plans and simple roofs are significantly cheaper than complex shapes. Every extra corner or roof junction adds cost.
Every steel beam and load-bearing wall removal adds £1,500–£5,000. Work with existing walls where possible.
Standard windows and doors cost a fraction of bespoke sizes. Roof lanterns and bi-folds add kerb appeal but also add £3,000–£12,000.
Last-minute changes once work has started are one of the most common causes of overspend. Lock in your specification early.
Extension quotes can vary by 20–40% for the same scope. BestBuilders matches you with up to 3 vetted local builders — free.
Planning Permission & Building Regulations
Many single-storey rear extensions fall under Permitted Development rights — no planning application required if they stay within size, height and material limits. Double-storey extensions, properties in conservation areas and listed buildings typically need full planning permission (8–10 week process).
All extensions require Building Regulations approval regardless of planning status — covering structure, fire safety, energy efficiency and drainage. Your builder manages the building control process and provides the completion certificate.
Full planning guide on the extensions page →Does a House Extension Add Value?
A well-built extension consistently delivers strong ROI in UK property markets. Estate agent and RICS data suggest:
Get Accurate Quotes for Your Extension
Online guides give you a benchmark — accurate pricing requires a builder who has seen your property and plans. BestBuilders matches you with up to 3 vetted local extension builders for free.
House Extension Cost FAQs
Extension Costs by Town
Each town page shows local extension builders, town-specific cost ranges, recent project examples and nearby areas. Compare 3 free quotes from vetted local specialists.