A house extension is one of the most popular ways to add space and value to a UK home. Whether you're planning a kitchen extension, extra bedroom, or a full double storey extension, understanding the realistic costs in 2026 will help you budget properly and avoid nasty surprises.

This guide covers everything: costs per m² by extension type, regional price differences, what's included in the build cost, planning permission, and a full sample quote breakdown for builders.

House Extension Cost at a Glance — UK 2026

Extension Type Typical Size Cost Range
Single storey rear extension 15-30 m² £25,000-£60,000
Single storey side return 8-15 m² £15,000-£35,000
Double storey rear extension 30-60 m² £40,000-£90,000
Kitchen extension 15-25 m² £25,000-£55,000
Wrap-around extension 20-40 m² £35,000-£80,000
Over-garage extension 15-25 m² £20,000-£45,000
Conservatory (modern) 10-20 m² £10,000-£30,000

Key Insight: The cost per m² is higher for smaller extensions because the fixed costs (foundations, structural openings, building regs) are spread over less floor area. A 15 m² extension might cost £2,000/m², while a 30 m² extension could be £1,400/m².

Extension Cost Per m² — UK 2026

The cost per square metre is the most useful way to estimate extension costs:

Extension Type Budget Spec Mid-Range High Spec
Single storey £1,200-£1,500/m² £1,500-£1,900/m² £1,900-£2,500/m²
Double storey £1,000-£1,300/m² £1,300-£1,600/m² £1,600-£2,000/m²
Loft conversion £1,000-£1,400/m² £1,400-£1,800/m² £1,800-£2,500/m²
Basement conversion £1,500-£2,000/m² £2,000-£3,000/m² £3,000-£4,500/m²

Budget spec means standard finishes (vinyl flooring, basic kitchen units, painted plaster). High spec means underfloor heating, bi-fold doors, engineered oak flooring, and a high-end kitchen or bathroom.

Pro Tip: Double storey extensions cost less per m² than single storey because you're sharing foundations, roof work, and structural costs across two floors. A double storey extension is typically only 40-60% more than a single storey — not double the price. For more on quoting large building projects, see our loft conversion quoting guide.

Extension Cost by Region — UK 2026

Location is one of the biggest factors affecting extension cost. Labour rates and land values vary significantly:

Region Single Storey (per m²) 20 m² Extension
London £1,800-£2,800 £36,000-£56,000
South East £1,500-£2,200 £30,000-£44,000
South West £1,400-£2,000 £28,000-£40,000
Midlands £1,200-£1,800 £24,000-£36,000
North West £1,100-£1,700 £22,000-£34,000
North East £1,000-£1,600 £20,000-£32,000
Scotland £1,100-£1,700 £22,000-£34,000
Wales £1,000-£1,600 £20,000-£32,000

For builder day rates across all UK regions, see our builder day rate UK 2026 guide.

What's Included in an Extension Build Cost?

A typical extension quote should include these elements:

Cost Element % of Total For a £40,000 Extension
Foundations & groundwork 10-15% £4,000-£6,000
Walls (blockwork, insulation, brick) 15-20% £6,000-£8,000
Roof structure & covering 10-15% £4,000-£6,000
Windows & doors 8-12% £3,200-£4,800
Electrics (1st & 2nd fix) 5-8% £2,000-£3,200
Plumbing & heating 5-10% £2,000-£4,000
Plastering & decoration 8-10% £3,200-£4,000
Flooring 5-8% £2,000-£3,200
Kitchen/bathroom fitout 10-20% £4,000-£8,000
Professional fees (architect, engineer) 7-12% £2,800-£4,800

Additional Costs to Budget For

These are often missed when budgeting for an extension:

  • Planning permission — £258 per application in England (if required). Free if using Permitted Development
  • Building Regulations — £400-£1,200 depending on the size and complexity of the extension
  • Structural engineer — £400-£800 for calculations on steel beams and foundation design
  • Architect or designer — £1,500-£4,000 for full drawings and planning application
  • Party wall agreement — £700-£1,500 per neighbour if building within 3m of a shared boundary
  • Skip hire — £250-£400 per skip (most extensions need 2-4 skips)
  • Scaffolding — £500-£1,500 depending on size and duration
  • Contingency — Always budget 10-15% extra for unexpected costs (ground conditions, drainage, matching bricks)

Pro Tip: The biggest surprise cost on extensions is usually ground conditions. If foundations need to go deeper than expected (tree roots, clay soil, drains), it can add £2,000-£5,000. A good builder will flag this risk in their quote. Learn how to present these details professionally with our quote vs estimate guide.

Planning Permission — Do You Need It?

Many single storey rear extensions fall under Permitted Development (PD) rights, which means you don't need planning permission. Here are the key limits:

Rule Detached Semi-Detached / Terraced
Max depth (PD) 4 metres 3 metres
Prior approval depth Up to 8 metres Up to 6 metres
Max eaves height 3 metres 3 metres
Max overall height 4 metres 4 metres
Side extension width Half the width of original house Half the width of original house

You WILL need planning permission for: double storey extensions, side extensions in certain areas, extensions in conservation areas or listed buildings, and anything exceeding PD size limits.

How Long Does an Extension Take?

Stage Single Storey Double Storey
Design & drawings 2-4 weeks 3-6 weeks
Planning permission (if needed) 8-12 weeks 8-12 weeks
Building Regs approval 2-4 weeks 2-4 weeks
Construction 10-14 weeks 14-20 weeks
Total project 4-8 months 6-12 months

Sample Quote — 20 m² Single Storey Kitchen Extension

Here's what a typical itemised builder's quote looks like for a mid-range kitchen extension on a 3-bed semi in the Midlands:

Item Cost
Foundations (strip footings, 1m deep) £4,200
Floor slab (insulated, DPM, concrete) £2,800
Blockwork walls, cavity insulation, facing brick £5,500
Steel beam (RSJ) — supply & fit £1,400
Flat roof (fibreglass GRP, insulated) £3,200
Bi-fold doors (3-panel aluminium, 2.4m) £3,000
Velux roof window x2 £1,200
Electrics — 1st & 2nd fix (12 points, consumer unit) £2,200
Plumbing — hot/cold supply, waste, radiator £1,800
Plastering — walls & ceiling £1,600
Decoration — mist coat & 2 coats emulsion £800
Flooring — engineered oak (supply & fit) £1,800
Drainage connection & building control £1,200
Skip hire (x3) £900
Scaffolding £600
Build Total (excl. kitchen) £32,200
Kitchen units, worktops & appliances (mid-range) £6,000
Grand Total £38,200

That works out to £1,910 per m² for a mid-range finish in the Midlands.

For Builders: Presenting a clear, itemised quote like this wins more jobs. Customers feel confident when they can see exactly what they're paying for. If you're still writing quotes on paper or WhatsApp, try a proper quoting app — it takes minutes and looks professional.

How to Save Money on a House Extension

  • Get 3+ quotes — Always compare at least 3 quotes from experienced builders. Make sure they're quoting for the same specification so you can compare like-for-like
  • Build in winter — Builders are quieter between November and February, so you may get better prices. Foundations can be tricky in wet weather, but experienced builders manage this
  • Keep it simple — A flat roof costs less than a pitched roof. Standard windows cost less than bi-folds. Every complex detail adds cost
  • Project manage yourself — If you're confident coordinating trades, managing your own project (hiring trades directly) can save 15-25% vs using a main contractor. But it's more work and risk
  • Use Permitted Development — Staying within PD limits saves £258 on planning fees and weeks of waiting
  • Don't move the drainage — Building over or near drains adds significant cost. Design around existing drainage where possible
  • Supply your own kitchen — Buying a kitchen from IKEA or Howdens yourself can save 20-30% vs having the builder supply it

Does an Extension Add Value to Your Home?

A well-designed extension typically adds 50-75% of its cost to your property value, and in some cases 100% or more:

  • Kitchen extension — The best ROI. A £40,000 kitchen extension can add £30,000-£50,000 to a property in the right area
  • Extra bedroom — Adding a 4th bedroom to a 3-bed house is one of the strongest value-adds, especially in family areas
  • Extra bathroom — An en-suite to the master bedroom can add £15,000-£25,000 to value for a relatively low build cost
  • Open-plan living — Kitchen-diner extensions are highly sought after by buyers and add strong value

Important: The added value depends heavily on your area. In London and the South East, extensions almost always pay for themselves. In areas with lower property prices, the maths is tighter. Always check what similar extended houses are selling for on Rightmove before committing.

Frequently Asked Questions

How much does a house extension cost in the UK?

A single storey rear extension costs £25,000-£60,000 in the UK in 2026 (£1,200-£2,200 per m²). A double storey extension costs £40,000-£90,000 (£1,000-£1,800 per m²). Prices vary significantly by region — London is typically 20-40% higher than the Midlands and North.

Do I need planning permission for a house extension?

Many single storey rear extensions fall under Permitted Development rights and don't need planning permission, provided they meet size limits (up to 3m depth for semi-detached, 4m for detached, or up to 6m/8m under prior approval). Side extensions, double storey extensions, and extensions in conservation areas usually need a full planning application (£258 in England).

How long does it take to build a house extension?

A single storey kitchen extension typically takes 10-14 weeks to build. A double storey extension takes 14-20 weeks. Add 8-12 weeks for planning permission if required, and 2-4 weeks for Building Regulations approval. Total project timeline from design to completion is usually 6-12 months.

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