House Extension Cost UK: How Much Does an Extension Cost? The 2026 Guide

Working out your house extension cost is one of the first steps when planning a home improvement project. Whether you want extra kitchen space, a larger living area or a dedicated home office, understanding how much an extension costs in the UK helps you set a realistic budget and avoid costly surprises.

This guide provides a full breakdown of extension costs for every common type, from single storey rear extensions to wrap around and side return projects. All prices are based on 2026 UK averages with regional adjustments for London and the South East.

Average House Extension Cost Per m2 (UK, 2026)

The most useful way to compare house extension cost per m2 is by specification level. The table below shows national averages across the UK in 2026:

These figures include foundations, structure, roof, electrics, plumbing and basic internal finishes. They do not include kitchen units, bathrooms or specialist features such as structural glass. London and the South East typically add 20 to 40 per cent to these figures.

Single Storey Extension Cost

A single storey extension cost depends primarily on the floor area and your chosen specification. Single storey extensions are the most popular type of house extension in the UK, offering a straightforward way to add a kitchen-diner, family room or home office.

If you are planning a single storey extension in London, see our dedicated London guide for local costs, planning rules and design ideas specific to the capital.

Kitchen Extension Cost

A kitchen extension cost is typically higher than a standard living room extension because of the additional plumbing, electrics, extraction and fitted kitchen installation. Budget between GBP 1,500 and GBP 2,500 per m2 for the building work, plus GBP 5,000 to GBP 25,000 for the kitchen itself.

Kitchen extension costs are heavily influenced by the specification of appliances, worktops and cabinetry. A basic IKEA-style kitchen will cost around GBP 5,000 installed, while a bespoke handmade kitchen can exceed GBP 25,000.

Side Return Extension Cost

The side return extension cost is popular with owners of Victorian and Edwardian terraced houses. The side return is the narrow alley running alongside the kitchen, typically 0.9 to 1.5 metres wide. Filling this space transforms a galley kitchen into an open-plan room.

A typical side return extension of 8 to 15 m2 costs between GBP 15,000 and GBP 35,000 depending on length, roof type and finish level. Because the footprint is relatively small, the cost per m2 is slightly higher than a standard rear extension at GBP 1,400 to GBP 2,200 per m2 due to the proportionally higher fixed costs (foundations, steelwork, building control).

Side return extensions almost always fall within permitted development rights, making them one of the quickest and easiest ways to gain extra floor space.

Rear Extension Cost

A rear extension cost is the benchmark against which most other extension types are measured. Rear extensions project out from the back of the house and are the most common type of home extension in the UK.

For a standard single storey rear extension, expect to pay GBP 1,200 to GBP 2,200 per m2. A 4-metre-deep rear extension across a 5-metre-wide house (20 m2) typically costs between GBP 30,000 and GBP 50,000 at mid-range specification.

Rear extensions under 3 metres deep on detached houses, or up to 6 metres on semi-detached and terraced houses, may qualify for permitted development under the prior approval process. This is faster and cheaper than a full planning application.

Wrap Around Extension Cost

A wrap around extension cost combines a rear extension with a side return or side extension into an L-shaped layout. This maximises the ground floor area and is particularly popular on semi-detached and detached properties.

Wrap around extensions typically cost between GBP 1,300 and GBP 2,400 per m2. A medium-sized wrap around of 30 to 40 m2 costs between GBP 50,000 and GBP 95,000 depending on specification. The additional cost over a simple rear extension reflects the more complex foundation layout, increased steelwork and the larger roof area.

Most wrap around extensions require planning permission because they extend beyond the original rear wall in two directions. Budget GBP 258 for the planning application fee and GBP 1,500 to GBP 3,000 for architectural drawings.

Double Storey Extension Cost

A double storey extension costs between GBP 1,000 and GBP 1,800 per m2, which is lower per m2 than a single storey because you share foundations and roof costs across two floors. A typical 20 m2 double storey extension (40 m2 total floor space) costs between GBP 50,000 and GBP 90,000.

For a comprehensive guide to design, planning and construction, see our double storey extension guide.

House Extension Cost Breakdown: What Is Included?

Understanding where your money goes helps you make informed decisions about specification and budget allocation. The following breakdown shows the typical proportion of costs for a standard single storey extension:

Professional fees include architectural drawings (GBP 1,500 to GBP 3,000), structural engineer calculations (GBP 500 to GBP 1,500), building control fees (GBP 400 to GBP 900) and any planning application fees (GBP 258 if required).

Factors That Affect House Extension Cost

The final cost of your extension will vary depending on several key factors:

Location: Labour rates vary significantly across the UK. London and the South East are 20 to 40 per cent above the national average. The Midlands and North of England tend to be at or slightly below average.

Size and complexity: Larger extensions benefit from economies of scale. However, complex shapes (L-shaped, curved walls) and multi-level designs add cost through additional steelwork and foundations.

Ground conditions: Clay soils, high water tables and proximity to trees can require deeper or more complex foundations. Tree root protection may add GBP 2,000 to GBP 5,000 to foundation costs.

Access: Restricted access for deliveries and machinery increases costs. If materials must be carried through the house rather than delivered directly to the build area, expect a 5 to 10 per cent premium.

Specification level: The difference between basic and high-spec finishes can double the cost per m2. Bi-fold doors, lantern roofs, underfloor heating and structural glass all add significant cost.

Existing services: Drains, gas pipes and electrical cables that run through the extension footprint need diverting. Drain diversions can cost GBP 1,500 to GBP 4,000 depending on complexity.

Planning Permission and Building Regulations for Extensions

When Planning Permission Is Not Required

Many house extensions fall under permitted development rights and do not require a planning application. Single storey rear extensions can extend up to 3 metres from the original rear wall for attached houses, or up to 4 metres for detached houses, without any application. Under the prior approval process (also called the Larger Home Extension scheme), you may extend up to 6 metres (attached) or 8 metres (detached) by notifying your local authority and giving neighbours 21 days to comment.

When Planning Permission Is Required

You will need planning permission if your extension covers more than half the garden area, is higher than the existing roof, extends forward of the principal elevation facing a highway, or if your permitted development rights have been removed. Properties in conservation areas and listed buildings have additional restrictions.

The householder planning application fee in England is GBP 258 (2026). Decisions typically take 8 weeks.

Building Regulations

Building regulations approval is required for every house extension, even those that do not need planning permission. Your extension must comply with Part A (structure), Part B (fire safety), Part L (energy efficiency), Part P (electrics) and Part M (access). Building control fees typically range from GBP 400 to GBP 900 depending on the size of the extension.

How to Reduce Your House Extension Cost

• Stay within permitted development: Avoiding a planning application saves GBP 258 in fees and 8 weeks of waiting time. Design your extension within permitted development limits where possible.
• Choose a flat roof over pitched: A flat roof is simpler and cheaper to construct. Modern flat roof systems with EPDM rubber or fibreglass are durable and low maintenance.
• Use a design and build contractor: A single company handling both design and construction can reduce costs by 10 to 15 per cent compared to hiring an architect and builder separately.
• Simplify the foundations: Standard strip foundations are significantly cheaper than piled foundations. Avoid building close to large trees where possible.
• Standardise window sizes: Standard-sized windows and doors are cheaper than bespoke. Consider aluminium bi-folds in standard sizes rather than custom.
• Phase the work: If budget is tight, complete the shell and basic fit-out first, then upgrade finishes later. This is easier and cheaper than ripping out completed work.
• Get multiple quotes: Always obtain at least three detailed quotes from reputable builders. Compare line by line, not just the bottom line total.
• Build in spring or summer: Builders are often busier in autumn and winter. Starting your project in spring can mean competitive pricing and fewer weather delays.

Extension Cost Comparison: All Types at a Glance

The following table compares the cost of extension types to help you decide which option offers the best value for your home:

For timber frame options, see our timber frame extension guide for detailed costs, pros and cons.

Frequently Asked Questions

How much does a house extension cost in the UK?

The average house extension cost in the UK ranges from GBP 1,200 to GBP 2,500 per square metre in 2026. A typical 20 m2 single storey rear extension costs between GBP 30,000 and GBP 55,000 at mid-range specification. London and the South East are 20 to 40 per cent more expensive than the national average.

How much does a kitchen extension cost?

A kitchen extension costs between GBP 1,500 and GBP 2,500 per m2 for the building work, plus GBP 5,000 to GBP 25,000 for the kitchen units, worktops and appliances. A medium-sized kitchen extension of 20 m2 typically costs GBP 38,000 to GBP 70,000 in total.

How much does a single storey extension cost?

A single storey extension costs between GBP 1,200 and GBP 2,200 per m2 at national average rates. Common sizes range from 9 m2 (GBP 14,000 to GBP 22,000) to 48 m2 (GBP 70,000 to GBP 115,000). London prices are typically 20 to 40 per cent higher.

What is the cheapest type of house extension?

The cheapest type of house extension is usually a single storey rear extension with a flat roof and basic specification, costing from GBP 1,100 to GBP 1,800 per m2. Side return extensions also offer good value for terraced houses because the foundations and one wall already exist. Timber frame extensions can also be cost-effective at GBP 1,000 to GBP 1,700 per m2 with faster build times.

Do I need planning permission for a house extension?

Many house extensions fall under permitted development rights and do not require planning permission. Single storey rear extensions up to 3 metres (attached houses) or 4 metres (detached) are generally permitted. The Larger Home Extension scheme allows up to 6 metres (attached) or 8 metres (detached) under prior approval. Extensions that exceed these limits, cover more than half the garden, or are on listed buildings or in conservation areas will need planning permission.

How much does a side return extension cost?

A side return extension typically costs between GBP 1,400 and GBP 2,200 per m2. Because the floor area is relatively small (usually 8 to 15 m2), the total cost ranges from GBP 15,000 to GBP 35,000. Side returns are particularly popular on Victorian terraced houses and almost always fall within permitted development.

How much does an extension cost per m2 in the UK?

The average house extension cost per m2 in the UK in 2026 is GBP 1,200 to GBP 2,500 depending on specification. Basic builds start at around GBP 1,100 per m2, mid-range projects average GBP 1,600 to GBP 2,000, and high-spec extensions with premium finishes can reach GBP 2,500 or more. For a complete breakdown of loft conversion costs as a comparison, see our dedicated guide.

How long does a house extension take to build?

A typical single storey extension takes 10 to 14 weeks from breaking ground to completion. A double storey extension takes 14 to 20 weeks. Add 4 to 8 weeks beforehand for design, planning (if needed) and building regulations approval. Weather, ground conditions and material availability can all affect the timeline.

Get a Detailed Quote for Your House Extension

At houseUP, we provide fixed-price quotes for house extensions across London. Our design and build service covers everything from initial concept to final handover, with no hidden costs. Explore our house extension services or contact us for a free consultation and detailed estimate.