House Extension Cost UK 2026

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✅ Updated April 2026

House Extension Cost Guide for UK in 2026

A complete breakdown of house extension costs in the UK for 2026 — from planning fees to build costs, what affects the price, and how to budget accurately.

📅 2026 Guide 🏴󠁧󠁢󠁥󠁮󠁧󠁿 England ⏱️ 10 Min Read

A house extension is one of the most popular and cost-effective ways to add space and value to your home in the UK. Whether you are planning a rear kitchen extension, a side return, a double storey addition, or a loft conversion, understanding the full cost before you begin is essential. Get it wrong and you risk running out of budget midway through your build — one of the most stressful situations any homeowner can face.

This guide gives you a thorough, realistic breakdown of house extension costs in the UK for 2026 — including build costs per square metre, planning fees, architect fees, and all the other associated costs that are easy to overlook when budgeting.

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How Much Does a House Extension Cost in the UK in 2026?

The cost of a house extension in the UK varies enormously depending on the type, size, specification, and location of the project. As a general guide, extension costs in England in 2026 typically range from £1,500 to £3,500 per square metre for the build cost alone — before professional fees, planning costs, and fitting out.

£1,500–£2,000

Per m² — Basic single storey

£2,000–£2,500

Per m² — Mid range extension

£2,500–£3,500+

Per m² — High spec / London

These figures represent the construction cost only. Your total project budget will also need to account for planning fees, architect fees, structural engineer fees, building regulations, and internal fitting out — all of which are covered in detail below.

Cost by Extension Type — 2026 Estimates

The type of extension you are building has a significant impact on cost. Here are the most common extension types with typical 2026 cost ranges:

Extension Type	Typical Size	Cost Range 2026
Single storey rear extension	15–30 m²	£25,000–£60,000
Double storey rear extension	30–60 m²	£50,000–£120,000
Side return extension	10–20 m²	£20,000–£50,000
Wrap around extension	25–50 m²	£45,000–£100,000
Loft conversion	20–40 m²	£30,000–£75,000
Garage conversion	15–25 m²	£10,000–£30,000
Basement conversion	20–50 m²	£60,000–£150,000+

What Affects the Cost of a House Extension?

The wide range in extension costs reflects the many variables that influence the final price. Understanding these factors will help you budget more accurately and make informed decisions about where to spend and where to save.

📍 Location

London and the South East consistently attracts build costs 20–40% higher than the rest of England. A 20m² rear extension that costs £35,000 in the Midlands might cost £50,000 or more in London. Labour costs are the primary driver of this regional variation.

📐 Size and Complexity

Larger extensions cost more in absolute terms, but the cost per square metre often decreases slightly as the project grows. Complex shapes, awkward access, or structural challenges such as removing load-bearing walls will increase costs significantly.

🏗️ Construction Method

Traditional brick and block construction is the most common and typically the most cost-effective method. Timber frame can be faster and sometimes cheaper. Steel frame is more expensive but allows larger open spans. Modern methods like SIPs panels can offer speed advantages.

✨ Specification and Finish

The internal specification — kitchen units, flooring, bathroom fittings, glazing quality — has a huge impact on total cost. A basic finish might add £15,000 to a 20m² extension; a high-end specification could add £40,000 or more for the same footprint.

🪟 Glazing

Bi-fold doors, roof lanterns, and skylights are popular choices for rear extensions but add considerable cost. A set of bi-fold doors can cost £3,000–£10,000 depending on size and specification. A roof lantern adds £2,000–£6,000. These features transform a space but must be budgeted for separately.

Planning Permission Costs for a House Extension

Before you can build your extension, you may need planning permission — and if you do, there is a statutory fee to pay. From 1 April 2026, the planning application fee for a householder extension is £548 for a single dwellinghouse. This fee is paid to your Local Planning Authority when you submit your application.

In addition, applications submitted through the Planning Portal carry a service charge of £75.83 + VAT. Your total planning application cost is therefore approximately £640 including the portal service charge and VAT.

Many smaller rear extensions fall within Permitted Development Rights and do not require planning permission — meaning no planning fee is payable. However, even Permitted Development extensions require Building Regulations approval, which carries its own separate fee.

Use our free Planning Portal Fee Calculator to find out instantly whether your project needs planning permission and exactly what the fee will be.

Architect and Professional Fees

Professional fees are one of the most commonly underestimated costs in an extension project. You will typically need an architect or architectural technician to produce your planning drawings and building regulations drawings, and you may also need a structural engineer.

Professional	Typical Cost	What For
Architect / Architectural Technician	£800–£3,000	Planning and building regs drawings
Structural Engineer	£500–£1,500	Beam and foundation calculations
Planning Consultant	£500–£2,000	Complex or borderline applications
Party Wall Surveyor	£700–£2,000	If work affects shared walls
Building Control / Building Regs	£400–£900	Technical compliance sign-off

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Complete Cost Breakdown — Single Storey Rear Extension Example

To give you a realistic picture, here is a complete cost breakdown for a typical 20m² single storey rear kitchen extension in the Midlands in 2026 at a mid-range specification:

Cost Item	Estimated Cost
Build cost (20m² at £1,800/m²)	£36,000
Architect / architectural technician	£1,500
Structural engineer	£700
Planning application fee	£548
Planning Portal service charge + VAT	£91
Building regulations	£600
Kitchen fitting and appliances	£8,000
Bi-fold doors (3m set)	£5,000
Flooring and decoration	£3,000
Contingency (10%)	£5,500
Total Estimated Cost	£60,939

How to Finance a House Extension

With total extension costs often running to £40,000–£100,000 or more, most homeowners need to consider how they will finance the project. The most common options are:

• Remortgage — releasing equity from your property is the most popular option, often at a lower interest rate than other borrowing. Your lender will need to be satisfied that the extension adds sufficient value to justify the additional borrowing.
• Further advance — borrowing additional funds from your existing mortgage lender without a full remortgage. Often quicker and simpler than remortgaging.
• Personal loan — suitable for smaller extensions, typically up to £25,000. Higher interest rates than secured borrowing but faster to arrange.
• Home improvement loan — specialist secured loan product designed for home improvements. May offer longer terms than personal loans.
• Savings — the cheapest option if you have sufficient funds available, as no interest is payable.

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Does a House Extension Add Value?

In most cases, yes — a well-designed and properly executed house extension adds significant value to a property. The value added depends heavily on the local property market, the type of extension, and the quality of the work. As a general guide:

• A single storey rear extension creating a kitchen-diner typically adds 5–8% to property value
• A double storey extension adding a bedroom and bathroom can add 10–20%
• A loft conversion adding a bedroom adds 10–15% in most areas
• A garage conversion to habitable space adds 5–10%

However, it is important to avoid over-capitalising — spending more on an extension than the value it adds to the property. This is more likely to occur in lower-value areas where property prices limit the return on investment. In higher-value areas, extensions almost always recoup their cost and more.

How to Reduce the Cost of Your Extension

There are several practical ways to manage and reduce the cost of your extension project without compromising on quality:

💡 Get at least three builder quotes

Builder quotes for the same job can vary by 30–50%. Always get at least three quotes and make sure they are priced on the same specification.

💡 Use an architectural technician not an architect

For straightforward extensions, an architectural technician produces drawings of comparable quality to a fully qualified architect at a lower fee.

💡 Keep the design simple

Complex rooflines, unusual shapes, and intricate details all add cost. A simple rectangular extension with a flat or mono-pitch roof is almost always more cost-effective than a complex design.

💡 Supply materials yourself

Some builders will allow you to supply certain materials — particularly kitchens, bathrooms, and flooring — directly, which can save money if you shop around.

💡 Always keep a contingency budget

Set aside at least 10–15% of your total budget as a contingency for unexpected costs. Extension projects almost always encounter some unforeseen issues — particularly with older properties.

Finding a Builder for Your Extension

Choosing the right builder is arguably the most important decision you will make during your extension project. A poor choice of builder can result in substandard work, budget overruns, delays, and even legal disputes. Always use a builder who is:

• Registered with a reputable trade body such as the Federation of Master Builders (FMB)
• Listed on a vetted trades directory with genuine customer reviews
• Able to provide references from recent, similar projects
• Fully insured with public liability insurance
• Willing to sign a proper written contract before work begins

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Conclusion

A house extension is a significant investment — but when planned and executed well, it adds both space and value to your home. The key to a successful project is thorough upfront budgeting that accounts for all costs, not just the build itself. Planning fees, professional fees, building regulations, and internal fitting out can add 30–50% to the basic build cost, so always budget for the full picture.

The first step in budgeting for your extension is calculating your planning fee. Use our free Planning Portal Fee Calculator to get an instant, accurate estimate based on the latest 2026 statutory rates — it takes less than a minute.

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Frequently Asked Questions

A 4 metre rear extension on a typical semi-detached or terraced house with a width of around 4–5 metres gives a floor area of approximately 16–20m². At mid-range costs of £1,800–£2,200 per m², the build cost alone would be £29,000–£44,000. Adding professional fees, planning costs, and fitting out, a realistic total budget for a mid-range 4m rear extension is £40,000–£65,000 in most parts of England outside London.

Many house extensions fall within Permitted Development Rights and do not require planning permission. Single storey rear extensions up to 3m (terraced or semi-detached) or 4m (detached) typically do not need permission. Larger extensions, side extensions, and extensions in Conservation Areas usually do require planning permission. Use our free Planning Portal Fee Calculator to check what applies to your project.

A typical single storey rear extension takes 8–14 weeks to build once work starts on site. This does not include the time needed for planning permission (8 weeks determination period) or building regulations approval. From initial design to completion, the full process for a straightforward householder extension typically takes 6–9 months.

Yes. VAT at 20% is charged on most building work in the UK, including house extensions. This means a build quote of £36,000 plus VAT would cost £43,200 including VAT. Always check whether quotes from builders include or exclude VAT. Some smaller traders who are not VAT registered will not charge VAT, but VAT-registered builders must add it. Planning application fees from the council do not include VAT; the Planning Portal service charge does.

Technically yes — self-managing a build or acting as your own project manager can save 10–20% on costs. However, it requires significant time, knowledge, and organisational ability. You will need to source and manage individual trades (groundworker, bricklayer, roofer, plumber, electrician) separately and coordinate the build sequence. For most homeowners without construction experience, using a main contractor is strongly recommended.

In most cases yes. A well-designed rear extension creating a kitchen-diner typically adds 5–8% to property value. A double storey extension adding a bedroom and bathroom can add 10–20%. However, the return depends on your local property market and the quality of the build. In lower-value areas, it is possible to over-capitalise — spending more than the value added. Always research comparable sold prices in your area before committing to a large extension budget.

From 1 April 2026, the householder planning application fee for an extension to a single dwelling is £548. Applications submitted through the Planning Portal also carry a service charge of £75.83 + VAT (20%). The total planning application cost is approximately £640. Use our free Planning Portal Fee Calculator to get an exact estimate for your specific project.

Yes — virtually all house extensions require building regulations approval, even if planning permission is not needed. Building regulations cover the structural, insulation, drainage, and fire safety aspects of the build. You apply to your local Building Control body either via a Full Plans application (before work starts) or a Building Notice (48 hours before starting). A completion certificate is issued when the work is signed off — keep this safe as it will be required when you sell the property.

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