How much does a house extension cost in London? 2026 Guide

We are often asked the same question by clients who come to us here at CDD – one that is very common among most London homeowners: Are we better off moving or extending our current home?

In 2026, property prices and the costs associated with moving have continued to rise making the choice of extending an existing house increasingly attractive.

But when it comes down to working out the sums, in reality: how much will an extension cost?

The question of a house extension in London is highly dependent on a host of factors, ranging from size, specification and the complexity of the scheme that is proposed.

It is important to keep in mind that while headline figures can provide useful starting points, they rarely reflect the realities of building in a dense urban city, where constraints such as access, planning policy and impact on neighbouring properties can all influence the final budget.

In this article, we have put together a realistic guide that will give you a good idea of an extension cost in London and includes key points to consider such as average house extension costs, cost per square metre and factors that you need to keep in mind that impact the overall spend.

If you’d like to discuss your project costs with us, you can contact us here

Let’s dive in shall we?

Extension cost in London: A quick overview

When beginning to look at the costings for a house extension project in London, a typical cost per square metre is usually a basis for an overall cost projection.

While every project is different, this calculation gives a useful benchmark for understanding overall budgets, helping you to understand from the outset a ballpark figure for your design.

In 2026, the estimate for an extension cost has been between the range of £2,800 and £4,500+VAT or more per square metre. As mentioned, this price is heavily dependent on the specification and design complexity of your extension.

This means that the average cost of a single-storey house extension ranges between £30,000 and £140,000 and for a two-storey house extension the cost ranges from £108,000 to £180,000.

Smaller extensions of around 15-20m2 may fall towards the lower end of this with side extensions coming in between £1,800 and £3,000 per square metre, while larger or more design-led projects, can exceed these figures, including double storey extensions which can exceed these figures.

It is important to keep in mind that these figures relate to construction costs only. You will need to factor in professional fees, planning permission, other costs and interior fit-outs on top of this to understand your total cost and how much you should budget for if you do want to carry the project forward.

Cost per square metre for extensions in London

While we have outlined a general range above for an extension cost estimate, it is useful to break this down further based on the level of specification and complexity involved.

As a guide in 2026:

1. Basic specification: £2,800-3,200 per square metreThis covers a straightforward, standard specification extension with minimal structural complexity and a more functional design approach.
2. Mid-range specification: £3,200 – 3,800 per square metreThis is the most common bracket for London projects and includes good quality finishes, glazing elements and moderate structural interventions.
3. High-end/bespoke: £3,800-45000+ per square metreThis covers an architecturally-led design with large amounts of glazing, bespoke detailing and technically demanding extensions.

It is also worth keeping in mind that in London, costs will always sit at the upper end due to site constraints, labour costs and general logistical challenges that need to be considered.

Also keep in mind that construction costs typically cover only about 60% of the total project budget.

House extension cost by type

When considering different extension types, each type of extension comes with its own cost profile, level of complexity and typical use.

Below is breakdown of the most common types of extensions in London and what you can expect in terms of pricing:

1. Single storey extension:A single storey extension is the most common type of addition. Within the single storey extension category is the rear extension, an effective way a lot of London homeowners add more space and increase their property value.The single storey extension typically sit within the mid to upper range of house extensions.A rear extension is often single storey and most cases are designed to create an open plan living space combined with a kitchen and/or dining area; perfect for everyday family living and for entertaining guests.The approximate cost: £60,000-120,000+
2. House side extension:Common in Victorian and Edwardian terraced houses, a house side extension utilises the narrow strip of land to the side of the property, dramatically improving layout and natural light in the existing house. The space can be used for a whole host of reasons but one we’ve found to be most common is to carry out a kitchen extension and add a utility room – a great addition for any home.The approximate cost: £50,000-120,000+
3. Double-storey extension:As a more substantial addition, two storey extensions increase space for both the ground and first floor of the property. While the overall two storey extension costs more than most, it can sit more efficiently within broader house extension ranges when assessed on cost per square metre.The approximate cost: £120,000-250,000+
4. Basement extensionTypically at the higher end of all house extensions, the basement extension is one that is complex and highly site-dependent. These are usually carried out to add space for a living room where above-ground expansion is limited.The approximate cost: £4,000-7,000 per square metre

Each of these extensions costs will be dependent on several factors from design ambition, specification and site constraints.

Keep the above figures in mind as a useful cost guide to give a broader understanding of what you could be facing if you decide to go forward with a new extension on your property.

What impacts the cost of a house extension in London?

The cost of a house extension in London can vary depending on a number of interrelated factors. While cost per square metre provides a useful benchmark, not all extensions follow the same cost profile.

Understanding how different project types – from a small to medium extension to a large extension or more complex extensions affect overall cost will help you plan more accurately and account for potential costs from the outset.

1. Size and scaleThe size and scale of your project will have a direct impact on the cost. A large extension will naturally require a greater upfront investment, although it may benefit from a more efficient cost per square metre. A medium extension by contrast, often sits within the most typical cost range for London projects.
2. Not all extensions are equalIt is important to remember that not all extensions are constructed the same way. A simpler design, like a basic rectangular layout for a rear extension, will keep construction straightforward and reduce costs, whereas more complex extensions involving structural alterations or multiple levels will carry additional costs.
3. Structural requirementsOpening up the rear of a property often needs significant structural steel. The wider the span and the more open the space, the greater the structural input and associated cost.
4. Specification and finishesThe level of finish can dramatically influence budget. Bespoke joinery, high-end glazing and premium materials will push the project to the upper cost end. Material costs need to be factored in early on as prices can vary.
5. Site constraints and accessRestricted access is a common issue in London. Properties without side access can increase labour costs and complexity, particularly for larger extensions, where the movement of material becomes more challenging.
6. Ground conditions and below-ground workUnforeseen issues such as poor soil conditions, drainage complications or nearby trees can result in more complex foundation solutions, adding to early-stage potential costs.
7. Planning permission and design complexityMore involved schemes, particularly those in a conservation area or sensitive neighbouring conditions – may require additional design development, reports and coordination, all of which can impact final cost.
8. VAT costsExtensions usually attract a full VAT rate of 20%, which can add a substantial amount to your final bill.

How to estimate the cost of your extension

Estimating a house extension cost in London doesn’t need to be too complicated.

Using simple calculations is a smart way to get a realistic picture of potential costs before committing to a design.

1. Start with a size and cost per square metreA straight forward calculation is often the most cost effective approach. Remember: Extension size (m2) x cost per m2 = construction costThis can provide rough average house extension costs and allow you to compare different extension types – such as single storey extension, side return, double storey, kitchen extension (etc) – and see where your budget fits.
2. Factor in professional fees and approvalsConstruction costs are only part of the total. Be sure to include:
   • Budget an additional 10–15% of construction costs for professional services for either an architectural designer or architectural technician and structural engineer (10-15%)
   • Planning permission, permitted development rights and building control
   • Contingency (10-15 percent) for unexpected issues that could increase costs.

   Also do not forget, VAT of 20% must be added to most builder quotes, which can add £10,000-£20,000 to the total cost.

3. Consider finishes and fit-outHigher specification materials to premium kitchens can all push the budget up, while simpler finishes can help keep your project cost effective.Combining size, type, specification and professional fees gives a practical starting point to understand average house extension costs and is a smart way to plan before design work begins, helping ensure your project remains on track.

Does a house extension add value in London?

Adding a home extension can significantly improve both the functionality and property value, but the impact is dependent on various factors. Location, design quality and type of extension all play a role in determining the potential return.

In general, well-designed extensions that enhance space, natural light and the flow of a property are more likely to add value. For example, a semi detached house with a thoughtfully executed rear or side home extension often sees a great uplift in market appeal compared to properties without an expansion.

Factors affect the value include:

• The size of the extension and how well it integrates with the existing home.
• The quality of finishes and design
• The functionality added – such as a kitchen extension, additional bedrooms, walk in shower etc.

For homeowners looking to maximise impact, multi storey extensions can be a highly effective way to add both floor space and long-term value, particularly in dense London neighbourhoods where ground-level space may be limited.

Overall, a home extension can be a very cost effective way to enhance living space and increase property value – provided it is well-planned, proportionate to the existing building and built to a high standard. Another great guide for house extension costs guide can be found at check a trade.

What about planning permission?

When considering a home extension in London, understanding planning requirements is essential. Some extensions may fall under permitted development rights, meaning you can build without submitting a full planning application.

However, more substantial projects, particularly multiple storey extensions often require formal approval.

If your project affects a neighbouring property, party wall agreements will need to be considered before work can go ahead. This ensures that both parties’ rights are protected and can help prevent disputes and hidden costs later on down the line.

High quality architectural drawings and detailed drawings are usually required for planning permission. These not only illustrate your design but also demonstrate compliance with local planning policies and building regulations.

Investing in accurate drawings early on can save money in the long run by reducing the risk of revisions or delays.

Even if planning consent is not required, you will still need to comply with building regulations which cover structural safety, fire protection, drainage and energy efficiency.

Factoring in other costs such as these is important when calculating your total budget, and to give you an idea, planning application fees in England have recently increased since April 2026 to roughly £633, with building control inspections adding £600 to £1,800+ (Depending on the complexity and scale of the project).

Submitting your planning application yourself can save you money on fees typically charged by experts but we really do recommend asking for help especially from designers that have extensive experience in these types of projects.

Understanding these requirements before you start the project ensures that your home extension proceeds smoothly, meets all legal obligations and avoids any unnecessary surprises down the line.

Conclusion: Planning your extension budget

Understanding an extension cost and the factors that can influence the project is key to a successful addition.

From a single storey extension to loft conversions and double storey projects, careful planning ensures you stay on budget and achieve a high-quality result.

By considering design, specification, site constrains, planning and building regulations early on, you can create a cost-effective home extension that increases both the living space and property value for your home in London.

Give us a call today or get in touch here if you would like to discuss a potential extension on your home and we will be happy to share our thoughts!

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