Extending your London home: a guide to house extension costs

Sep 24

Space is always at a premium in London, and when it comes to extending your home, that can be both a challenge and an opportunity.

If you’re thinking of adding more room to your home – whether through a stylish loft conversion or a practical extension – you’re not just creating more space but potentially adding significant value to your property.

When it comes to extending your London home, one of the first things you might wonder is how much it costs. The answer depends on a huge range of factors, from the type of extension you’re looking for to materials, fittings, and other features.

Read on to explore the main factors that drive the cost of house extensions in London – insights that will help you plan your project from start to finish.

House extension prices in London

The average cost of a single-storey extension in London falls between £1,800 and £3,000 per square metre.

This means that a typical 20m2 extension will cost between £ 36,000 and £60,000.

But here’s the thing: you need to put that cost in a local context. For example, did you know that just one square metre in Kensington and Chelsea is worth some £20,000 – ten times the national average?

So, in theory, adding just a small extension in this area could boost your property’s value by considerably more than the extension cost itself. A small 20m2 extension might cost £60,000, but be worth £400,000 when you eventually come to sell your property.

Now, it’s very rarely that simple or linear in practice. But, it goes to show that while extending in London and other expensive areas might cost more, it could also boost the return on investment (ROI) associated with the extension.

House prices in some London boroughs are two to three times the national average, which can reflect in extension costs.

Factors that affect house extension costs

Let’s take a look at some factors which affect home extension costs, as they can vary widely depending on extension types, location, features, and more.

The size and type of extension

The size of your extension is the biggest determinant of overall cost. It’s simple: the larger the extension, the more expensive it will be.

Additionally, the type of extension you choose will impact the price. For example, a simple rear extension will usually cost less than a more complex wraparound or side return extension.

Location

London covers a huge area, but it’s doubtful whether the costs of the extension vary widely based on location alone.

However, if you live in a high-value area like Kensington or Knightsbridge, etc, you might expect to pay more.

Moreover, the build itself may be more complex for more expensive, unique properties – particularly if they’re built in one of London’s many conservation areas or are listed.

Materials and finishes

The materials and finishes you choose for your extension will also impact the overall cost.

High-end options like bespoke joinery, premium appliances, and luxury flooring will increase the price tag. If you opt to upgrade other elements of your home, e.g. the plumbing or electrical system, this will also ramp up costs.

Features like high-end bi-fold patio doors, roof lights, and roof lanterns can also increase costs.

bright house extension — While a worthwhile investment, high-end finishes can increase the cost of extension.

Build complexity

As we’ve mentioned already, if your extension requires major structural work or has a complex design, the cost will inevitably increase.

This is especially the case for older Victorian homes (common in London) or listed or otherwise protected properties, where you might be required to match materials or conform to certain styles.

Professional fees

Finally, remember to factor in the cost of professional services when budgeting for your house extension.

Depending on the specifics of your project, you'll need to hire an architect to design your extension and manage the planning permission and building regulations processes, as well as structural engineers, party wall surveyors, and other specialists.

Budgeting for your London house extension

So, how much does an extension cost in London when we put all of the above together?

As a rough guide, here's what you can expect to pay for different types of extensions for some typical projects:

Basic single storey rear extension cost (20m2): £36,000 to £60,000
High-end single storey rear extension cost (30m2): £72,000 to £120,000
Basic double storey side extension (40m2): £90,000 to £150,000
High-end double storey side extension (60m2): £180,000 to £300,000
Cost of kitchen extension London (with fittings, 20m2): £50,000 to £100,000

Of course, these are solely ballpark figures. The actual cost of your extension will depend on numerous other factors, from the costs of your chosen contractor to the design, materials, and overall specifications.

Properly budgeting for your extension is much simpler when you work with a friendly, experienced architectural firm like Humphreys and Sons.

We'll help you develop a detailed budget for your project and ensure that you get the best value for your money. It’ll give you control and predictability over the project.

Final thoughts

Extending your London home is, of course, a major investment, but it can also be a great way to add value to your property and improve your quality of life.

Plus, for reasons noted, it might unlock a substantial profit when you come to sell your home.

Working with an architecture firm with specialist knowledge in London and the surrounding areas can often prove decisive to the project’s success.

At Humphreys and Sons, we have years of experience designing and delivering high-quality house extensions across London.

Unlike other companies, we have architects, party wall surveyors, and other key expertise in-house.

We provide one point of contact for the entire project, from planning to design and helping you find a builder. We can manage the planning process, help you find reputable builders, and manage building regulation approval.

We'd love to hear from you if you're considering a home extension in London.