House Extension Cost in 2024

If you’re thinking of adding extra space to your home, the first thing that comes to mind is the price. Unfortunately, if you’re looking for a straight quote for the cost of an extension, you won’t find one. A lot goes into estimating an extension cost because of the sheer number of variables, from the size to the materials.


That said, there are ways to approximate the average cost and that’s exactly what we’re going to do. We’ve put together a comprehensive method for gauging a new extension cost. With our help, you can start assembling a budget and moving forward with your quality extension.

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Understanding the Basics of Home Extension Cost

Getting your head around house extension costs is tricky and oftentimes, daunting. So, it’s extremely important to first understand the basics of a home extension cost and to take note of aspects that you may not have thought about before. The basic factors that influence building extension costs include: 


  • Size and design (e.g. 24m2 garage extension)
  • Number of storeys
  • Location
  • Quality of the foundations
  • Whether the extension includes major plumbing (bathroom or kitchen)
  • Specialist features, like smart technology, etc. 
  • Type of windows (e.g. single, double, or triple-glazed)

Most of you will be aware of these factors when determining the cost of a house extension in the UK. However, some other crucial expenses that you might not consider are:


  • Architectural fees (can cost up to 10% of the total extension price)
  • Engineer expenses
  • Surveying costs
  • Assessment fees, such as tree surveyors or ecology surveyors
  • Insurance costs
  • Planning permission fees
  • VAT


When pricing a house extension, it’s important to be aware of the costs involved in the five key stages of the build. 


  1. Planning stage
  2. External construction stage
  3. Internal construction stage
  4. Interior design stage
  5. Decorating and final touches stage


Approaching your extension in this way makes the process more digestible, leading to a more comfortable process, especially when it comes to budget.

how much does an extension cost
house extension cost

The Importance of Accurate Cost Estimation

Without an accurate house extension cost estimate, you can grossly misjudge the overall price of the project. For example, under-budgeting often causes projects to be put on hold because money runs out. An accurate estimation allows you to make a plan to extend your home and stick to it. 


It also helps you to prioritize the most important aspects. For example, you may want a particular external wall finish but the budget isn’t there. So, you might put off this part of the job until you have more money. An accurate cost estimation is a critical starting point.

Key Factors Influencing Extension Costs

We briefly touched on the different things that affect the cost of an extension in the UK. Now, let’s take a detailed look at the most influential factors.

Size and Scale: Impact on Overall Costs

The size and scale of an extension have perhaps the biggest influence over the final price. Compared to small extensions,  bigger projects take longer, need additional labour, and require more materials and machinery. 


Take a wrap around house extension for example. The L-shape makes it a larger and more complex build than a regular square rear extension, typically adding to the price. 


Peculiarities in design may also have an impact. Complex designs tend to be more difficult to build and again, take longer.

how much do extensions cost
cost of an extension

Location Variations: Regional Cost Differences

Just like when you’re buying or renting, location has a significant impact on the cost of extensions. House extensions in London are likely to be more expensive than similar-sized construction projects in smaller towns in other parts of the country. 


In locations with higher living costs, labour expenses also tend to be higher. In large cities where there’s a high demand for construction work, there’s a good chance that you’ll have to pay a premium.

Material Choices: Balancing Quality and Cost

Materials and labour are undoubtedly the two biggest costs when building an extension. All extensions require the basics, like concrete, bricks, lumber, and clay. These materials must be sourced and transported to your site, which also contributes to the cost. 


Although these types of materials are standard, others aren’t. For example, you may want a particular type of stone or finish on your double storey extension, like limestone. This will come at a higher cost than a painted concrete wall. 


The same principle applies to interiors too. Flooring materials can affect the overall cost of a house extension. On a budget, you’ve got to find a balance between the quality of the work and the price.

house extension cost per m2
side return extension london

Types of Extensions and Their Costs

To answer the question, “How much does an extension cost?”, you must first determine what type of extension you want. There are several different types of extensions, and depending on the one you go for, the costs can vary significantly. 

Small rear extensions are among the most common add-ons in the country. So, to give you an idea of an average extension cost, here’s what you can expect.

Small Extension Cost

Extension Details



Average Cost

15m2 Extension Cost




24m2 Extension Cost




30m2 Extension Cost




Extension Costs UK

Not every extension is a small one, however. You can approximate average extension costs in the UK according to their size. Let’s take a look at the costs. 


  1. Small rear extension should cost between £35,000 and £48,000 (see the table above for the breakdown)
  2. Medium rear extension costs approximately £50,000 to £65,000
  3. Large rear extension can cost from £65,000 to £100,000 and above
Average House Extension Costs

If you know exactly what type of extension you want, it’s easier to get a more accurate price estimation. Here’s a breakdown of the cost of building an extension in the UK according to the construction type. But remember, the pricing can change depending on various aspects, like material selection. 

Extension Details

Single Storey Extension

Double Storey Extension

Side Return Extension

Wrap Around Extension

Small (15-30m2)





Medium (30-40m2)





Large (>40m2)






While you shouldn’t take these figures as quotations, they give you a good indication of how much you’ll need to set aside for your extension.

Single Storey Extensions: A Detailed Cost Analysis

Single storey extensions are the easiest and most common household add-ons. However, the cost of a ground floor extension can still vary significantly. A small, 15m2 could cost as little as £22,000, but you could easily surpass £50,000 with a large one of over 40m2. 


The single storey house extension cost per m2 usually falls somewhere between £1,500 and £4,600.

Two Storey Extensions: Investment and Returns

You might not realize that the cost to build an extension that’s two storeys high isn’t necessarily twice the price of a single storey building. Sure, the price of this extension will be higher, but the additional costs usually aren’t excessive. 


Along with the first floor, you’ll have to account for the second storey floor joists, walls, interior fixtures, and the roof. Plus, you’ll likely have to cover a fee for scaffolding. 


According to our estimations, the cost of building an extension with two storeys is between £25,000 and £85,000. An average two storey (30m2 to 40m2) would be approximately £45,000 to £75,000.

how much are extensions
london side return extension

Side Extensions: Maximizing Space Cost-Effectively

A side return extension is similar to a regular rear extension but it’s located on the side of the building rather than at the back. They’re generally built to make an existing room bigger. If you have kitchen extension ideas, a side extension could be right up your alley. 


Alternatively, you may be interested in a side extension to build a single office, home gym, or even a conservatory. Side extensions can be more tricky than rear ones because of the location. However, this isn’t always the case. You can build a side extension for the same price as a regular one. 


We estimate an average side extension to cost somewhere between £47,000 and £78,000, depending on the complexity of the job.

Conservatories and Orangeries: Affordable Alternatives?

Conservatories and orangeries are nice additions to houses but are not direct alternatives to extensions. Conservatories are generally smaller and contain more glass. They’re often cheaper due to the size and material costs.

Loft Conversions: Transforming Unused Space

If you don’t have the available space to build an extension but you’ve got an attic or elevated ceilings, you may consider transforming this unused space into a loft. Lofts are extremely versatile and can be used in several ways, such as bedrooms, offices, or kids’ playrooms. 


Although lofts don’t involve the same building work as extensions, they typically incur significant costs. The main reason for this is that the space has to be temperature-controlled. You can expect to pay between £20,000 and £40,000 for a loft (15m2 to 30m2).

extension cost per m2
small extension cost

Breaking Down the Costs

By now, you should be well on your way to answering the question, how much do extensions cost? Next, we break down how much your extension will cost to give you a clearer picture of exactly where the money goes.

Architectural and Design Fees Explained

Many homeowners rely on architects to handle their extension design. This approach often streamlines the process and provides you with several design options. However, using an architect will mean your extension plans cost around 8% to 10% of the entire build. 

Some architects are open to flat rates so it’s best to shop around to find an architect you like. Alternatively, you can go with a construction company that handles all aspects of the extension, including the design.

Builder and Contractor Costs: What to Expect

Approximately 50% of your extension cost in the UK should go to the builder or contractor. This will cover the cost of labour and the equipment that they use. 


However, if you use the contractor for additional services, like design, carpentry, or stonework, you may have to allocate more funds to them. 


The building materials take up approximately 40% of the costs on average. Again, if you opt for more expensive materials, this percentage could be higher. 


Between the design and planning work, materials, and labour costs, there is 100% of your building costs. But, this doesn’t include interior design and putting the finishing touches.

small extension cost
extension builders cost

Interior Fit-Out: Finalizing Your Budget

We don’t include the interior design in the build cost because this is where prices can vary. You can essentially spend as much or as little as you want on things like flooring, furniture, countertops, and decorative pieces. It’s important to carefully budget this alongside the build.

Additional Financial Considerations

Even if you’re careful with assessing the cost of a house extension in the UK, it’s easy to forget something and misjudge your budget. So, here are three additional financial considerations that you must pay attention to.

VAT on Extensions: A Cost Guide to Calculations

Builders and tradespeople must charge the standard 20% VAT. However, many of them don’t include VAT in their quotes so make sure to check to avoid an unwanted issue. 


You may be able to avail of a reduced VAT rate for energy-conserving products. Be sure to confirm all VAT rates before you finalize your budget and start scheduling work.

Planning Permissions and Legal Fees

Planning permission is typically inexpensive in relation to the total cost of building an extension but it’s still worth including in your budget. You should also consider that it can take several months to clear. In some cases, you may require legal assistance to push it through, adding further costs. 


It’s generally a good idea to have a legal professional look over any contracts that are part of the deal. These costs should also be accounted for.

cost of extension per m2
cost of extension uk

Long-Term Value: Does an Extension Increase Property Value?

Aside from the additional space, the biggest advantage of building an extension is the value it adds to your home. Extensions can increase the value of your property by up to 20%, depending on the type of extension and the size and location of your home.

Expert Insights: How to Save on Your Extension

The cost of an extension is significant but it doesn’t have to cause you financial duress. With the right approach, proper planning, quality builders, and good decision-making, you can shave off a lot of money on your build. Here are our expert insights on how to cut the cost.

Choosing the Right Builders: Fittra's Competitive Edge

Seeing as builders usually account for the biggest chunk of extension costs, choosing the right ones is critical. If you’re London-based, going with an end-to-end building company, like Fittra, should be your first decision. 


We’re not only a reputable NHBC-registered building construction company, but we also have long-term partnerships with architects, engineers, consultants, and designers. We can cover every aspect of your extension from the very first step to the last.

DIY and Project Management: Balancing Risks and Rewards

Going the DIY route and personally managing your extension project can save you thousands of pounds in labour and material costs. However, it comes at a big risk and sacrifices a lot of time. 


Without experience or expertise, you may walk into several pitfalls, like hiring poor tradespeople, using the wrong materials, or going with a bad design.

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Material Sourcing and Budgeting Tips

One of the most effective ways of reducing your spending is to consider alternative places for sourcing material. There’s a virtually endless conveyor belt of second-hand items that are more than suitable for extensions. 


While we don’t necessarily recommend reusing structural items, you can most definitely reuse furniture, wood, doors, etc. Look online or attend a local auction to secure good deals.

Fittra: Your Go-To Construction Company in West London

With over 10 years of experience, Fittra is a quality-first construction company that provides end-to-end solutions for first-time builders and homeowners. Our five-step comprehensive process ensures that you’re fully looked after from the initial meeting to securing warranties upon completion.

Why Choose Fittra for Your Extension Project

There are many reasons to choose Fittra for your upcoming extension project. Here are some factors to consider before you make your decision.

NHBC-Registered with a Decade of Expertise

As an accomplished NHBC-registered company, we’re licensed to provide quality work throughout the UK. We leverage our team of expert builders along with our network of building professionals to ensure clients are satisfied with every project.

average cost of an extension
average extension cost

Comprehensive Services from New Builds to Renovations

The Fittra name is renowned for all extension types, loft conversions, and renovations. However, our comprehensive service offerings go far beyond work on existing houses. Our other services include: 


  • New builds (end-to-end services)
  • Household refurbishments
  • Commercial buildings
  • Development and investment projects
  • Window supply and installation
  • Door supply and installation

Exclusive Client Advantages: Why Fittra Stands Out

In addition to our experience, reputation, NHBC accreditation, and wide range of services, Fittra has even more to offer. Here are some of the exclusive client advantages you can look forward to when working with us. 


  • Free consultations throughout the entire process
  • Free schedule of works
  • In-depth breakdowns of extension costs
  • 5% deposit
  • Accreditations: NHBC, NFB, FMB, TrustMark, and Considerate Constructors
  • A fully integrated team ensures smooth processes
  • Contactable at all times
  • Trade discounts
  • £10 million employer’s liability insurance
  • £5 million products and liability insurance
extension costs
24m2 extension cost

The Fittra Process: Smooth, Stress-Free, and Efficient

The Fittra process is a five-step action plan that we’ve refined and perfected over the years. From day one, we are there to take the reigns, ensuring a stress-free journey. However, we’ll never make decisions without your say-so. Here’s a snapshot of what we do. 

Free Consultation and Full Architectural Design

We start all projects with a free consultation on-site. During this meeting, we’ll ask you some straightforward about your extension. If you need help with architectural design, we can help to provide that service. 


Once the meeting has concluded and you’re ready to proceed, we’ll begin our official proposal. This will be a comprehensive but easy-to-digest document between 12 and 18 pages. We will then give you time and space to make your decision independently.

Project Management and Legal Support

Once the project is underway, our experienced team will handle everything with professionalism, involving you at each step. We’re available at all times as consultants, even when it comes to legal issues. 

Client Satisfaction: Our Portfolio Speaks for Itself

At Fittra, we pride ourselves on delivering a high-end product on time and on budget. If you need further convincing, check out our client testimonials. 

how much is an extension uk
how much does an extension cost uk


With Fittra, there are no guessing games. Our quality services and transparent processes ensure a first-rate experience from our first consultation. When we wrap up your extension, we’ll make sure to complete all snagging and provide you with warranties and certificates. 


The best part? Our tried and tested approach allows us to offer competitive prices. We take our work seriously and client satisfaction is the number one priority.

Summarizing the Cost of Extensions

Estimating the cost of an extension in the UK is the first, and one of the most important steps in the entire process. Getting a professional building company, like Fittra, on board at this time can help ensure your estimations are accurate and that you stay on budget throughout the project. 

Extension costs vary depending on the project size and type, the location of your property, and the materials you choose. However, you must also consider things like planning permission, legal fees, architectural costs, and VAT. 


There’s always a way to save money and get the most out of your extension, like hiring experienced builders to guide you through the process.

Making the Right Decision for Your Home

Armed with the knowledge of extension processes and costs, all that’s left to do is make a decision. Regardless of what direction you’re leaning, we invite you to schedule a consultation with Fittra. It won’t cost you a penny and it could help you on the way to creating your dream home. 


Reach out with any queries via email, phone call, or through the form on our website.

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Why Work With US

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at all stages

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with project manager

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liability insurance

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of works

Integrated, In-house Team Will Work On Your Project

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Benefits from our trade discounts
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£5M public & products liability
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10% only deposit payments
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Detailed cost breakdown

In the End You Will Get




Our Projects

FAQ: Answering Your Key Questions

If you still have questions regarding house extension costs in the UK, don’t worry. We’ve assembled a list of our most frequently asked questions, ensuring no stone is left unturned. 

How much does an extension cost in the UK?

How much does it cost? A UK extension can cost anywhere between £22,000 and £100,000. However, a common small extension can cost as little as £15,000. A medium size single storey extension (30m2 to 40m2) usually costs between £40,000 and £60,00).

How much does a house extension cost per m2?

The cost per square per meter varies. However, a single story extension, one of the most typical add-ons in the UK, usually costs between £1,000 and £4,600 per m2, giving you an average of £2,800 per m2. 

How much will my extension cost?

Extension costs in the UK depend on several factors, like size, building type, site location, and materials. However, the average mid-size project usually costs between £30,000 and £40,000 from start to finish. You can use an extension cost calculator for a more accurate estimate, or give us a call.

How to calculate house extension cost?

The best approach to calculating your extension cost is to get an estimation of the extension price per m2. Since the average cost per m2 is £2,800, simply multiply this number by the size of your planned extension (in m2). 

How much does a structural engineer cost for a house extension?

Structural engineer costs vary depending on who you choose. Some charge based on what they’re inspecting on the site, while others have a flat fee. £1,000 per project is a standard cost, but it’s common for structural engineers to charge between £500 and £2,000.

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