How Much a Rear Extension Costs in London

A house with a rear extension

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February 14, 2024

There are a lot of reasons to love rear extensions. As well as adding living space to a home, a rear extension allows for a lot of flexibility and creative freedom with interior design. Regardless of whether your extension project is solely on the ground floor or it extends upstairs too, understanding the costs involved is key.

Average Costs for a Rear Extension in London

Total Costs

The average extension cost in London is between £65,000 and £100,000, and there are a variety of things that impact this cost. The average size of a rear extension is 45m2 and you should expect to pay between £85,500 and £135,500 for this. The cost is lower in other parts of the UK, with the average cost falling to between £74,250 and £117,000.

Day Rates

Builders in London charge an average of £240 to £300 per day. This can vary depending on their experience and expertise. More often than not, the day rate of a rear extension is ignored, with the majority of tradespeople providing an overall cost for the project.

Cost Per Metre Squared

It’s possible to determine the cost of your rear extension by considering how large it will be and estimating the total cost based on the average cost per square metre.

In London, the cost per m2 of a rear extension is between £1,900 and £3,000. In the rest of the UK, the cost per m2 of a rear extension is between £1,650 and £2,600. This means that a rear extension of 30m2 will likely cost between £57,000 and £90,000 in London.

Types of Extension and Costs

One Storey Rear Extension

A London single storey extension costs between £1,900 and £3,000 per m2. There are a lot of things that go into the cost of a one storey rear extension, which is why it’s important to look at averages.

Two Storey Rear Extension

The cost of a two storey rear extension is unlikely to be double that of a one storey rear extension. For example, a 30m2 extension costing £60,000 will not double if you add in another storey. Instead of being £1,900 to £3,000 per m2 for a one storey extension, you should expect to pay an average of £3,000 per m2 for a two storey extension.

This is because a lot of the work has already been done, the same foundations and roof will be needed regardless of whether an extension is one storey or two. However, the cost of scaffolding and the inconvenience of working at a height will need to be taken into account.


The average cost of a 3m x 3m conservatory is around £15,000 in the UK. A small and relatively basic conservatory can range from £5,000 – £10,000, whilst larger conservatories can cost up to £25,000.

A conservatory with a tiled roof

Kitchen Extension

If you are adding a kitchen extension, you could expect to pay an average of £1,900 and £3,000 per m2 in London. This is for the materials and labour of building the extension itself, and then the cost of the kitchen needs to be added on.

The average cost of a new kitchen is between £6,000 and £20,000. The price varies so much due to the number of things that impact the cost, such as the quality of the fittings, appliances and flooring.

New Bathroom

Once you have paid for the extension itself, you will need to factor in the cost of adding in everything that a new bathroom needs. This includes a sink, toilet, bath or shower, taps, cabinets, shelving, mirrors and lighting. The cost of this can quickly add up, and that’s without factoring in the cost of flooring.

It’s hard to give an exact cost of a new bathroom extension, as there are a lot of variables. For example, you could cut costs by choosing tiles that are only £10 per m2, or you could choose some that are much more expensive.

You could also increase the cost by adding in an expensive bath or smart features. It’s not out of the realm of possibilities that you could end up spending up to £15,000 on a bathroom.

Factors That Affect Costs

A couple calculating costs in the kitchen

House Size & Type

Larger houses usually involve more construction work, materials and labour, which impacts the overall cost of a rear extension. The specific type of house – such as a detached, semi-detached or terraced property – can also influence construction complexity and therefore, the cost.

Type of Extension

The type of extension – whether that’s a single storey, two storey, or wrap around extension – will significantly impact costs, as more work and materials are required. Adding extra features such as large windows and bi-fold doors will also increase the cost.

If an Architect is Required

If an architect is involved in the project – for example, if they are needed to create detailed plans and designs – you can expect to incur additional costs.

Costs Breakdown

There’s a lot that makes up the cost of a rear extension in London, and a lot of your budget will go towards labour and materials. But, there are other costs to consider.

  • Labour – For a medium rear extension of 25m2, the cost of labour varies from £17,000 to £29,000 depending on where you live in the UK.
  • Materials – You should expect around 20% of the overall cost to go on materials.
  • Glazing – You should expect around 15% of the overall cost to go on glazing.
  • Architect – You should expect around 4% of the overall cost to go on an architect.


You should expect around 5% of the overall cost to go on fittings.

Additional Costs

There are other costs to consider. For example, structural engineers can take up to 3% of the budget and contractors can take up to 30%. You are also likely to spend money on a surveyor, administration fees and VAT.

Reasons to Build a Rear Extension

There are a number of benefits that come with building a rear extension. It’s an effective way of transforming your home and enhancing your overall living experience.

More Living Space

One of the main reasons for building a rear extension is the expansion of your living area. Whether you need a larger kitchen, an additional bedroom or a larger family room, a rear extension provides a lot more living space.

Improved Functionality

Rear extensions provide an opportunity to transform your existing space, turning it into a more functional place to live. You could create an open plan layout, connect indoor and outdoor living areas, or add unique rooms such as an office or gym.

Added Property Value

A professionally designed and built rear extension can significantly increase the value of your property. The additional square footage makes your home a lot more attractive in the property market, and it’s more likely to attract potential buyers if it’s larger.

Natural Light

By extending towards the rear of your property, you can add larger windows, skylights or glass doors to your property. This helps to bring natural light into the home, whilst also providing views of your garden or outdoor space.

Bespoke Design

There’s a lot of creative freedom to be had when you add a rear extension to your home. You can tailor the design to your specific preferences and lifestyle. Whether you prefer a modern and minimalist aesthetic, or a more classic and traditional look, a rear extension can be tailored to align with your vision.

Growing Families

As a family, you can quickly outgrow your living space, but a rear extension gives you a way to accommodate this growth. It can add additional bedrooms, play areas or communal spaces to your home.

Cost Effective Solution

Compared to the cost of moving to a larger property, building a rear extension is a cost effective way to extend your living space. It allows you to maximise the potential of your current home without the expenses and stress associated with relocation.

Planning Permission

A rear extension can significantly extend your property’s living space, but it’s important to consider planning permission to ensure a smooth process from start to finish. Though not always, planning permission is required for a lot of extension projects, especially when the extension’s size and design may impact your home’s appearance and structure. Several factors contribute to the need for a planning application for rear extensions. These include the extension’s height, size and design. Additionally, how close it will be to neighbouring properties will also be taken into account.

DIY Vs Hiring a Professional

There’s no denying the excitement that comes from adding a rear extension cost to your property, but the entire project is a big one, and this can be daunting. One of the biggest things to decide is whether to take on the task yourself, or whether to hire a professional builder.

It doesn’t matter if you are planning to add a one storey extension or a two storey extension, a large extension or a small extension, you need to decide how to approach things.

Do you want to try and manage things yourself, or do you want to enlist the help of a professional? We have taken a look at the pros and cons of both options.

A part built extension

The Pros of a DIY Rear Extension

Boosted Self Esteem

Completing a DIY project can be incredibly rewarding, especially if you are using the project to improve your skills. This leads to a sense of achievement when you step back and see building success, which boosts self esteem. It could even motivate you to tackle other building projects yourself.

Cost Effective

One of the biggest advantages of building a rear extension yourself is the cost savings. By handling the build yourself, you will avoid a lot of labour costs, which can reduce your budget significantly and save you a lot of money in the long run.

Learning Curve

A DIY building project can be an educational experience, especially one as large as building a rear extension. It’s a good way to gain valuable DIY skills and construction knowledge, much of which you are likely to use again in the future.


When you are doing a DIY project, you can work to your own schedule, without needing to worry about the availability of a professional builder. You can do the work at a time that works for you, rather than moving things around to accommodate someone else.

The Cons of a DIY Rear Extension

Limited Extension Knowledge

Unless you are a professional builder or you have a lot of experience in the industry, you are likely to have limited building knowledge. It’s unlikely that you will have built a rear extension before, or even helped on a similar project. There is always a risk that a DIY building project will lead to mistakes along the way.

Quality Issues

Even with all of the right tools and equipment, DIY extensions are unlikely to achieve the same standards as extensions built by professionals. This means that you could end up with a rear extension that is noticeably less impressive than other aspects of your property.

Safety Concerns

A lack of building experience can lead to safety concerts, especially in regards to the structural integrity of the extension. You might find yourself faced with unexpected challenges and setbacks, without the proper knowledge of how to handle them safely.

Time Consuming

It’s not uncommon for DIY projects to take a lot longer to complete than a project handled by a professional. This is because you will probably need to learn as you go. There’s a risk that you could be without a finished rear extension for much longer than you would like.

The Benefits of Hiring a Professional Builder

There are many benefits to getting a professional in to build your extension:

  • Professional Expertise – When you hire a professional builder, you benefit from them having years of training and experience in building rear extensions. This is experience that you are unlikely to have. A professional builder knows how to finish a house extension to a high standard, meet deadlines and provide a bespoke building service.
  • Time Savings – Due to their expertise, knowledge and experience, professional builders are able to work faster. This means that your building project can be completed quickly, minimising disruption to your daily life, and without missing deadlines.
  • High Quality – With a professional builder, you can relax, knowing that you can expect a high quality and impressive finished project. Professionals have access to the right tools, equipment and materials, and this helps them to achieve a high quality result.
  • Safe and Secure – Thanks to their experience and knowledge, a professional builder is able to build a rear extension that’s safe and secure. There’s no need to worry about the structural integrity of the build, or that it won’t stand the test of time.

The Downsides of Hiring a Professional Builder

These are the bad points to hiring a professional builder:

  • Cost – One of the biggest downsides of hiring a professional builder is the cost. As well as paying for the materials needed for the extension, you also need to pay for labour. The cost of labour makes up a significant portion of your extension budget, and paying for labour could mean that you need to cut costs elsewhere.
  • Builder Availability – You might have to schedule your build around the availability of your chosen builder. This is likely to be true if you choose a popular builder. This could delay your project, preventing you from getting started straight away, and leaving you without additional living space for longer than you would like.

Finding a Builder

A smiling tradesman

The Page

There are a number of ways to find a builder, including using The Page. At The Page, you can send an enquiry detailing the specific requirements of your project, and you will be matched to a list of professional builders who are up to the challenge.

Let The Page curate you a free bespoke shortlist of tradespeople suited to you and your job.

When it comes to building a rear extension, there are a lot of variables to take into consideration, and projects can vary in complexity. But, at The Page, it’s always possible to find a tradesman who can match your requirements.

All of the tradesmen at The Page are personally met and vetted, and their previous work has been assessed. You can choose any of the recommended builders, knowing that your project is in good hands.

Online Search

When you are starting a construction or renovation project, finding a skilled and reliable builder is key. One of the easiest ways to do this is with an online search, which can be done using search engines and tradespeople directories. An online search provides a convenient and efficient way to find builders in your local area.

With a simple online search, one that’s tailored to your project specifications, will present you with a list of local builders, each with their own online presence. You’ll be able to see their previous building projects, client testimonials and services offered. This online presence allows you to assess their expertise and reputation before contacting them.

Ask Around

Asking around and getting recommendations from friends, family, neighbours and colleagues can be an effective way of finding a reliable builder. Personal recommendations and reviews often come with genuine insights into a tradesman’s workmanship, communication and overall professionalism. By asking around, you can find out about firsthand experiences and gain a clearer understanding of a builder’s reputation within the local area.

Friends or family who have had positive experiences with a builder are likely to provide recommendations, giving you a good starting point for finding a skilled and reliable tradesperson. By asking around, you hire a builder in confidence, based on the positive experiences shared by people that you know and trust.

Free Tradesperson Shortlist

Let The Page curate you a bespoke shortlist of tradespeople suited to you and your job.

Our Promise

We started The Page because we both had terrible experiences with tradespeople in London and were adamant we would help others to avoid this.

We also know that things can go wrong sometimes, so our promise is to be here to sort things out when they do.

As a result, along with our strong relationships with all our tradespeople, we have our good deed piggy bank, which we give out at our discretion if something goes wrong.