How Much a Rear Extension Costs in London

In the ever-evolving landscape of London property, rear extensions have become a popular solution for homeowners seeking additional space without the hassle and expense of moving. But with construction costs fluctuating and regional variations across the capital, understanding the true cost of extending your home can feel like navigating a financial maze.

Whether you’re planning a single-storey kitchen extension or dreaming of a two-storey addition with architectural flair, this comprehensive guide will break down exactly what you can expect to pay in 2025.

Average Costs for a Rear Extension in London

Day rates

Builders in London charge an average of £250 to £400 per day. This can vary depending on their experience, expertise and specific skill set. More often than not, however, contractors will quote for the extension project as a whole rather than work off a day rate, which is typically an easier way of working for both parties.

Cost per metre squared

In London, the average cost lowest cost per square metre of a rear extension is about £1,900 and the average highest cost £3,000. In the rest of the UK, the average cost per square metre of a rear extension is between £1,650 and £2,600.

It’s possible to determine the cost of your rear extension by considering its size and estimating the total cost based on the average cost per square metre.

Total house extension costs

The average size of a rear extension in London 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.

Types of extension and costs

One-storey rear extension

A London single-storey extension costs on average between £1,900 and £3,000 per m², depending on the complexity of your project, finish and materials used. If you have a desired square footage or budget in mind, you can use these averages to calculate an estimate for your extension.

Two-storey rear extension

You can expect to spend 50-70% more on a two-storey rear extension than on a single-storey with the same footprint, however, the cost per metre squared is lower. Many of the fixed costs (like design, planning and site setup) will be the same and you’re utilising the same foundations and roof.

For double-storey extensions in London, the average cost typically ranges from £1,500 to £2,200 per square meter of total floor area (combining both floors). So for a 90m² space, the cost could be around £150,000 for a simple design and basic materials, reaching up to about £190,000 for a more complex extension with premium finishes.

Conservatory

The average cost of a 3m x 3m conservatory is around £15,000 in London. A small and relatively basic conservatory could set you back £10,000 while larger conservatories can cost upwards of £30,000. Materials, roof type, site prep and the complexity of the project will impact the cost in addition to size.

Kitchen extension

The average cost of a kitchen extension is £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 and materials, whether or not you’re buying new appliances or integrating them, and the varying prices of flooring and tiles.

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 in an 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; with these, it’s possible that you could end up spending up to £15,000 on a bathroom.

Factors that affect costs

Location

London is the most expensive area of the UK to do work to your home, but where you are in the city plays into the cost too. Prices tend to be higher in central London boroughs and slightly lower in outer London areas.

House size and 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 side-return extension – will significantly impact costs as this choice determines the complexity of the project and labour costs, and the type and quantity of materials needed.

Client supplies vs. included project costs

An important thing to be aware of for rear extensions is what materials and fittings are listed as client supply in your quote vs. what’s included in the cited cost. Often, what’s included in the project fee covers more basic or standard features, and if you want something more high-spec or bespoke this will be for you to purchase additionally. For example, basic aluminium doors might be included in your quote but crittall doors might not be. So to get an accurate estimate on costs before you press go on the project, it’s key to know exactly what fittings and details you want, and go through your quote line by line with your contractor to understand what’s included.

Costs breakdown

The majority of your budget will go towards labour and materials, but there are numerous other professional fees and site costs to factor in, too. If you opt for premium materials, then they can significantly increase your total spend, but the below is based on using middle-range materials:

• Labour – Around 30% of the overall cost.
• Materials – Around 20% of the overall cost.
• Fittings – Around 5% of the overall cost.
• Glazing – Around 15% of the overall cost.
• Architect – Around 4% of the overall cost.
• Admin fees – Around 1% of the overall cost.
• Structural Engineers – Around 3% of the overall cost.
• Surveyors – Around 2% of the overall cost.
• VAT – An additional 20% of the total project cost.

Additional costs

Depending on your property, and project scope and size, there are other costs to be aware of and budget for your rear extension, including:

• Planning permission application – £200-£500
• Building regulations approval – £500-£1,000
• Party wall agreements – £700-£1,500 per neighbour
• Tree surveys – £350-£750
• Drainage modifications – £1,500-£5,000

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 the way you live in it, plus you get to put your own stamp on the space.

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 house extensions provide an opportunity to make your living space more functional and tailored to your lifestyle. 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 hassle and price of moving to a larger property, building a rear extension is a relatively cost-effective way to extend your living space. It allows you to maximise the potential of your current home without the expense and stress associated with relocation.

Planning permission

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.

Find a builder for your rear extension with The Page

The Page specialises in matching homeowners with trusted, hands-on builders (our Pagers) who can bring visions to life at competitive prices – without sacrificing quality. We’ve already done the hard work of finding exceptional contractors: we team up with Pagers not just for their skills but their approach to project management and communication.

Get started by booking a free call with our renovation coach, Thomas. An experienced renovator, he understands the practical and emotional aspects of extending your home and can offer support at every step. He’ll help you understand the process, connect you with the right Pager and professionals for your extension, and ensure you feel confident and supported from start to finish.