single storey rear extension

How much does a house extension cost

In 2024, the average cost of a house extension in London is between £65,000 and £100,000. Most Londoners choose double-storey extensions because it adds extra space.

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*Proficiency completed rear and side return project at Queen's park, North London.

Cost to build:

Factors Affecting Construction Costs

How much does an extension cost in London? What is the average cost of house extension? In order to come up with the answers to the questions like this you should know the factors that influence extension prices.

Many factors have an impact on building costs like:

  • House extension type
  • Building regulations
  • Complexity of design
  • Size of an extension project
  • Materials used
  • Quality of works
  • Labour costs
  • Soil type
  • Location
  • Additional costs

Costs depend on the size of the construction and choice of quality. The bigger your construction is - the more you have to pay for it.

rear kitchen extension with glazing and skylight

There are small, medium and large size extensions. The cost of small extension varies between £16,000 and £21,000. However, if you are interested in medium or large size projects, you have to set the budget of £26,000 - £34,000 and £52,000 and £100,000 respectively.

It better to make all the necessary calculations ahead of time. Take advantage of an extension cost calculator to determine build cost per square metre for different types of extensions. So, you’ll understand how much money you will need to realize the project successfully.

How Much Does It Cost to Build Different Types of Home Extensions

There are different types of house extensions in London. Kitchen extension and loft conversion are the most popular. You can add either a single storey extension or two storey extension. A lot of people also choose to have basement extensions built. Ground floor rooms look very attractive and create additional living space. A basement extension will be an excellent addition to your house. Now, let’s talk about the costs of home extension building projects in the city of London.

Side or rear extension

People choose to extend their homes in order to create more space. Rear and side extensions are trendy among homeowners. In most cases, homeowners decide to build some single storey rear extension ideas just because they have enough land to be used in the backyard.

Are you interested in side extensions? In this case, you should think about your neighbours. Will your new construction overlook their house? You will have to talk to your neighbours first in order to find out what they think about your plans.

The overall cost of the construction does not depend on the type of project. The cost of an extension is determined by other factors such as the number of doors and windows you are planning to have in the extension. The project will be more complicated and expensive if you want to have a new bathroom or kitchen in the building.

Single rear house extension

Two or single storey extension

Building a double storey extension is considered to be a more profitable investment in comparison with a single storey extension. It can add more value to your property. But how much will this type of home extension cost you? Some experts believe that the construction of a double storey extension is less expensive than a single storey one. A single storey extension of 20 square meters will cost you about £22,000. In this case, building the second floor will cost £11,000. The extension will cost you £ 33,000 in the low end.

There are also builders who take into account the amount of space when pricing house extensions. Such builders tell the cost per m2. In this case, building a two storey extension may cost you up to £100,000.

Single storey extensions seem more difficult due to the groundwork and foundations. However, two storey extensions include some additional costs you can avoid if you prefer a single storey construction – using scaffolds, work at height and roof replacement.

It’s important to know that the quality affects the building costs of single and double storey extensions. Let’s say that you are interested in a small side extension in London. In this case, you must be prepared to pay approximately £35,000, £40,000 and £50,000 for an extension of basic quality, medium quality and premium quality, respectively.

Masonry or timber frame extension

Masonry construction is the building of structures from individual units such as bricks and blocks. This type of construction has been extremely popular in London for a very long time. Timber frame construction is gaining popularity as well. This method uses wood in the construction of walls, ceilings and floors. If you are planning to extend your house this way in the nearest future, choose the method that suits you most. But if the cost is the most decisive factor for you, don’t worry. Both methods have a similar price tag.

Building a garden room

Extending a house with a garden room is a perfect way to add value to your house. This room will connect your garden space to the house, providing extra space and stunning views. The total cost of adding a garden room depends on a wide variety of factors. The quality of the materials you are planning to use is one of the most critical factors you should consider.

Adding a bungalow

Adding a bungalow to a house is a great way to create extra living space in your house. If you need to know how much does an extension of this type costs, find out the cost per m2 in your area. Then you will be able to calculate the price.

small kitchen extension

Kitchen or bathroom

Extension costs will increase if you would like to have a kitchen or bathroom. Kitchen cabinets as well as kitchen appliances, can be costly. The price of these products depends mainly on their quality. Extending a small kitchen will cost you £7,000. With a more extensive kitchen and quality in mind, it will be closer to £50,000. When it comes to fitting a bathroom, you will have to pay between £3,000 and £10,000.

Extra costs

If you plan your budget, keep in mind that the prices mentioned above do not include VAT. The current VAT rate in the UK is 20%. So, you should add this amount to your quotes. Plus, some of the rooms in your house can be affected by the construction process. Once the construction is complete, the rooms may need refurbishing. In this case, the cost of an extension cost will increase significantly. You will not have to spend too much money if the existing rooms need only decoration.

Costs of Extension Building Works

In order to build an extension, you have to get different types of construction works done. During the construction process, you will have to project manage it. You will need to deal with different specialists such as a project manager, structural engineer, building control expert and, of course, extension builders. Read on this guide to get insights into project costs.

Designing an extension

The architects will prepare all the necessary drawings for building specialists. Architectural fees are usually 4% of the overall cost. You must be prepared to pay from £800 to £1500 for architectural drawings. The fees depend on the complexity of your home extension project.

Getting planning permission

Homeowners need to find out if they need to get planning permission or not before getting started with a project. The good news is that permission is not necessary for small extension. However, more significant extensions will require permission.

Ask either a team of local architects in London, a structural engineer or your local authority about the regulations that an extension built in your city has to comply with. Also, clarify all of your questions about a planning application.

It largely depends on the size of the construction. Property owners usually need to get planning permission for building large extensions. Getting permission will cost you about £200.

Even if you don’t need to get planning permission to start building an extension, there are some regulations that you will have to follow. Building regulations are standards for the construction and design of any building project. It’s crucial to stick to those regulations while planning your extension.

You have to make sure that the building will be safe, posing no risk to other people. Plus, your extension has to be water and energy-efficient. Building regulations approval will cost you about £300 – £500.

extension with flat roof

Laying the groundwork for the construction

There is no doubt that laying the groundwork is an extremely important step for building your house extension. The site for the extension has to be prepared properly. Otherwise, you can’t be sure that the extension is really safe.

UK residents pay approximately £30 - £50 per cubic metre for the groundwork. However, it’s hard to predict how much you will have to pay to get this type of work done. The reality is, some complications may arise during this process. For example, removing tree stumps as well as tree roots can make the process more difficult and expensive.

Building the shell of an extension

Once the site is ready, you can start building the extension. The shell is the basic structure of extensions. What type of building material are you planning to use during the construction process? Is it going to be timber, block or probably brick? In fact, you can choose the materials that you consider most appropriate for the building. Don’t worry about costs. These types of materials are available at similar prices.

When the frame is done, a roof can be built. Keep in mind that a subfloor and plasterboard need to be added as well. It will make the whole structure watertight.

contemporary living room

Fitting out the extension with important things

After adding the extension to your house, you may want to fit it out with some necessary things. Among the features that you can add to your interior design are:

  • ground floor
  • doors
  • internal decorations
  • radiators
  • electrical sockets
  • lighting
  • underfloor heating
  • heating controls
  • kitchen fixtures
  • switches
  • skirting boards
  • waterproofing
  • external windows

Estimating how much money you will have to pay to get all these things done can be hard. High-quality products are more expensive in comparison with the products of poor quality. So, it’s up to you to decide what sum of money you can spend to fit out the extension. Take your preferences into account and stick to your budget.

Finishing works

When your extension is ready, you will definitely want to finish it. It’s worth noting that finishings, fittings and other types of property renovations increase extension costs. In fact, you can spend as much money as you can afford on buying carpets, tiles, kitchen, bathroom or some other furniture.

Today, there is a wide selection of products you can choose from. The products have different quality and are sold at absolutely different prices. You must be prepared to pay about £85 per square metre to get wall and ceiling finishes done. So, the price for a new kitchen ranges between £7,000 and £25,000. If you are planning to purchase a bathroom, keep in mind that the minimum price you will have to pay is £3,000. You can also buy a more expensive bathroom with a huge price tag that exceeds £10,000.

Benefits of Building Extensions for Properties

Extensions allow London residents to add extra space to their homes and avoid the stress of moving. In reality, it’s a lot easier and much cheaper to extend an existing house rather than to buy a new one and move out.

The statistics show that extending a UK house adds about 5% - 13% to property’s value. So, you’ll be able to sell your home at a much higher price if such a need arises in the future.

It’s also important to note that extensions make homes unique. So, your extended home will look like no other.

Building London extensions is a serious process that requires time and dedication. This means if you plan, the project will go smoothly. Let’s look at the different factors that affect house extension costs.

On average the price of building and extension per square metre in London and outside London is about £1,200 - £2,000. The project’s cost per m2 largely depends on the type of construction you want to add to your home.

Clara Annesley

Clara Annesley is an interior design and construction content writer. She holds a BSc (Hons) in Architectural Design Technology from the University of West London. Clara specialises in residential construction for topics like health & safety, architectural design and writing cost guides for renovation and remodelling projects.