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How Much Does an Extension Cost To Build in 2023?


Last year I asked our architects to design lots of extensions for my house and the brief was simple: ‘I’d like my dream home’.

The plans were simply stunning but after working out the construction costs I was a little shocked.

As I speak to homeowners on a daily basis, I thought it might be useful to create this a guide that explains exactly how much it costs to build and extension in the UK.

It gives the average construction costs of lots of different types of popular extensions – I hope you find it useful.

Table of Contents

1. How Much Does an Extension Cost?

If you have only just decided you would like to extend your home, I would suggest starting with a rough calculation. First of all, calculate approximately how big you would like your extension to be in m2. Once you have that total, multiply it by the average construction cost per m2.

Construction cost/ m2 in 2022 = £1,500.00 – £2,500.00

So, if you calculate that your ground floor rear extension will be approximately 30m2, the total construction cost will be between: £45,000 – £75,000.

You may be thinking, ‘That is a huge range’, and you would be right.

Bear in mind there are a huge number of factors affecting construction cost:

  • Single or two storey extension?
  • The size of the extension?
  • Modern vs traditional?
  • Quality of the build: basic, average or stunning?
  • Would you like a lot of glazing?

1.1. much does a single storey rear extension cost?

Quality of FinishMinMax


One of the biggest determining factors when planning your extension is the type of finish you would like. There are some builders out there who will spend a little less time on the finish, but they will also charge less. There are also some very high-end builders, who will be meticulous and spend far longer on the detail, but they also charge more to compensate for the extra time spent.


Where you live also plays a huge part in how much builders will charge to build your extension.

If you live in the north, the average cost/m2 might range from £1,500 – £2,000/m2.

This means that a 30m2 extension could cost anywhere between £45,000 – £60,000.

In the South and South East, construction costs are typically higher, ranging from £2,200 – £2,500/m2.

This means that the average cost of a 30m2 extension could cost between £66,000 – £75,000.

1.2. How much does a single storey side extension cost?

Quality of FinishMinMax

House shape

The cost of single storey side extensions are determined by the depth of your house. If you have quite a wide house that is also quite shallow, the cost will be less. If you have a narrow house which is quite deep however, your side extension would have a larger footprint which would cost more to build.

Due to economies of scale, smaller side extensions typically cost a little more per m2 when compared to larger extensions.

Room Types Created

It’s also important to consider what type of room you will be including within your single storey side extension. If you simply want to increase the size of an existing living room, the overall cost might be low.

If you would like a new home office, downstairs bathroom or utility room, the materials and labour involved will be more and will therefore result in a higher overall construction cost.


Before starting you project, it’s important that you set a clear budget as extension costs can quickly spiral out of control. It’s also important to have a contingency fund. We would recommend that you leave 10 – 15% of your total budget in reserve just in case you need it. Extensions can be complex and there are often unforeseen circumstances which may require some creative thinking by your architect and cost more money for your builder to solve.

1.3. How much does a two-storey extension cost?

Quality of FinishMinMax

In the South and South East, construction costs are typically higher, ranging from £2,200 – £2,500/m2.

Quality of Finish

As with all extensions, the quality of the finish is a huge factor. For example, what type of room will you have on your first floor? If it is just a bedroom the costs to carpet and paint could be relatively inexpensive. However, if you’d like a huge bathroom with a free standing bath and stunning porcelain tiles, the costs of materials can jump quite drastically.

1.4. How much does a conservatory cost?


Unlike extensions which are typically brick built, conservatories are usually made from uPVC which is relatively inexpensive.

Construction Time

Materials for this type of extension are pre-fabricated off site. This means that the time it takes to erect them is far quicker when compared to the time it takes a brick layer to construct an extension.


These cost slightly more than conservatories. The quality of the build will be far better than uPVC and in terms of aesthetics, these extensions can look stunning.

Orangeries can be more expensive than uPVC extensions because they are often partly built of brick. The windows will also usually be made of hardwood, and rather than a simple rooflight, they are often fitted with elaborate roof lanterns. This option offers far more daylight but also adds a sense of space due to the higher ceiling.

1.5. How much does a loft extension cost?

Loft Conversion TypeMinMax
Velux Loft Conversion£5,000£20,000
Dormer Loft Conversion£30,000£60,000
Hip to Gable Loft Conversion£40,000£65,000
Mansard Loft Conversion£50,000£70,000

Velux Loft Extension

If you’re thinking of adding some much-needed extra space to your home, this is certainly the most cost effective. As you’re working within an existing structure there will be less labour and fewer materials required.

You may need to strengthen the floor and increase the insulation between your rafters, but in the grand scheme of things this is a great cost effective option.

Dormer Loft Extension

This type of loft extension is slightly more than a Velux extension because your builder will need to remove part of your roof in order to add the new dormer window. This will take more time, and the cost of the materials is also more expensive.

The advantage of this type of loft extension is that it creates more usable living space in your loft because it raises the overall height in the area directly below the dormer window. So, the bigger the dormer, the more space you can create within your loft.

Hip to Gable

This option is one of the most common types of loft extension found in the UK. It involves removing the side part of the roof of your house and changing it to a vertical brick wall known as a ‘gable’.

Through Permitted Development you can also merge this option with a rear dormer window. This creates a huge amount of usable space in your loft.

The size of the space will be determined by the size of your existing loft and how high your ridge is. The larger this space, the more you can create. If you own either a terraced or semi-detached house, this option will create a master bedroom and ensuite shower room.

Mansard Roof

This type of loft extension is less common and often found on more traditional houses. They consist of a horizontal roof and the rear section can be almost vertical.

This helps create a huge amount of space within the loft but can cost more than the three options described above to build, as it involves rebuilding your entire roof.

1.5. How much does a garage conversion cost?

Garage TypeMinMax
Attached Garage£7,500£12,500
Integrated Garage£10,000£20,000
Detached Garage£15,000£25,000

Attached Garage

This type of garage is an independent structure, but part of the garage relies on the structural support of an adjacent building which would typically be your house.

This type of garage is the most common to convert. As it is already connected to the main house there will be less construction work involved. You can often use the electrics and plumbing from the main house that can easily be run into the new space.

It is also a very economical way to create some much-needed living space as you need only knock down the dividing wall to increase the size of your living room.

Integrated Garage

An integrated garage is where your car is parked within the main structure of your house with an internal door leading to the main living space.

These are also very common and the cheapest cost to convert because they are already part of the main house. This means you would not need to rebuild either the roof or wall as you might do on an attached or detached garage.

Detached Garage

A detached garage is where your car is parked in a separate structure which is therefore not connected to your house.

This type of garage conversion is the most expensive of all three options, but often far less when compared to a side extension. This is because it is more challenging to add water, electrics and plumbing as it is not attached to the main house. This is however the perfect space if you’re looking to create a nice home office with fewer distractions from people in the main house.

For more information, check out our dedicated post How Much Does a Garage Conversion Cost?

1.6. How much does a basement extension cost?

Basement Extension Cost: £4,000 – £5,000/ m2

This type of extension is the most expensive because of the amount of construction work and expertise required to carry out the work.

There is a huge amount of groundwork to carry out including: Excavating the basement, underpinning the existing foundations, and damp proofing.

Extras: The above costs budget for mid-range finishes. With basements however, there are lots of opportunities to add some fantastic features. For example, if you would like a little bit more natural light you could opt for some walk on roof lights. These are also the perfect space for a home gym or cinema.

1.7. How much does a new bathroom cost?

Bathroom SizeAverage Cost
Small Bathroom£7,500
Medium Bathroom£10,000
Large Bathroom£15,000

Labour: This is one of the biggest costs when creating a new bathroom. It will include removing all the old fixtures and fittings as well as all old tiles. Your plumber will then prepare the first fix which is usually running all the new pipe work and putting the shower tray, toilet and bath in place.

Your tiler will then need to carefully lay all your tiles whether they be on the floor or wall. Once that has been completed your plumber will come back for the second fix where they add things like taps and shower heads etc.

Materials: These can range depending on the quality of the product. On average, the cost of a toilet, shower, sink and bath could be as little as £3,000, which is far less than the labour involved in fitting them.

Tiling: Bear in mind the cost of tiles can vary hugely. It is also worth considering that if you opt for elaborate mosaic tiles these will be more expensive than larger porcelain tiles. They also take someone more skilled to apply correctly.

1.8. How much does a new kitchen cost?

Kitchen SizeMinMax
Budget Kitchen£6,000£10,000
Mid Range Kitchen£10,000£20,000
Bespoke Kitchen£20,000£50,000 +

Budget: There are so many budget kitchen companies to choose from including: Ikea, Home Base or B&Q.

The carcasses and doors will probably be made of MDF rather than solid wood, and the worktop would be either vinyl or laminate rather than more granite or real wood options found below.

These kitchens are usually prefabricated and therefore require a little more creativity and labour from your kitchen fitter to make it look perfect.

Midrange: These types of kitchens are made to order and have slightly higher quality materials and quality of finish when compared to more budget options.

The carcasses will be more robust and made of veneered MDF or birch ply. You might also expect to find solid timber of plywood doors.

This type of company will also have better customer service. They may have more designers and include a few home visits. Best of all, some include 3D renders of what your kitchen might look like when finished.

Bespoke: This type of kitchen is fully custom made, meaning the fit will be absolutely perfect, giving you more options to make efficient use of every nook and cranny in your kitchen.

This type of kitchen is often handmade with solid wood carcasses and doors. There is a limitless supply of different materials but the final kitchen will be jaw dropping.

2. What factors affect the cost of building an extension?

Construction System

The way you construct your extension can have a huge impact on the overall cost of your project.

One of the cheapest options is concrete blockwork which all builders are familiar with. An ever more popular method is to use structural insulated panels. The overall cost of the materials is slightly more than concrete blockwork but your builder will certainly save on labour.

As these panels are also well insulated, there will be a lifetime of future cost savings as it will cost less to heat your home which makes this option one of the most sustainable.

Cladding Choice

In the UK, the majority of houses are brick built, but there is a new option becoming increasingly popular. In this route, the main structure of your house is made from concrete blockwork but then covered in a specific type of material, and there are a huge number to choose from. Stone, wood and even zinc.

This use of modern materials can really help your extension stand out from the crowd and give you something unique and personal to you.

Build Route

DIY: We don’t really see this option undertaken often as it involves quite a bit of technical knowledge and skill. It involves you carrying out most of the work yourself and purchasing all the relevant materials with the help of various tradespeople along the way.

Hiring a main contractor and subcontractors: This option means you have slightly less involvement compared to the DIY route mentioned above. It involves you managing the project yourself by hiring all the different contractors.

For example, you might find a builder to lay the foundations and build the shell. You could then liaise directly with a roofing supplier, who often fit their own products. You would then find a plumber, electrician and painter, and manage them all accordingly. This method takes quite a bit of time and effort but the total cost saving can be substantial.

Building using a main contractor: This option is the most expensive but requires far less of your time. You hire one builder who would arrange for his own team to carry out the building works but would subcontract out some elements. Ultimately, one person would be responsible for the entire project which gives you more peace of mind if any problems arise.

3. How can you save money when building an extension?

3.1. Get Professional Advice

The first and most important step is to find yourself a good architect. They have a huge amount of experience in creative design, planning in your local area, and also dealing with builders.

It may be that you have lots of great ideas and your architect can work with you to refine them. They can even prepare lots of different options to see which works best.

Inevitably, there can always be small unforeseen issues on site when building extensions. Your architect will be the perfect person to fix them and will play an important part in your project.

3.2. Choose a simple design

We know that after watching an episode of Grand Designs the aspirations of homeowners goes sky high. Oddly shaped roofs and huge expanses of cantilevered glass can look fantastic but can cost a huge amount of money.

Keeping the design and shape of your extension a bit more conventional will save you a huge amount of money. This is where your architect can also help. There are hundreds of ways to make an extension appear unique without spending a fortune.

3.3. Stick to your budget

Once your architect has prepared the initial design and confirmed that it is within your budget, do your best to make as few changes as possible, as that is when costs begin to mount up. Builders are often in a strong bargaining position when agreeing to changes as they know you have few other options.

Always remember to agree a fee upfront for whatever changes you are looking to make throughout the project.

3.4. Manage the project yourself

Builders will usually charge an extra 15% – 20% on top of labour and building materials to manage the project themselves. You can save a huge amount of money by managing the project yourself and sourcing the various tradespeople.

This route will certainly take you slightly longer to complete your extension, but the reward is saving quite a bit of money.

3.5. Source your own materials

If your builder has offered to purchase all the materials for your extension, the chances are they are marking those prices up which is common practice.

To avoid this, you could go to a builders merchant yourself and purchase the various building materials you might need as well as hiring skips yourself.

4. FAQs

4.1. How long will my extension take?

The time it takes to construct your extension really does depend on your design and size.

For an average size single storey rear extension and loft conversion, we would expect the entire construction process to take no longer than 5 – 7 months.

4.2. How long does a planning application take to be approved?

After your architect has submitted your planning application or certificate of lawful development, it will take your local authority approximately 1 week to validate your application. After that, the statutory time period for determining that application is 8 weeks.

4.3. Should I move out of my home during the work?

This is a question we are often asked. The answer really comes down to the cost of moving out of your house and also the scale of works.

If you’re proposing a relatively straightforward single storey rear extension then your builder can build the majority of the extension and knock through into your main living space when complete. This causes minimum disruption to you and your family.

If your extensions are bigger in scale and involve removing your roof and part of your rear wall, it makes it far more difficult to live in the house comfortably during construction works.

4.4. Do I need planning permission for my extension?

This is not an easy question to answer. But to help, our team have written a separate blog which goes into this question in a lot of detail. Click here to read the article.

4.5. Will an extension add value to my property?

‘Nationwide’ recently prepared some research to calculate how much certain types of extension can increase the property value. They found that an extension creating a double bedroom and en-suite can add up to 23% to the value of your property. An extra bathroom could add up to 6% and an additional bedroom can add 12%.


After your house, building an extension could be the second biggest purchase you ever make.

Going through the design, planning and construction process can be quite complex.

Our team help homeowners go through the process every day and have a huge amount of experience. If you have some ideas you would like to discuss, or you’re wondering how much your extensions might cost, please feel free to book a free 30 min consultation with us.

We’re always here to help.

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