How much does an extension cost in 2021?

Sarah Parker
Authored by Sarah Parker
Posted: Monday, April 12, 2021 - 15:02

Average house extension prices are between £15,000 to £125,000, but this varies depending on the type of extension you want to have.

Find out more about the cost of extensions in this guide from the National Association of Building Contractors.

How Much Does an Extension Cost?

Vetted Builders

Let's get straight to the point.

The average house extension cost is around £15,000 to £125,000.

Well hang on, that's quite a large price range but let us explain:

The cost of an extension will be dependent on the type of extension you choose to have, its size and the quality of finish.

For instance, a single-storey extension would cost around £21,000 to £100,000. This type of extension is normally used for small additions, such as a bathroom or adding more space to a kitchen or living room.

Now that's still a huge price range, so let's break it down a bit more.

A small single-storey extension (15m2) has an average price range of £20,000 up to £30,000 with the range mainly being affected by the quality of finish and your location.

Whilst a large single-storey extension (50m2) can cost anywhere from £70,000 up to £100,000. As you can see, it's difficult to place a small price range on home extensions.

You could also have a double extension. This features two floors and helps you to expand the space in both the downstairs and upstairs of your home. The cost of a double extension ranges from about £42,000 to £200,000.


There are also more specific extensions you can have.

This includes kitchen extensions which range from £10,000 to £60,000 and bathroom extensions which cost between £6000 and £30,000. You can also enhance the front of your home with a porch extension (typically built around your front door) which costs approximately £1500 to £7000.

The location of your extension will also determine the price.

Rear extensions cost around £15,000 to £80,000 and are located at the back of a property. These are normally added to homes with a large garden space.

If you have room at the side of your home, you may also want to consider a side extension for £20,000 to £120,000. You could even consider a side-return extension - which is usually a small space on the side of your home that can be utilised to expand rooms in your home. A side return extension costs around £9000 to £28,800.

For properties with more exterior space, you can choose a wrap-around extension for £40,000 to £180,000 - which covers two or more sides of the home.

Another popular option is a prefab extension. This is a pre-constructed building which is built in a factory then shipped and installed quickly by a professional team of tradespeople. This normally costs between £18,000 and £65,000.

House Extension Prices

Looking to understand the costs of adding an extension onto your home in more detail?

Let's break it down a little.

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Single-Storey Extension Cost

A single-storey extension is a popular house extension and is normally used to enhance the space of a kitchen or living room on the first floor of a property.

The cost of single-storey extension installation is around £1400 to £2000.

The table below sets out the single-storey extension cost for different sizes of basic, average, and premium quality builds.

Size (m²)




Small (15m²)




Medium (25m²)




Large (50m²)




Double-Storey Extension Cost

A double-storey extension is built across two floors of a property, increasing the space available to your family by allowing you to add on multiple rooms.

The cost of an extension with two floors is between £1400 to £2000 per square metre.

The following table lays out the costs for different sizes and qualities of double-storey extensions.

Size (m²)




Small (30m²)




Medium (60m²)




Large (100m²)




Rear Extension Cost

A rear extension is installed at the back of a property where there is a large garden area.

The cost of a rear extension is around £1000 to £1600 per square metre.

The table below sets out the prices for different-sized basic, average, and premium rear extensions.

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Size (m²)




Small (15m²)




Medium (25m²)




Large (50m²)




Side Extension Cost

Side extensions are a popular choice for properties with a lot of space on the left or right hand side of the home.

Typically, side extensions cost around £1000 to £2000 per m².

The following table shows the different-sized buildings for basic, average, and premium side extensions.

Size (m²)




Small (20m²)




Medium (40m²)




Large (60m²)




Side-return Extension Cost

Properties with a small amount of space at the side of their home which is used to store bins, bikes, and more may benefit from a side return extension.

The price of a side-return extension per square metre is around £1500 to £2400.

The table below sets out the different costs of basic, average, and premium side-return extensions.

National Association of building contractors vetted builders

Size (m²)




Small (6m²)




Medium (9m²)




Large (12m²)




Kitchen Extension Cost

Want to have a larger kitchen in your home?

Your best option might be to have a kitchen extension.

This would cost about the same as a single-storey extension, at around £1400 to £2000 per square metre plus £200 to £1000 per m² for the actual kitchen installation itself.

The table below sets out the costs for small, medium, and large kitchen extensions for various budgets.

Size (m²)




Small kitchen extension (6m²)




Medium kitchen extension (9m²)




Large kitchen extension (18m²)




Small kitchen suite (6m²)




Medium-sized kitchen suite (9m²)




Large kitchen suite (18m²)




Bathroom Extension Cost

A bathroom extension is ideal if you only have one bathroom in your property, or you want to have a bigger bathroom.

The cost of house extension installation with a bathroom is based on single-storey extension prices. This means that it costs about £1400 to £2000 per m² along with the bathroom installation price which is around £500 to £1000 per square metre.

The following table outlines the costs of installing different-sized bathroom extensions with varying qualities.

National Association of building contractors accredited builders  

Size (m²)




Small bathroom extension (3m²)




Medium bathroom extension (6m²)




Large bathroom extension (9m²)




Small bathroom suite (3m²)




Medium-sized bathroom suite (6m²)




Large bathroom suite (9m²)




Porch Extension Cost

A porch extension is a great way to enhance the entrance of your home.

On average, porch extensions cost around £500 to £800 per m².

The following table highlights the prices for uPVC, wood, aluminium, and brick porch extensions.

Size (m²)





Small porch extension (3m²)





Medium porch extension (6m²)





Large porch extension (9m²)





Prefab Extension Cost

On a tight budget?

If you don't have a huge budget, then a prefab extension may be your best option - as it will save on labour costs due to it being prefabricated before installation.

The average cost of a prefab extension is around £1200 to £1700 per square metre.

The below table sets out the costs for basic, average and premium prefab extensions.

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Size (m²)




Small prefab extension (15m²)




Medium prefab extension (25m²)




Large prefab extension (35m²)




Wrap-around Extension Cost

A wrap-around extension is one of the largest extension types, which is normally attached to the rear and side of the home.

The average cost per square metre of a wrap-around extension is £2000 to £3000.

The table below shows the prices of different-sized wrap-around extensions on a basic, average, and premium budget.

Size (m²)




Small wrap-around extension (20m²)




Medium wrap-around extension (40m²)




Large wrap-around extension (60m²)




What are the Additional Costs of An Extension?

What other costs might you need to consider when building an extension?

Let's tackle that now.

Extension Architect Costs

You may want to consider hiring an architect to design and plan your new extension, while also making sure that it meets building regulations.

The cost to hire an architect is around £18 to £30 per hour.

Extensive Extension Groundwork Costs

Before building an extension, it is important that the ground is fully prepared.

This will ensure that the extension is installed on a robust foundation, while also preventing issues in the future.

If the existing ground is not level, then it may need to be excavated. This can cost around £30 to £50 per cubic metre.

Another extensive type of groundwork is installing drainage. This may involve removing old pipes and installing new pipework and drains to ensure that your new extension has proper drainage.

The cost to install new drainage is around £700 to £1000.

Extension Planning Permission Costs

Before building an extension, it is important that your plans align with planning permission regulations. This will help you avoid paying around £200 for an application.

To avoid incurring extra costs, you'll need to make sure that your extension meets the following criteria:

  • The extension must not cover over half the property’s land.
  • It must not exceed the highest point of the roof on the existing property.
  • Extensions within two metres of the property’s edge should not have eaves that exceed a height of three metres.
  • The work must not include installing raised platforms, balconies, chimney flues, vent pipes or any work to the roof (including cladding).
  • Side extensions must be single-storey and cannot exceed four metres in height from the side elevation.
  • Extensions on the side of a property must not exceed more than half the width of the building.
  • Single-storey rear extensions must not extend more than four metres beyond the rear wall of a detached property or three metres for any other.
  • Double-storey extensions must not extend over four metres from the back wall of the property and must not be within seven metres of a property boundary.
  • The roof pitch of a double-storey extension must match the existing house as closely as possible.
  • All windows on the top floor of a double-storey extension must be obscure-glazed if located on a side elevation.


Extension Building Regulation Costs

The majority of house extensions do require building regulations approval before they can be constructed.

And what's this cost?

The cost of building regulations approval will vary depending on your local council, as all local authorities charge different amounts for building regulations approval.

There are various elements of an extension that must be signed off by a professional in order to deem the property safe.

This includes:

  • Internal and external walls

Some extensions require external walls to be built below ground level to ensure the superstructure (the extension) is supported properly.

The new walls that are built will make up the substructure which normally includes blocks and mortar or brick. To ensure that the whole structure is protected, it is vital that the substructure is resistant to frost and substrates which are found in the ground.

  • Flooring

To ensure that the flooring installed in your extension meets building regulations, it is important that it has a sound structure to support a significant amount of weight.

It should also be heat and moisture-resistant to insulate, ventilate, and protect the extension flooring from damp, weed growth, and contraction.

  • Ventilation

One of the most important elements you need to consider when installing a new extension is ventilation. This will improve the air quality in your home and promote better health.

To ventilate your property, you could simply install windows - especially in rooms with poor ventilation such as a bathroom. You can also install trickle ventilation in all the rooms of your home by applying it to window frames.

  • Foundations

Before building an extension, it is important that there are robust foundations in place (which are normally made up of concrete).

The soil used should be able to hold the weight of the foundations and extension structure. It should also be dug deep enough to ensure that movement and contraction does not affect the extension. This is especially important for a double-storey extension, as more soil and foundations are required to support a larger extension.

What are the Supply Costs of an Extension?

Planning a DIY extension? Or maybe you just want to buy the supplies yourself?

In this case, you'll need to consider the following supply costs:

Supply description

Average supply cost

Skirting board

£2 to £8 per m²


£3 to £16 per m²

Wood flooring

£35 to £90 per m²

Laminate flooring

£7 to £26 per m²

Wall tiles

£9 to £45 per m²

Floor tiles

£12 to £50 per m²


£3 to £20 per litre

Plug sockets

£2 to £120 per socket

Light switches

£1 to £200 per switch


£14 to £400

Underfloor heating

£60 to £100 per m²

Internal doors

£70 to £500

External doors

£35 to £900


£100 to £320

Door handles

£3 to £220

Light fittings

£2 to £80 per light

Tradesmen Costs for an Extension

Here's another important thing.

When calculating the final cost of your house extension, you'll need to consider the labour costs charged by all the different tradespeople who will be working on your house extension.

Let's go through them now.

Vetted Builders

Builders have one of the most important jobs when constructing an extension.

They're involved in building the foundations and the actual structure of the extension to ensure it is robust and stable.

The average cost to hire a builder for your house extension is around £130 to £260 per day or £10 to £15 per hour.

Extension Plumber Cost

If building a kitchen or bathroom extension, you will need to hire a qualified plumber to install sinks, toilets, appliances, and more.

Plumbers cost around £200 to £400 per day for full bathroom installations or £30 to £60 an hour to plumb a sink.

Extension Electrician Cost

To get your extension fitted with lighting and electrical appliances, you will first need to hire a professional electrician to ensure everything is compliant and working properly.

The average hourly rate for an electrician is typically around £30 to £60 per hour or £170 to £340 a day.

Extension Plasterer Cost

To prepare your walls and ensure any paint or wallpaper lasts, you will need to hire an expert.

You should expect to pay £150 to £220 per day for a tradesman to plaster your new extension. They may only charge an hourly fee between £10 and £20 to re-plaster walls.

Extension Painter and Decorator Cost

Once the plastering is complete, you will need to bring in a decorator to paint or wallpaper the internal and external walls of your new house extension.

The average tradesperson will charge around £15 to £40 per hour to paint one or more walls.

For multiple rooms, you should expect to pay £100 to £300 a day.

Extension Carpenter Cost

If you're installing a new extension, you should hire a professional carpenter to help with the construction and installation of the new extension.

On average, a carpenter costs around £160 to £240 per day.

You may also want to hire them for smaller jobs - such as fitting a new door frame. In this case, they'll usually charge you an hourly rate of £20 to £40 an hour.

Extension Kitchen Fitter Cost

For a new kitchen extension, you should consider employing a kitchen fitter - who will install your kitchen cabinets and worktops.

For a small kitchen, you should expect to pay an hourly fee of £10 to £20, while a large kitchen may be subject to a day rate of between £120 to £200.

Extension Window Fitter Cost

Once your extension is built, you should hire a professional window fitter to install fully insulated windows.

For one or two windows, you should expect to pay around £8 to £20 an hour.

For a full set of windows on a double-storey extension, you'll likely be charged between £100 and £200 per day.

Extension Labourer Cost

Another important tradesperson you may need to hire is a labourer - who will assist with the majority of jobs onsite including building foundations, the extension structure, and more.

The average day rate for a labourer is £100 to £160 a day or £7 to £15 per hour.

Extension Bricklayer Cost

To ensure your extension is built properly, you will need to hire experienced bricklayers who know what they're doing.

Bricklayers tend to charge around £130 to £180 a day for a large extension or £12 to £20 per hour for a porch extension.

How Long Does It Take to Build an Extension?

Building an extension can be quite time-consuming.

Because of this, it's important that you understand the timeframes involved in building the type of extension you want.

Here's a quick break down:

Extension type


Single-storey extension

2 to 3 months

Double-storey extension

4 to 6 months

Side extension

2 to 3 months

Side return extension

1 to 2 months

Rear extension

2 to 4 months

Wrap-around extension

4 to 6 months

Kitchen extension

2 to 6 months

Bathroom extension

2 to 4 months

Prefab extension

8 to 10 weeks

Porch extension

3 to 14 days

How Much Does Each Stage of an Extension Cost?

There are various stages involved in building an extension

Each of these stages has individual costs that you need to know about (and budget for).

Let's dive into them now.

Building Regulations & Design

So, what's first?

You'll need to start by ensuring that your extension design is compliant with current building regulation and planning permission rules.

But what if your current plans do not adhere to the conditions?

In that case, you will need to change your plans or pay around £200 or more to apply for your extension to be approved.

Once approval is obtained, then you can plan your extension in more detail.

For this, you'll need the help of an architect. Their main job is to draw up plans for the structure - which include the dimensions and the aesthetic of the extension, making sure the whole time that they adhere to the regulations.

Typically, an architect costs between £18 and £30 per hour.

Foundations and Groundworks

Before any building can begin, it is important that the ground is fully prepared.

This can be a costly process, as it involves excavation. This can cost from £30 to £50 per cubic metre, with the total cost being dependent on the size of the land you're working on.

You may also have to remove pipework and install new drainage. This will ensure that your extension is protected against flood and damage, while also ensuring water flow is running and connected to the main property (if it needs to be). This work tends to cost between £700 to £10,000.

Once all the groundwork is complete?

Only then can the foundations be set to provide a stable framework for the installation of the extension. Installing foundations is a time consuming component of the project, and will normally cost around £100 to £130 per cubic metre.

Building the Outer Shell

Once everything's prepped and the foundations are in place, what's next?

The next stage involves building the outer frames and walls - using timber, bricks, blocks, and tile materials.

The average price of an extension outer shell is around £800 to £900 per square metre for professionals to install a weatherproof extension shell.

Fixtures and Fittings

The last stage is all about the finishing touches.

This will include installing furniture (like kitchen cabinets and worktops) which will cost between £1000 and £7000.

For a bathroom extension, you should expect to pay around £1400 to £6000 for brand new bathroom fixtures.

At this stage, your windows and doors will also be installed. This costs around £400 to £600 per window, while door installation is typically priced at £40 to £90 per door.

You'll also want to fit indoor and outdoor light fixtures at this point, which normally amounts to £60 to £1000 if fitted by a professional electrician.

How Much Does an Extension Roof Cost?

One of the most important elements of a house extension is the roof.

But you have a few types of roofs that you can choose between.

Let's cover them now.

Duo-pitched Extension Roof Cost

A duo-pitch roof (or a gabled roof) is a simple roof design which features two slopes which meet at a ridge.

Duo-pitch extension roofs tend to cost around £3000 to £4500.

Flat Extension Roof Cost

A flat roof is type of roof that is almost completely flat except for a slight slope which allows for drainage.

A flat extension roof typically costs around £1200 to £2000.

Catslide Extension Roof Cost

A catslide roof is a type of pitched roof which slopes downwards further on one side than the other.

The average cost of a catslide roof is around £1000 to £1500 per square metre.

Mono-pitch Extension Roof Cost

A mono-pitch roof or a lean-to roof is a single-sloped roof. This continues to be one of the most affordable roof solutions.

The installation price for a mono-pitch extension roof is normally between £2000 and £3200.

Hipped Extension Roof Cost

A hipped roof has four sloped sides which all have an equal length and meet at a central ridge.

The cost of a hipped extension roof is typically around £4000 to £4500.

Cost of Removing an Extension

What about if you want to remove an old extension? How much should that cost you?

To ensure the extension is removed properly, you should consider hiring professionals. This will cost around £500 to £3000.

On top of this, the cost of waste removal will be between £70 and £300.


Q: How close to my boundary can I build an extension?

A: If you're planning to build a double-storey extension, it should be no closer than seven metres from the boundary.

For any other type of extension that is within two metres of the boundary, it cannot exceed more than three metres of the existing property’s eaves.

Q: How much does an extension add to the value of a house?

A: Adding an extension onto your home can result in a return of investment up 23% for an additional bedroom and en-suite, while a bathroom extension can add up to 6% to the value of the property.

Q: What size extension can you build under permitted development?

A: If you want to save money on obtaining planning permission and building regulations approval, then you will need to build an extension that meets permitted development.

If you’re planning to build a single-storey rear extension, then this should not exceed a depth of 3 to 4 metres. Other extensions can extend to 6 metres, and detached properties can even extend their home up to 8 metres.

Q: How deep does footing need to be for a single storey extension?

A: Single storey extension foundations must be at least 200mm deep to ensure that the structure is completely secure and robust.