Rear Extension Cost

By Sam
Last updated 30th October 2023 - Reading time: 0 mins
At a glance
  • The average cost to build a rear extension is around £40,000
  • The job will take approximately 12 - 16 weeks to complete
In this article, you'll discover the following:
  • A complete pricing breakdown which includes cost factors to consider, the types of rear extensions available, along with what such a task usually involves
  • How long the job should approximately take and a general overview of what kind of jobs can be performed
  • How to find and hire a builder

Want to know how much a rear extension will cost?

Rear extensions can cost anywhere between £30,000 to £100,000 to build, depending on the quality of finishes you choose and the size of the rear extension.

That's why we've created this price guide - to show you exactly how much a rear extension will cost you!

However, if you're looking for a builder quote - we've got you covered there too.

For all the information you need - check out the guide below!

rear house extension

Extension Cost Calculator

Average Cost to Build a Rear Extension:

Depending on the complexity of the job, it usually takes: 12 – 16 weeks


How Much Does a Rear Extension Cost?

When you are planning an extension one of the hardest things to do is to come up with a budget. To do this, it is helpful to know the average cost of an extension. It depends on what sort of extension you will be building.


For a single-storey, the cost of a house extension is between £30,000 - £40,000.

For a double-storey extension, the prices are £40,000 - £50,000.

For a wraparound extension, the costs can be between £50,000 - £100,000.

There are a lot of factors that can affect the building extension costs. The main factors are the size of the extension, along with the type of extension you have chosen. Finally, the finish you choose will have a significant impact and can more than double the costs if high-quality finishes are chosen throughout.

Cheapest option for remodelling a sunroom tacked onto rear kitchen? Our house was extended at some point to add a sun room to the back of the house. The original rear window was left in the back wall to let in light to the kitchen. It doesn't look very pretty and the sun room feels tacked on. What's the cheapest way to remodel it and modernise it slightly? What would a builder suggest?

"There a few options to get more light in, remodel the kitchen units around, get a bigger window or a french door for more light, best option would be get a skylight window put on you roof."

Rear Extension Prices

Total Cost Based on Type of Extension

Type of Extension Average Cost
Single-storey £30,000-£40,000
Double-storey £40,000-£50,000
Wrap-around £50,000-£100,000
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Cost per square metre

Number of storeys Basic quality finish(m²) Good quality finish(m²) High quality finish(m²)
Single-storey £1000-£1700 £1700-£1900 £1900-£2200
Double-storey £950-£1500 £1500-£1850 £1850-£2200
Double + loft £900-£1300 £1300-1750 £1750-£2200
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Labour Costs and Timescales

The labour costs and time scales will vary a great deal depending on exactly what work you have done in your extensions. The following sections should give you some rough estimates on average costs and timescales.


Timescale and Labour Cost Based on Type of Extension

Type of Extension Duration Average Labour Costs
Single-storey 11-13 weeks £18,000-£22,000
Double-storey 15-18 weeks £28,000-£30,000
Wrap-around 10-14 weeks £28,000-£60,000
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Cost Per Hour of Different Tradesmen

Tradesman Averate Cost Per Hour
Builder £20-£35
Tiler £20-£40
Plumber £35-£45
Electrician £50-£120
Plasterer £30-£50
Decorator £20-£50
Joiner £35-£45
Roofer £30-£40
Kitchen fitter £20-£30
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Supply Only Costs

Rear Extension Windows Cost

Type of Window Size (cm) Average Cost
Velux 98 x 78 £325-£375
140 x 78 £400-£450
180 x 78 £500-£550
uPVC 60 x 90 £250-£400
90 x 120 £400-£600
120 x 120 £650-£900
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Rear Extension Bathroom Cost

Fixture Average Cost
Bath £80-£500
Shower £60-£1000
Shower enclosure £100-£550
Toilet £50-£500
Sink £50-£300
Taps £50-£100
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Rear Extension Kitchen Cost

Item Average Cost
Units with doors and handles £3200-£5500
Worksurfaces £200-£2000
Appliances £1500-£5000
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Rear Extension Lighting Cost

Type of Lighting Average Cost
Pendant/chandelier £40-£100
Simple ceiling or wall lamp £40-£60
Recessed spotlights £80-£110
Dimmable ceiling or wall lights £80-£100
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Rear Extension Heating Cost

Type of Lighting Average Cost
Traditional radiator £60-£150
Electric radiator £150-£300
Underfloor heating £100-£150 per square metre
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Rear Extension Flooring Cost

Type of Flooring Average Cost per square metre
Vinyl £7-£20
Wood £17-£70
Carpet £4-£40
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Additional Costs

Whenever you get extensive work done on your home, there are a lot of unexpected costs that can arise. Some of these additional costs can come from the choices you make about your extension, whereas others come from the existing condition of your home.

House Rewire Cost

When you are getting your extension built, you will need to have an electrician in to sort out the electrics for the new part of your home. If when they check your electrics, they find a problem, this will need to be fixed. Otherwise, you won’t pass building control inspections.

If you need a full house rewire it can cost on average £4,000. If you just need a new consumer box fitting, this will cost on average £500.

New Boiler Cost

If your extension includes any new plumbing, such as a toilet, bathroom or kitchen, then you might also need to invest in a new boiler.

If your current boiler isn’t able to cope with the extra strain, there is no choice but to upgrade. The average cost to install a new boiler is £1,500 - £2,500.

Removing Drains Cost

Depending on where the extension is located in comparison to your existing drains, you might find that you need to remove or relocate your drains. To check if it is necessary, you may need to carry out a drain survey first of all. The average cost for this is £250. If you need to move a drain, this can cost around £200 - £1,300.

Underfloor Heating Cost

When you are getting an extension built, you might want to take the opportunity to include underfloor heating in the new area of your home. Underfloor heating is easy to install if done during building work and can be an efficient way to heat your home. The average cost to lay underfloor heating is £800.

Adding a Kitchen Cost

If you are going to have a kitchen fitted in your extension, then you will need to add around an extra £3,000 - £10,000. This will cover the installation costs as well as the appliances cabinets, doors, and worktops.

Adding a Toilet Cost

If you are going to add a toilet to your extension, this will add to the cost of your extension. For a simple downstairs bathroom, the average cost is £100 - £200.

Adding a Bathroom Cost

If you are getting a new bathroom installed in your extension, then you should add around £2,000 - £6,000 to the price for your extension. This will cover labour costs as well as the supply of a bath shower, toilet and tiling.


Adding a Utility Room Cost

A utility room that houses a sink and has plumbing for washing machines and dryers will add extra costs to your extension. You should budget £3,500 - £7,000.

Site Survey Cost

Before you begin construction on your home, it can be a good idea to get a full survey done of the site and existing property. This will highlight any issues that might arise during the project and can prevent costly surprises. The average cost is £600 - £1,400.

Architect Cost

Having an architect produce plans for your extension will usually cost 7-15% of the cost of the extension. This does give you a plan and a clear vision to share with your contractors. To get planning drawings done usually costs £2,500- £3,000.

Structural Engineer Cost

If your construction requires the use of any reinforced steel beams or shifting of load-bearing walls, then you will need the services of a structural engineer. This can cost from £400, - £4,000, depending on the complexity of the work.

Planning Application Cost

If you need to apply for planning permission, it costs £206 to apply.

Building Regulations Cost

You will need to pay a fee to get you extension certified as meeting building control regulations. The fee covers several site visits and paperwork. This will cost £500 - £1,000.

Insurance Cost

If you are planning to move out while to work is completed, then you might find that your home insurance will not cover you during the project. You will need to get an insurance bundle to cover, employer’s liability, public liability and construction. This usually costs around £500 - £800.

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Cost Factors of Building a Rear Extension

When you get an extension done, there are a lot of factors that can affect the cost. With many big projects are few costs are fairly fixed, like the cost of a roof, and foundation. It is all the other decision that you make that have the biggest impact on the final budget.


Size of Extension

Unsurprisingly the bigger your extension is, the more it is going to cost. However, your cost per m² of space will decrease if you build over more than one floor.

Type of Extension

This is somewhat tied into the size of your extension, but generally speaking, a single-storey extension is the cheapest option, a double-storey extension is more expensive, and the costliest option is a wraparound extension.

Quality of Finish

Once you have the basic extension built, you will need to make a decision about the finish. Things like flooring, kitchen units and bathroom fittings. The quality of these that you choose will have a huge impact on the final cost. If you decide to go for all bespoke fittings and expensive materials, you can end up doubling your final price.

What's Involved in Building a Rear Extension?

There are several stages involved in building an extension. Depending on exactly what is being done, the plan will vary. For a single-storey extension, you can expect the work to go as detailed below. For a two-storey extension, you should add in an extra 3-5weeks.


Weeks 1-2

It usually takes about two weeks to prepare the site for building. This includes digging out the footing and pouring the foundations. If any drains need moving this will begin as well.

Weeks 3-4

The damp proof course will be installed at this point and the drains fitted. A con-crete floor structure will be installed during these weeks.

Week 5

The external walls will begin to go up. The cavity wall insulation will also be in-stalled, and the new walls tied to the existing ones.

Week 6

The internal walls will be constructed, and the cavity wall insulation finished.

Weeks 6-7

The roof structure will be built. If there are any windows going into the roof the spaces for them will be prepared at this point.

Week 7

The roof coverings will go on the membrane will be laid and then covered with the material of your choice.

Week 8

Windows and doors will be installed at this point. This will make the extension fully waterproof, allowing internal work to speed up.

Week 9

As soon as the extension is watertight, the first fix will start. This is the basis for the plumbing and electrics. Some of the internal carpentry will also begin.

Week 10

The extension will be joined to the existing house. This involved breaking through the old external wall. It can be a messy job.

Week 11

The plaster will go on the walls and be given time to set.

Week 12

This is the big week when the second fix of electrics and plumbing go in. The flooring will be laid, and kitchens or bathrooms can be fitted.

Week 14

Any issues that need fixing will be addressed at this point. It often takes a bit of time to find all the small issues, so keep a list to share with your builder.

Do I Need an Architect for a Rear Extension?

You do not have to have an architect to build an extension. However, it is a good idea to think about using one. Although it can be a little pricey to hire an architect, it can actually make the whole project cheaper.


When you have full plans drawn up you are in clear agreement with your con-tractors about what the finished product will look like. This prevents changes mid-build and can prevent unforeseen problems.

Some builders may not want to complete the project if an architect has not been involved, whereas others will be happy to complete the work without one. You will need to discuss this with any builders you consider hiring.

Can I Build a Rear Extension Myself?

Building an extension is a big job with a lot of different skills required. Unless you are a professional builder, it is probably not a project you should attempt to DIY. This is especially true in regard to the electrics.


There are parts of the project that you may be able to complete yourself, to keep costs down. Easier aspects of the work include decorating, installing kitchen units, laying flooring and tiling.

If you wish to complete some of the work, yourself to keep costs down, you should communicate clearly with your lead contractor. They will need to plan when all the work is done, so they need to know what you intend to do and when you will be doing it. It is best to have these conversations early on.

Choosing a Rear Extension

When it comes to a rear extension, you have three main options. A single-storey extension, a double-storey extension or a wraparound extension.

Single-storey Rear Extension Cost

A single-storey rear extension can be used to extend an existing downstairs room or add an extra room onto the house. The average cost for this type of exten-sion is £30,000 - £40,000.



  • ✔ Can cause minimal disruption
  • ✔ Can easily add skylights to minimise lost light


  • ✖ Can make interior rooms darker
  • ✖ Only gives a small amount of extra space

Double-storey Rear Extension Cost

These extensions add on at both ground and first-floor level. This usually allows for an extra bedroom and/or bathroom along with space for a new kitchen. The av-erage cost for this is £40,000 - £50,000.



  • ✔ Gives the most extra space


  • ✖ Big project
  • ✖ Can make interior rooms darker, especially downstairs

Wrap-around Rear Extension Cost

A wraparound extension is one where you are adding on to both the back and side of your home. It adds an extra L shaped living area to your home. The average cost is £50,000 - £100,000.



  • ✔ Get a larger area without sacrificing as much garden space
  • ✔ Can increase the amount of light in you downstairs living areas


  • ✖ Much bigger project

What Are The Benefits of a Rear Extension?

Increase Value of Property

The more space there is in your home, the more it is worth. Most extensions will add more value to a home than you spend on getting them done.

More Space

The main reason to get an extension built is to increase the space in your home. You can add extra rooms or extend existing ones. Either way, you will have more space to live.

Don’t Have to Relocate

When a family is growing homes can begin to become crowded. Whether this is because of children being born or parents coming to live with you. An extension can provide you with extra space without the need to move and uproot your lives.


Updated House

Adding an extension will allow you to rearrange your home so that it has a more modern layout and spacing. Homes can feel dated depending on how they are ar-ranged. An extension is an opportunity to change the feel of your home.

Brings More Natural Light into Home

Building an extension gives you opportunities to allow more natural light into your home. It is possible to do the opposite if care is not taken. However, if you take the opportunity to add windows and consider the layout, your home will feel bright-er.

Adds a Link to the Outdoor Area

A well-conceived extension can join your garden to your home. With considered use of glass, you can create a bridge between the two spaces. Doing this can make your home feel even larger, and it can increase the usefulness of your garden.

Will a Rear Extension Increase the Value of My Home?

Adding extra space to your home is a great way to increase its value. Adding a rear extension can add on average £59,000 to the value of your home. This figure will vary depending on the size of the extension and the location of your home. For instance, in the south of England, an extension will add more value than in the north.


Can I Have a Rear Extension?

In order to get a rear extension, you need to have space for one. The more space you have at the back of your home, the bigger the extension you can build. If you need to use more than half your garden space, you will need to apply for planning permission.

Other limits on getting an extension include listed buildings. If your building is listed or you live in a conservation area or a national park, then there may be re-strictions on the work you can get done. If you have any doubt, you should speak to your local council.

Planning Permission for a Rear Extension

One of the first things that many people worry about is planning permission. There have been a lot of changes to the rules surrounding planning permission, which means that it is required in fewer circumstances. So, check if you need it be-fore you start to worry about it.

Do You Need Planning Permission?

A lot of extensions will be covered under permitted development regulations. The following list identifies the maximum you can do before you need to apply for plan-ning permission:

  • A single-storey extension on a detached property can extend 8m to the rear.
  • The maximum height of a single-storey extension is 4m
  • A double-storey extension on a detached property can extend 3m to the rear
  • A double-storey extension can be no closer than 7m from the rear property boundary
  • Any extension should be of the same or similar material as the existing proper-ty
  • You can use no more than half the garden space for the extension

If your planned extension goes beyond these limits, you may need planning permission. Most councils offer a consultation service that will help you identify if you need to apply for planning permission. There is usually a cost of £50 - £150 for this.

If you live in a conservation area or a national park, then these limits may be smaller.


Applying for Planning Permission

If you need planning permission, you can apply through your local council or through the online planning portal. You can make the application yourself, or you can ask your architect or designer to do it for you.

It costs £206 to apply. You will need to check what documents your local council requires for a submission. Most local councils offer a support service with a small fee. This usually involves a meeting with a council employee. They can guide you through what you need and any potential problems.

One your application is submitted you will receive confirmation of the ap-plication. It will take around eight weeks for you to get an answer. During this time anyone can lodge objections to your plans. If you are rejected, you can see the reasons why. You can then amend your plans and reapply.

Building Regulations for a Rear Extension

When you have large scale work done on your home like an extension, you will need to comply with building regulations. This will involve site visits and paperwork. Most builders will be able to arrange and complete this for you. However, make sure you confirm with your builder that they will be doing this as you are ultimately responsible.

Depending on the scale of the work it will cost £50 - £100 to get certi-fied. There will usually be two site visits so that the quality of construction and the wiring and plumbing can be checked. You will receive your certification in the mail after work is completed.

Rear Extension Heating Cost

Deciding how to heat your new extension is a big choice. You can add radiators into your existing system, but if your boiler is not able to cope with the added load, you may need to replace it with a new boiler.

Wet radiators aren’t your only option. Underfloor heating can also be added, which will free up any wall space too.

Rear Extension Flooring Cost

When you get an extension built, it is like a blank slate on which you can choose any finish. When it comes to flooring, you can choose from any option.


There are so many options to choose from when it come to flooring, depending on your budget and tastes:

Rear Extension Kitchen Cost

If you are going to have a kitchen, then there will be a lot of decisions to make all of which affect the kitchen extension cost.

A bigger kitchen is the number one reason why a rear extension is added to a home, as many older homes have a small kitchen.

Chec out our new kitchen cost guide to find out how much they cost on average.

Rear Extension Exterior Cost

There are a few options for the exterior of your rear extension. However, if you are not applying for planning permission, you will need to plan to use the same ma-terials as the rest of the house. If you apply for planning permission, you may be able to have more choice.

Rear Extension Roof Cost

There are a surprisingly large number of options when it comes to the roof for your extension. If you have a single-storey extension, there are more options open to you.

Keeping Extension Costs Low

This is a big project to undertake, and it brings a lot of costs with it. Here are some rear extension ideas that can help to keep your costs down.

Simple Design

The design of your space will have a big impact on the cost. For example, any rooms that have anything other than 90-degree angles in them will cost extra. Try to keep your plan as simple as possible, and this will help to keep costs low.

Off-the-shelf Furnishings and Fittings

Where possible, try to use standard sizes for your furnishing and fittings. As soon as you start choosing more bespoke items, your costs will begin to rocket.

DIY The Parts You Can

There are a lot of parts of this project that you can do yourself. Speak to your builder beforehand and negotiate which parts you can do yourself and when you will be doing the work. You can easily save around 30% of your labour costs by doing the easier parts yourself.

Avoid Complicated Groundworks

One of the fixed costs in any building project is the groundwork. The more ex-pansive the footprint, the higher the base cost will be. Try to keep the new footprint as simple as possible. You can maximise your investment on the groundworks by building more storey. This will reduce your extension costs per square metre.

Accurate Design and Plan

Paying for an architect to design your extension can seem like a large cost up-front on the project. However, it can keep your total costs down. Having a plan means you are less likely to make changes partway through. It also means your quote will be more accurate, and there will be less chance of miscommunication. All of these things can push costs up.

Choose Your Builder Carefully

If you are trying to keep costs as low as possible, then discuss this with your builder before you start. If your builder is experienced at working with small budgets, then they will be able to guide you to make a decision that will keep you on track.

Alternatives to a Rear Extensions

If you are looking for ways to increase the living space in your home, there are a lot of options to consider. Which is best for you will depend on your budget and exactly what you are trying to achieve.

Side Extension Cost

These are a popular choice for terraced or semi-detached homes. Small side passageways can be used to expand existing rooms to make them more liveable. The average cost for a side extension is £30,000.



  • ✔ Can maximise your existing space
  • ✔ A more cost-effective option
  • ✔ Can easily add skylights


  • ✖ May need a party wall agreement with neighbours
  • ✖ Removing an external wall can be a big job

Conservatory Cost

Getting a conservatory is a cheaper and easier way to gain space in your home. However, a conservatory is not considered a liveable space. There are limits on the services you can have inside a conservatory. The average cost for a conservatory is £15,000.



  • ✔ Quicker and easier than an extension
  • ✔ Cost-effective
  • ✔ Range of options


  • ✖ Can be less useful than an extension
  • ✖ The temperature can be uncomfortable in summer and winter

Glass Room Cost

Glass rooms are similar to conservatories but have more glass in them. There is no brickwork at the base, and the panes of glass are much larger. The cost of a glass room varies between £15,000 - £70,000.



  • ✔ Great views
  • ✔ Contemporary style
  • ✔ Bespoke


  • ✖ High cost for the extra space
  • ✖ Can be cold in winter

Loft Conversion Cost

A loft conversion allows you to create extra room in your home by converting your attic space into a liveable space. There are several different types of loft con-version that create different amounts of space. The average cost of a loft conversion is £40,000, although you can get a basic loft conversion for as little as £15,000.



  • ✔ More living space without sacrificing garden space
  • ✔ Good return on investment in terms of home value


  • ✖ Not all homes are suitable for a loft conversion
  • ✖ May have to spend a lot to get a usable space

Pre-fab Extension Cost

A prefab extension is one that is built offsite and then lifted into position. They can be made of a range of different materials. Large ones may need to be fitted in sections. The average cost is £2,000 per m².


  • ✔ Less disruption at home
  • ✔ Quick installation process


  • ✖ Limited design options

Hiring Rear Extension Contractors Checklist

When you are hiring a contractor to build your extension, you will likely hire a builder to manage the whole project. There are a lot of moving parts, and it is important to have the right person for the job. Otherwise, it can become more expensive and take longer than needed.

Here are some things to look out for.

  • They should be able to show you examples of previous work they have complet-ed
  • They should be happy to let you speak to previous clients
  • Your contractor should be able to give you a break down of the expected costs. Don’t expect this on the day. They will likely have to go away and calculate all the costs to give you a realistic quote
  • A good sign is a contractor who takes notes when you discuss what you want do-ing. It means that things are less likely to get forgotten about
  • Check who will be responsible for organising building control visits. Most contrac-tors will do this but make sure it is clearly agreed as it is your responsibility


How much does an extension cost?

It depends on what sort of extension you get. It can cost any-where between £20,000 - £100,000.

How much does a double-storey extension cost?

Typically, a double-storey extension will cost be-tween £40,000 - £50,000.

Should I move out during an extension?

It is entirely up to you. If you can afford to move out during the work, it can allow the builders to complete the work faster and be a less stressful experience. It is not necessary to move out. It is a matter of how much disruption you can cope with.

How long does it take to get planning permission?

It usually takes around eight weeks for planning permission. You should bear this in mind when getting quotes from builders.

How long does it take to build an extension?

It depends on the size of the extension, but for a single-storey extension, it is usually around 12 weeks. While a double-storey extension, typically approximately 16 weeks.


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