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Cost of Installing a Rear Extension

Whether you plan to have a rear extension installed or are considering the possibility, this cost guide will prove very useful and provide you with the knowledge you need to make an informed decision!

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Introduction to the Cost of a Rear Extension

Are you thinking of having an extension added to the back of your home?

Would you like to add more space to your property but want to weigh up your options first?

Are you looking to build a terraced house rear extension or a semi-detached rear extension?

Regardless, in the following article, we’ll lay out the pros, cons and costs involved in having this type of extension constructed.

We’ll also contend with the timeframe and installation steps involved in building this type of extension.

Whether you plan to have a rear extension installed or are considering the possibility, this cost guide will prove very useful and provide you with the knowledge you need to make an informed decision!

Rear extensions are as the name suggests, an extension that is added to the end of a property, protruding outwards and providing the back of a home with more room.

Increased spaciousness and the possibility of creating a more aesthetically pleasing kitchen are among the reasons why rear extensions are popular with homeowners across the UK.

A rear extension can also add value to a property.

In addition, depending on its design and construction, this type of extension may improve the lighting conditions at the back of your home.

How Much Does Building a Rear Extension Cost?

The cost of a rear extension can vary greatly. However, the average cost is about £30,000.

If you’d like a small rear extension constructed, you can expect to pay about £15,000 to £20,000.

In the case of a mid-sized extension, this should cost around £30,000 to £35,000 while a large rear extension will likely come with a total bill of approximately £45,000 to £60,000.

These rear extension prices assume that you’re looking to have a single-storey rear extension built.

If you’d prefer a double-storey rear extension, you should add about 50-75% to the above estimates.

In this case, it would cost about £26,000 to £30,000 for a small rear extension, £50,000 to £57,000 for a medium rear extension or £70,000 to £100,000 for a large rear extension.

The labour cost should make up about 45% of your overall bill so around £12,000 to £15,000 for a medium single-storey rear extension or £20,000 to £24,000 for a medium double-storey rear extension.

As for the supply costs, this should make up about half of the total cost.

Again, based on a mid-sized rear extension, this part of the bill should land around £13,000 to £17,000 for a single-storey extension or £23,000 to £26,000 in the case of a double-storey option.

The final element of the cost is down to waste removal, which should end up about 5% of the full value or somewhere between £1000 and £3000.

There are many cost-affecting factors that will influence your final bill.

For instance, the type and size of the extension will matter the most in deciding how expensive the extension will be.

In addition, the type of room will also matter, whether it is a living room, bedroom, bathroom, kitchen or conservatory.

Further, the type of property you have, how accessible the proposed work area is and where in the UK you are located will also influence the cost.

In terms of the latter, this is because, in some parts of the country, labourers charge higher rates than in others.

In London and the southeast, you can expect to pay a higher labour cost than you would elsewhere in the country.

On the other hand, contractors in the north of England generally charge a lower rate than the nationwide average.

Size of Extension Single-Storey Cost Double-Storey Cost
Small £15,000 to £20,000 £26,000 to £30,000
Medium £30,000 to £35,000 £50,000 to £57,000
Large £45,000 to £60,000 £70,000 to £100,000

Types of Rear Extension

We’ll now break down the pros, cons and cost of various types of rear extensions based on what room it might be used for.

We’ll also look at which type is easiest to build, which is the cheapest and what type of rear expansion is the most expensive.

The price estimates laid out in each of the following subsections assume a single-storey extension.

Once more, you’ll need to add about 50-75% to the cost if you’d prefer a double-storey rear extension.

Among the types available are a rear kitchen extension, conservatory and rear bathroom extension.

Kitchen Rear Extension Cost

Many homeowners choose to add an extension to the back of their home to create a more useful and spacious kitchen.

In fact, it is the most popular type of rear extension.

The cost of having an entirely new kitchen installed will add about £5000 to £10,000 to your total bill making it the most expensive type.

If you are extending an existing kitchen, it may cost several thousand less.

On average, you’d probably pay somewhere in the range of £20,000 to £40,000 to have a rear kitchen extension installed.

Unlike with say a bathroom or living room extension, a kitchen extension will likely involve far more additional work once the actual structure is built considering that there will be many fittings and appliances to add.

The scale of any electric and plumbing work may also prove more challenging with a kitchen rear extension in comparison to any other type of rear extension.


✔ Can provide a large and beautiful kitchen space.

✔ Kitchen with improved lighting.

✔ Kitchen with greater utility.


✖ Kitchen fitting will add to the duration.

✖ Expensive.

Conservatory Rear Extension Cost

Another common use for a rear extension is a conservatory which is usually made up of a glass or tarpaulin roof and walls of the same material.

The purpose of this is to allow maximum light into the conservatory; hence why many would use it as a sunroom.

Conservatories are a great option if you’d like somewhere relaxing to socialise, read or enjoy the sun without being directly outside.

Of course, you could also use a conservatory to grow a wide range of plants.

If the weather is poor, conservatories are ideal for getting as close to the outdoors as you can without being directly exposed to the elements.

This type is the most expensive option as you’ll probably be looking at paying anywhere from £6,000 to £20,000 in total.

It could cost significantly more depending on the scale and quality of the conservatory.

However, generally, a conservatory is going to be smaller than say a kitchen or living room extension; thus, why the price tag should end up significantly less than the average rear extension in general.

There are many types of conservatories such as Victorian, Lean-to, Edwardian and Orangery greenhouses.

It’s vital that you do your research before picking a conservatory type as each come with their own pros, cons and features.


✔ Great way to create a sunroom.

✔ Perfect for growing plants.

✔ Relaxing environment.

✔ Less expensive than most extensions.


✖ Risk of choosing the wrong type of conservatory.

✖ Temperature inconsistencies.

Living Room Rear Extension Cost

It may also interest you to have a living room extension added to the back of your home.

It will cost an additional several thousand pounds to have a living room fitted once the installation is complete.

As with a kitchen extension, how much this will ultimately cost will depend on whether you are extending an existing living room or creating one from scratch.

Considering that the average living room is about 20 metres squared, you can expect this type of extension to cost approximately £25,000 to £35,000.

While a living room extension may involve more additional work than with a conservatory, the contractor will likely need to install fewer appliances and fixtures than would be the case with a kitchen or bathroom.


✔ Adds space and more function to a living room.

✔ Better aesthetic than most living rooms.

✔ Generally not as complicated as having a new kitchen added.


✖ More expensive than a conservatory.

Bathroom Rear Extension Cost

Another rear extension option is that of a bathroom.

On average, a bathroom extension will probably end up about 4-5m2 with a price range of £5000 to £15,000.

This makes bathroom extensions one of the more affordable types of rear extensions.

However, this is mainly down to the estimated size of a bathroom extension, and the appliances and fittings needed to complete the bathroom extension will still make up several thousand pounds of the total price tag.


✔ Relatively inexpensive.

✔ Great way to add a bathroom if you currently have limited space.

✔ Faster to build than most options.


✖ Installing appliances and fittings will influence the cost significantly.

Garage Rear Extension Cost

Lastly, you may wish to have a garage added to the back of your home.

Of course, if you want to actually house any vehicles, it’s important that there is a way for you to drive in and out of your property with a garage located at the back.

In addition, if any particular work is required (converting the side alley into an extended part of your front driveway), it’s essential that you know whether planning permission or/and building regulations approval will be needed.

You may prefer to use the garage for storage, DIY work, etc. Given that a garage extension will likely not be as large as most extensions, it will probably only cost you somewhere between £2000 and £4000.

Garages are probably the least common way to use a rear extension, although they would be the easiest to build given their relatively small scale.


✔ Cheapest.

✔ Easiest to build.

✔ Quickest to build.


✖ Back of your property may not be suitable for a garage.

What Does Building a Rear Extension Involve?

Let’s take a look at what building a rear extension entails. We’ll now break down each of the steps involved in the process:

1. Choosing a Material & Contacting a Professional

First and foremost, you’ll need to decide what type of extension and design works for you.

As shown in the above section, there are many options to choose from, each with their own costs and features.

Even if you know what you’d like to use your extension for, there are many possibilities when it comes to design.

Of course, you’ll also need to decide whether you want your extension to have one or two storeys.

2. Planning Permission and Building Regulations

There are a range of different rules that determine whether you’ll need planning permission for an extension.

Your permitted development rights (PDRs) grant that you may have work performed on your property without the need for planning permission approval given certain requirements are met.

Planning permission will not be necessary so long as the extension is:

  • Covering no more than half of the land area that surrounds the original dwelling.
  • No taller than the existing roof of the property or no taller at the eaves than the current eaves.
  • Not built ahead of the property’s principal elevation or if your home fronts a highway, the side elevation.
  • Not part of a construction project which includes the installation of a veranda, balcony, raised platform, chimney, soil and vent pipe, flue or microwave antenna or where the current roof is to be altered.
  • Not to include exterior cladding if your property is on article 2(3) designated land.
  • Not going to provide a significantly altered appearance (in terms of materials) compared with the exterior of your existing home.
  • Not constructed in an area where there might be an Article 4 Direction, planning condition or any other kind of limitation that restricts or voids your PDRs.
  • Added to a converted house or a house that was made via the PDRs which cover changes of use.

There are also rules specific to single-storey and double-storey extensions.

If you’d like to have a one storey rear extension added, to avoid planning permission being necessitated, it cannot be greater than four metres in height, extend beyond the back wall of the original dwelling by four metres or more if a detached house or three metres or more if you have any other type of house.

Assuming you are not living on a Site of Special Scientific Interest or article 2(3) designated land and have received prior approval, your single-storey extension may reach up to six metres in height for most houses or as much as eight metres if you live in a detached house.

The Local Planning Authority needs to be informed of your work in the form of a prior approval application should these specific circumstances apply to you.

As for a two-storey extension, for your permitted development rights to apply, it cannot extend past your home’s rear wall by three metres or more or be situated at a point that is seven metres or closer to any boundary that is on the opposite side of your home’s rear wall.

The roof pitch needs to match your current dwelling in a way that is as feasible as possible.

If there are any upper-floor windows fitted at a side elevation, they cannot open unless more than 1.7 metres from the ground and they need to be obscure-glazed.

If your property is on article 2(3) designated land and you want to have a two-storey extension constructed, householder planning permission will be needed.

If your PDRs cover an extension project, you could consider this a permitted development extension.

Either way, building regulations will apply.

There are two ways of going about this. On the one hand, you could submit a Building Notice, and while this is the cheapest option, there is a risk that you’ll only learn of compliance issues after the extension is built.

You can avoid this issue by submitting a Full Plan Submission instead.

3. Hiring Professionals

At this stage, you will want to hire professionals for the construction of your extension.

That is of course unless you’d like to build it yourself (see the next section; DIY Rear Extension).

You should look around and get some quotes from a range of builders or/and companies before choosing one.

Before making a pick, you should also ask about warranties and take a look at the opinions of their previous customers, if possible.

Sometimes getting a recommendation from a friend or family member is the best way to ensure that you’ve chosen a reliable contractor.

Once confident with your choice of builder(s), you should hire a professional designer or architect to draw up the specifics of the design.

An alternative to architects are architectural technologists who could also help you with the planning process.

You may also want to get in contact with a home designer if you are having a living room, kitchen or bedroom extension built.

Don’t forget to do your own brainstorming and go through some rear extension ideas of your own including bungalow rear extension ideas.

Before construction begins, you should also ask yourself whether you’ll want to temporarily leave your home during the course of the work.

4. Construction

Each type of rear extension will involve a different construction approach.

For example, a garage will be a more short-lived construction project, while a large living room or kitchen extension could take quite a bit longer to build.

As for materials, bricks and mortar are usually used to construct a rear extension, but some are made with alternative materials like timber, copper or even glass as is the case with most conservatories.

Building an extension can be broken down into the following phases:

  1. Laying the groundwork.
  2. Checking the drainage and any demolition work that is needed.
  3. Adding the walls and roof.
  4. Installing any doors and windows.
  5. New drainage connections.
  6. Interior work (e.g. painting, adding appliances, etc.)

Some of the key differences in constructing a rear extension will come down to the interior design and installations whether it be the shower, bath and toilet of a bathroom, the oven, fridge and freezer of a kitchen or the sofas, TV and curtains of a living room.

5. Additional Work and Clean Up

You may wish to have one or several extra jobs performed after having a rear extension added.

This would include the interior work such as installing light fixtures which might cost you somewhere between £100 and £200 or interior decoration which will probably set you back several hundred or several thousand depending on the extent and nature of the work.

Painting the insides of an extension will likely come with a price tag of about £1000 to £2000.

To have a radiator installed will likely cost between £150 to £300 per unit.

Moving a radiator will likely cost about £100 to £200.

For a heating system, the full installation cost could land anywhere from £3000 to £5000, although it will depend heavily on the type and fuel used.

You’ll also need to take into account the annual running costs of such a system.

DIY Rear Extensions

While it is possible to build a rear extension (or at least much of it) by yourself, there is some work which must be undertaken by a professional.

For example, any work that involves electrics has to be performed by a qualified contractor.

As for building regulations, you will need to submit either a Building Notice or Full Plan Submission to ensure that your extension is compliant.

Planning permission will be required under certain circumstances as laid out in the previous section.

Building Control will need to sign off on the extension once it is complete.

As with any DIY project, there are a range of dangers and hazards associated with building a rear extension.

On the one hand, if you make any serious errors in construction, this could mean that you’ll have wasted weeks or months working on the extension only to find that you need to hire builders to start the entire project from scratch.

As for more severe risks, when dealing with heavy materials, there is a danger of being struck by objects that could result in injury or even death.

There’s also the risk of falling from a ladder if working at a height.

For these reasons, you should wear appropriate safety equipment such as a helmet and safety gloves (when applicable) during the course of construction.

If you are unsure of anything, however, you should hire a professional.

Potential Problems and Pitfalls

Whether you build a rear extension by yourself or hire contractors to do so for you, there are a variety of disadvantages related to having a rear extension built.

Cons associated with rear extensions:

  • Expensive.
  • Particularly disruptive work (you may decide to live elsewhere during construction).
  • May render your back garden virtually non-existent.

Beyond that, there are a variety of repairs and maintenance that a rear extension will need.

To have the roof or part of the roof (e.g. individual roof slates) repaired will likely cost anywhere from £300 to about £1000 depending on the scale of the repairs needed.

In terms of maintenance, should you need to have the exteriors of your extension painted, you can expect to pay about £160 a day.

There are many ways in which you may need to maintain your extension over time.

For any gutters, you should regularly clean them, secure them if they become loose and seal any leaks that may arise.

To hire a professional to clean your guttering will likely cost between £50 and £150.

To repair or replace guttering may cost anywhere from £25 to £75 per m2 depending on the type.

In terms of exterior wall maintenance, you’ll need to remove dust and dirt that accumulates with time and clean/remove stains.

You can expect a bill of about £19 to £26 to have a professional perform this work for you.

You will need a party wall agreement with your neighbour should the extension involve the addition of steel supports, the digging of fresh foundations or a damp proof course.

Where required, to hire a party wall surveyor will come with a price tag of about £220 to £250 per hour.

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Sam J

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