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Cost of Building a Side Extension

Are you thinking of having a side extension added to your home? Want to know how much this should be priced at? In the following guide, we'll break down the average cost of constructing various types of side extensions such as single-storey, double-storey and kitchen side extensions.

10-14 weeks
Average price:

Introduction to Building a Side Extension

Whether you're thinking of building a side extension as a DIY project or intend to hire professionals to undertake the work for you, this article will prove very useful as you plan out your side extension project.

Adding a side extension to a property involves fitting new structure to a home by building outwards, extending beyond the side elevation. A side extension is often added at the back/side part of a property and involves extending the kitchen. Side extensions are popular because of the security, convenience, and efficiency they can provide.

We will now break down the costs involved in building a side extension.

How Much Does it Cost to Build a Side Extension?

It will cost approximately £40,000 to have a side extension added to your home, although this is a broad estimate. In practice, the cost will vary significantly depending on the type and size of the extension being built, as well as its quality.

First, let's consider how much it is to have a single-storey side extension constructed. In this case, for a basic 15m2 side extension it will cost about £22,500. Again, for a budget quality build, the cost rises to £30,000 for 20m2 side extension, £37,500 for 25m2, or £45,000 should it be 30m2.

Moving on to a mid-quality one-storey side extension, you can expect to pay £25,200 for a 15m2 construction, £33,600 for 20m2, £42,000 should it be 25m2, or £50,400 in the case of a 30m2 option.

As for a premium quality version, expect the total bill to be roughly £30,000 for a 15m2 extension, £40,000 for a 20m2 build, £50,000 if it is 25m2 or £60,000 for a 30m2 structure.

Moving onto double-storey side extensions, for a budget option, the side extension cost will land around £39,900 for a 30m2 project, £64,980 for a 60m2 build, or £81,000 for a 90m2 extension. Going with a mid-quality finish would set you back about £46,980 for a 30m2 extension, £70,000 for a 60m2 option, or £85,500 for 90m2.

As for a premium quality two-storey extension, it would cost around £54,900 for a 30m2 construction, £75,000 if it is 60m2 or £90,000 in the case of a 90m2 deluxe build.

For a side return extension, it will likely cost £35,000 for a 10m2 small side return extension, £52,500 for 15m2, or £70,000 should it be 20m2 in size. While a kitchen side extension will probably feature a price tag of about £22,500 for 15m2, £30,000 for 20m2, £37,500 for 25m2, or £45,000 for 30m2.

All of these estimates are broad approximations of course, and the total price will vary depending on the company/contractors you hire, among other factors. One of those other cost-affecting factors is the labour charge, which will vary from region to region. In addition, there are many different contractors you may need to hire as part of the full project, from builders to plumbers to electricians.

You'll need an appropriate tradesperson, ideally an architect, to help you plan out your extension before any work can commence. Architects tend to charge about £200 to £300 a day. Each labourer hired will cost around £50 to £150 daily.

Concrete and bricklayers usually charge about £70 to £80 a day. You'll be looking at paying around £100 to £150 to hire a window fitter, and you would likely face a similar rate to hire other contractors such as a plumber and kitchen fitter.

In general, contractors charge higher rates in the southeast of England compared with the rest of the country while, on the other hand, labourers cost less to hire in the north of England than in most places nationwide. On average, the labour cost will make up about 40-50% of the total project cost.

The overall cost will vary depending not only on the size, type and quality of the extension but also the room being extended (e.g. a bathroom or kitchen), the nature and extent of the groundworks, ease of access, and possible weather conditions.

As you can see, how much a side extension costs will depend on many different factors. Therefore, it is essential that you are aware of all of the possible costs involved, including possible extra and hidden fees.

We will now summarise the side extension prices with the four cost tables shown below.

Cost of Building a Single-storey Side Extension

Quality Size Total Cost
Budget 15m2 £22,500
20m2 £30,000
25m2 £37,500
30m2 £45,000
Medium 15m2 £25,200
20m2 £33,600
25m2 £42,000
30m2 £50,400
Premium 15m2 £30,000
20m2 £40,000
25m2 £50,000
30m2 £60,000

Cost of Building a Double-storey Side Extension

Quality Size Total Cost
Budget 30m2 £39,900
60m2 £64,980
90m2 £81,000
Medium 30m2 £46,980
60m2 £70,000
90m2 £85,500
Premium 30m2 £54,900
60m2 £75,000
90m2 £90,000

Cost of Building a Side-return Extension

Size Total Cost
10m2 £35,000
15m2 £52,500
20m2 £70,000

Cost of Building a Kitchen Side Extension

Size Total Cost
15m2 £22,500
20m2 £30,000
25m2 £37,500
30m2 £45,000

Types of Side Extension

As you can see, there are various options when it comes to choosing a side extension, including quality, size and type. In this section, we'll take a closer look at the latter, breaking down the features, pros & cons and estimated costs of each type of side extension.

This will help you narrow down your choices as you plan your side extension build. Side extensions may be entirely to the side or in part to the side of your home.

Single-storey Side Extension

Also described as a single floor side extension, this is the most popular type of side extension as well as being the most affordable, with an average cost of around £30,000 to £50,000 for a medium-sized one storey extension.

Single-storey extensions are ideal if you want to expand part of your home in order to create a roomier and functional space while avoiding the more extortionate costs of a double-storey or premium quality build. Though, you could have a premium quality single story extension constructed if you'd like, with budget and mid-quality options also existing.

Another notable advantage of a single-storey extension is that it's unlikely that planning permission will apply since more times than not, it will fall under permitted development.


✔ Least expensive

✔ Perfect for expanding a kitchen or bathroom

✔ Faster to build

✔ Usually covered by permitted development


✖ Doesn't add as much value to your home as a double-storey extension

✖ Less space than a two-storey option

Double-storey Side Extension

If you'd like a larger extension, opt for a two-storey build. With a double-storey side extension, you get more space, and of course an additional floor with the possibility of a second, alternative use for the extension. For instance, you might choose to have your kitchen expanded downstairs, but a bathroom expanded on the second floor, both as part of the same side extension.

Generally, double-storey extensions cost about £65,000 to £75,000 to build, though it will depend mainly on the structure's size and quality. Planning permission approval will be needed if you choose to go with a two-storey side extension.


✔ Larger than a single-storey extension

✔ Two floors with the option of a different use for each level

✔ Adds more to the resale value of your home

✔ A better, more standout street appearance


✖ Dearer option

✖ May bother your neighbours

✖ More complex construction with further safety concerns

✖ Not covered by permitted development

Side Return Extension

Next, we have the option of a side return extension. It involves expanding your home into the side alleyway. In most cases, a side return extension will be smaller than most with the average side return extension usually having a size of around 10-20 square metres.

This could be a single or double-storey extension, though it's a lot more likely to be the former. Plenty of period homes already have this type of extension.

It will likely cost between £35,000 and £70,000 to have a side return extension built. One of the most apparent advantages of a side return extension is that it avoids expanding into your garden. In many cases, an extension will significantly reduce the size of a back garden, if not entirely remove it.

Of course, by extending your home wholly to the side, you will block off the alleyway. Further, planning permission approval will most likely be needed.


✔ Usually not very expensive

✔ Perfect for a small extension

✔ Avoids using up garden space


✖ Blocks off the alleyway of a home

✖ Will probably require planning permission

Kitchen Side Extension

This type of extension could also fit into any of the three previous categories, although a double-storey kitchen is not especially common. You may want to expand a downstairs bathroom, in part, or fully to the side of your home, or a living room, or more than likely, a kitchen.

This is a particularly common type of side extension, and in total it would set you back about £22,500 to £45,000. It will again depend a lot on the size and quality of the extension. With an enlarged kitchen, you'll have more space for a large dining table, adding fixtures/fittings such as countertops or/and kitchen islands, as well as creating a roomier look.

Should you choose to have an extension built mostly with glass, this may significantly improve the room's brightness and tone. And of course, a kitchen side extension will also add value to your property.


✔ Perfect for creating a more spacious and functional kitchen

✔ May enhance the lighting of your kitchen

✔ More room to add kitchen fixtures/fittings


✖ Involves more interior work if new electrics, plumbing, etc. is required

What Does Building a Side Extension Involve?

1. Choosing a Design, Planning & Hiring an Architect

Before doing anything else, you need to know what type of side extension is right for you. This means taking into account any budget considerations, preferences, and anything you would deem necessary.

As shown in the previous section, the various types of side extensions each have their own advantages and disadvantages. It's imperative that you spend sufficient time making sure that you make the best choice. You may wish to consult an architect for advice, even at this stage.

However, you should do so either way for planning out the construction of your home's side extension. You could hire another appropriate professional if you would like, such as a professional designer, but an architect would be the best fit for the job.

As for the specific individual, you should find someone with plenty of experience and ideally lots of positive online reviews or references. You should also enquire about the insurances and guarantees which they may offer.

Another approach is to ask your friends or family members if they have any recommendations. For designing the side extension, it's important that you think of everything that you'd like included in its construction. So, for example, what utilities or other instalments will be required? This may include electrics or new appliances.

Whether you are trying to conjure up some stellar bungalow side extension ideas or exciting plans for a double-storey detached house side extension, brainstorming, consulting, and research are all essential aspects of planning a side extension right.

2. Legal Requirements & Side Extension Rules

You may need to submit a planning permission application in order to build a side extension. However, it will depend on whether or not your permitted development rights (PDRs) cover the work. Planning permission rules vary depending on the type of extension. Let's take a look at how permitted development applies for the various types.

Planning permission will be necessary if for any extension:

  • Half of the land area surrounding your home will be covered by the new extension in its current state.
  • The extension will end up being taller than either the highest part of your home's roof or it will be taller at its eaves than the current eaves of your property.
  • The height of the extension's eaves will reach more than 3 metres, in the scenario where the extension will be located within 2 metres of the edge of your property's cartilage.
  • The extension is going to be built beyond your home's principal elevation or its side elevation if your house faces out toward a highway.
  • You plan to include the installation of a chimney, flue pipe, veranda, raised platform, microwave antenna, balcony, soil and vent pipe, or any alteration to the existing roof.
  • Your property exists on article 2(3) designated land and cladding is planned as part of the construction.
  • The materials used for the exterior of the extension will have a distinct look from those already present on your home's exterior.

Permitted development does not cover the following for side extensions:

  • An extension with a height of 4 metres or more.
  • The construction of a double-storey side extension.
  • Will have a width that is greater than half the width of the original dwelling.
  • A side extension being built on article 2(3) designated land.

Even if your PDRs cover you, a building control officer will still need to approve and sign off on the work. There are various building regulations which cover the construction of a side extension. For instance, if you occupy a listed building, any proposed alterations will need prior approval. Beyond that, building regulations cover many other subjects, including drainage, access, electricity, fire safety, and insulation.

To start the process of complying with building control, you should contact your local council. Though it is not necessary, you may want to consider applying for a Lawful Development Certificate as this could come in handy at a future point. You can also contact the council if you have any doubts about the project's legal requirements in general.

3. Hiring Contractors

Once you are given the green light to build your side extension (if any approval was needed) you'll need to hire all relevant contractors for construction. Some tradespersons will be necessary such as labourers, bricklayers and an electrician.

However, others are optional, depending on the project at hand, including a plumber, carpenter, and kitchen fitter. Once again, you'll want to find experienced contractors with plenty of backing. If you want to have a kitchen side extension built, it may also be worth your while consulting a kitchen designer, before tying up your plans.

4. Laying the Groundwork & Construction

On the first day of practical work, the site must be cleared and prepared. Once this is complete, the foundation can be laid in a dug-up area. In most cases, a concrete slab will be fitted over several hours. Then the structure can be added to establish the outer layers of the extension. This process may involve adding a single-storey or two, depending on what option you've decided to go with.

In some cases, a side extension will be fully to the side of the house, and in other cases, it will partially exist beyond your home's side elevation. The process itself will vary depending on the material used, the size of the extension, and the quality of the build.

Once the skeletal structure has been complete, interior work can get underway, including but not limited to, the installation of appliances and fittings (e.g. range cooker, fridge, shower, fireplace, etc.), a gas boiler, and new pipework. Further interior work may be needed depending on the size and type of side extension concerned. For instance, if you've had a kitchen side extension built, you may want to have new kitchen cabinets added, the walls painted or wallpapered, or new, modern lighting fitted.

5. Clean Up and Further Work

Waste removal will be an ongoing part of the work, and it will likely make up around 5% of the total cost. As for the overall cost of having a side extension built, while we gave various estimations earlier, you will need to add any additional costs to these figures in order to know exactly how much you'll actually spend altogether. The foundations alone may cost several hundred pounds and could bring the total above an average if the work proves more difficult than expected.

If you are looking to have an entirely new kitchen installed, you can expect to pay anywhere from £5000 to £20,000 or around £400 to £500 per square metre. As for a new bathroom, this may set you back about £2500 to £7500 or roughly £700 to £900 per square metre. For a kitchen six lamp lighting grid, expect to pay just under £1000. To have four dimmed wall lights fitted will cost about £400 to £450. Whether it is a pendant, chandelier, or ceiling lamp for a single light installation, it's more likely to be about £100.

The cost of new flooring will vary significantly depending on the type chosen and the size of the area. To have a professional install wood flooring tends to cost somewhere in the range of £70 to £140 per m2. To have laminate flooring installed is likely to cost about £17 to £30 per m2 with the price of having a new bathroom or kitchen tiles fitted landing around £30 to £40 per square metre. For carpet fitting, it will probably set you back around £30 per m2, though the price will vary depending not only on the type of carpet but used but whether underlay is needed.

On the other hand, decorators would charge about £160 per day plus the cost of any supplies. To apply for planning permission costs roughly £150 to £250 while for building regulations, an application and inspection would probably cost around £200 to £250 in total.

DIY Side Extension Construction

You should only take on the bulk of work for a side extension building project if you have lots of prior experience and know precisely what the work entails. There is a lot involved in building a side extension, and any mistakes during planning or construction could prove very costly.

However, either way, there are many parts of the project that an average DIY enthusiast could certainly undertake. These tasks include much of the interior work, such as painting, decorating, and installing new flooring.

Even performing these sorts of jobs could save you hundreds, if not thousands of pounds in total. Regardless, certain aspects of building a side extension must be performed by professionals or those otherwise qualified to do so, including most electric work and gas appliance installations.

As discussed in the previous section, planning permission may apply, though it will depend on whether your proposed building project is covered by permitted development or not. Either way, building control would need to sign off on a newly built side extension, whether you've performed most of the work yourself, some of it, or none at all. Some additional work may require a professional to sign off on it too. For further information, check out Planning Portal's section on building regulations.

Like with any DIY work, building a side extension or working on its interior comes with a range of risks. Firstly, you might perform the work incorrectly. Should this happen, you'll have wasted time and still need to pay a professional to come in and finish the work for you.

However, there are also more severe risks associated with DIY work. Whether from carrying heavy loads, using sharp or otherwise dangerous equipment/tools, or dealing with harmful substances, there are many ways in which accidents can occur as part of a DIY project. For this reason, make sure you know exactly what is involved and take all of the necessary safety precautions.

Potential Problems and Pitfalls

There's no denying that having an extension built is disruptive and time-consuming work. Depending on the work's extent and nature, you may decide to move out temporarily and stay with friends or family.

There is also a danger that building a new side extension could cause issues or conflict with your neighbours, particularly if it is a large side extension. Beyond that, a side extension may block off the side alleyway of your home entirely, although this might be a preferable alternative to extending into your back garden, which would reduce its space.

If your applications for planning permission or building regulations are rejected, unfortunately, you will need to make changes to your plans and resubmit your proposals. You should receive a clear explanation as to why your application(s) were rejected.

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Sam J

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