Rear Extension Cost

Want to know how much a rear house 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 overall size.

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

Already know your spec and want to get a quote for a rear single-storey extension?

MyJobQuote can help! Enter a few of your project's details and watch the quotes roll in from builders near you.

But, if you want more information on what's available first - check out the guide below!

Extension Cost Calculator

How Much Is a Rear Extension?

How much do house extensions cost?

When planning an extension, one of the hardest things to do is come up with a budget. You can use our rear extension cost calculator above to get an estimated cost, but here's what you need to know in general:

• A single-storey extension costs between £30,000 and £40,000.
• A double-storey extension costs between £40,000 and £50,000.
• A wraparound extension costs between £50,000 and £100,000.

There are a lot of factors that can affect the rear extension costs, but the main ones are the size of the extension, the type of extension, and the finish - the latter of which can more than double the costs if you've opted for high-quality finishes throughout.

Rear Extension Prices

Average Rear Extension Prices

Let's take a closer look at the costs you can expect to pay during your extension project, starting with cost based on the type of extension first:

Single-storey extensions are the most affordable, while double-storey and wrap-around extensions require more structural work, which increases their costs.

Choosing the right extension type is based solely on your needs: if you want extra living space or a larger kitchen, a single-storey extension will be your best choice, but if you want an extra bathroom and bedroom, a double-storey extension will be better suited.

As with all home renovation jobs, the type of finish you choose will impact the price you pay, as will your location and whether or not your builders have good ease of access to the site.

Rear Extension Prices Per M²

Now, let's consider the price per m² for these different extension types:

An interesting point to note is that the cost per m² decreases with additional storeys, as the second floor shares the same foundations as the base floor and will share the same roof, reducing further costs.

This is why a double-storey extension with a loft conversion is the most cost-effective per m² when compared to a single-storey extension.

These costs include standard fittings and finishings but will fluctuate depending on your chosen style.

Rear Extension Labour Costs and Timescales

Labour costs and timescales will vary depending on the extent of the work involved in building your rear house extension. Here's a brief overview of what you can expect:

Labour Costs and Timescales Based on Type of Extension

Cost Per Hour of Different Tradesmen

Rear House Extension Costs (Supply Only)

Rear Extension Windows Cost

Rear Extension Bathroom Cost

Rear Extension Kitchen Cost

Rear Extension Lighting Cost

Rear Extension Heating Cost

Rear Extension Flooring Cost

Additional Rear Extension Costs

Unexpected costs can crop up whenever you get extensive work done on your home. 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.

Here's what you should bear in mind when calculating your house extension budget:

Structural Engineer Cost

Structural engineers differ from architects but work closely together, with engineers focusing on the stability of structures and their adherence to building regulations. Structural engineers cost from £400 to £4,000, depending on the complexity of the work, but are an integral part of the building process.

If your construction requires the use of any reinforced steel beams or shifting of load-bearing walls, you will need the services of a structural engineer.

House Rewire Cost

When you're getting your extension built, you're likely to encounter the cost of small electrical upgrades - such as adding in new sockets to your extension, adding light fixtures and switches - but, you may also come across larger, more extensive (and expensive) problems.

More electrical sockets and lights mean more electrical load - and if your electrician inspects your current system and thinks it won't be able to handle the extra demand, they may suggest you need a complete rewrite to meet current electrical safety standards.

Unfortunately, a full house rewire costs, on average, £4,000. This is most likely for older properties that haven't had their electrics touched in a long while, however, so if you've recently had work done and encountered no extra load issues, you're unlikely to need to budget for this extra cost.

On a smaller scale, if it's simply a case of your existing consumer unit not having the capacity to cater for your new electrical fittings, you may need a new consumer box. Prices for consumer units average around £500 depending on make and model.

New Boiler Cost

If your extension includes any new plumbing - such as a toilet, a full bathroom, or a brand new kitchen - you might need to invest in a new boiler if your current model is undersized for this additional demand. This might also be the case if you have an old boiler nearing the end of its lifespan.

In these cases, your heating engineer may suggest you need a new boiler fitted to mitigate these issues. To do so, the average cost to install a new boiler is between £1,500 to £2,500, depending on the efficiency of the model and the size.

Removing or Relocating Drains Cost

Depending on where the extension is located in comparison to your existing drains, you might need to remove or relocate your drains. To check if it is necessary, you may need to budget for the cost of a drain survey - £250 - so a drain specialist can advise on the best solution.

You're likely to encounter this extra cost if you're building close to existing drains or drain lines - especially if they're close to the extension foundations, as this could cause subsidence. Depending on how complex the drain layout is, and how deep the system is, you can expect costs of £200 to £1,300 to move a drain.

Underfloor Heating Cost

Underfloor heating is a luxury, but it can also be incredibly energy-efficient and space-saving if you want to keep your extension clear of heating clutter, such as radiators. The cost of underfloor heating can be anything from £800 to £7,700 depending on the type of system you choose and the scale of the area.

It can also be a cost-saving hack if your current heating system isn't adequate for your extension. If you choose a water-based system, you will still need your boiler to heat the water - but if you choose an electric system instead, you can run this entirely via electrical cables, mitigating the need to upgrade your boiler if your existing system isn't sufficient.

Adding a Kitchen Cost

Rear extensions can be modelled into new kitchens, adding more space, light, and value to your home. To do this, you'll need to add around an extra £3,000 to £10,000 to your budget to cover the cost of the kitchen installation, as well as the appliances, cabinets, doors, and worktops.

When installing a new kitchen, be mindful of your existing plumbing, heating, and electrics, and remember to factor in any necessary system upgrades if your original fittings can't handle the additional demand.

Adding a Bathroom Cost

A simple downstairs toilet costs £100 to £200 - but, if you're thinking of adding this to your rear extension, you'll need to ensure your budget allows for extra plumbing fees and any necessary drainage fees so that everything connects and works as desired.

If you're planning a larger project, such as adding an entire bathroom to your extension, you'll also need to bear these things in mind. For the cost of installing a new bathroom alone, you should add on £2,000 to £6,000 onto the price for your extension, which will cover labour costs as well as the supply of a bath shower, toilet and tiling.

Bear in mind that the bigger the project, the more likely it is that you'll need to carry out some additional plumbing work to ensure the water supply is sufficient.

Adding a Utility Room Cost

Utility rooms can be welcome additions to any home, providing a place to keep laundry and other household jobs out of the way. Depending on the exact spec of your utility room, you can expect to budget for an extra £3,500 to £7,000 for the cost of utility room installation.

These costs can increase with the volume of appliances you want added to the room. For example, if you want to include plumbing points for a sink and a washing machine, that will cost extra and calculate the additional energy load for a dryer.

Site Survey Cost

To mitigate potential problems during construction, it is a good idea to get a full survey done of the site, the land, and the existing property. Surveys are critical if you haven't updated your house in a long time or bought it without taking out a house survey and aren't sure if the previous owners had existing extensions done correctly.

The average cost of a house survey is around £600 to £1,400. Although it can seem like an unnecessary extra expense if the survey comes back showing nothing of note, it can help to save you spending on unexpected costs during the extension project - and by that time, problems are likely to be urgent and will cost more than a routine check-up would.

Architect Cost

If you're unsure where to start with your rear extension but have an idea you want someone else to bring to life, hiring an architect to produce plans can be a great way of making your ideas come to life.

Typically, the cost of an architect will make up 7 to 15% of the total cost of the extension, and to get planning drawings done usually costs between £2,500 to £3,000. Architects are particularly helpful if you're planning a multi-storey extension with lots of open space, and they can advise on the best placement of windows and doors to utilise available natural light to help make an enjoyable living space.

Planning Application and Building Regulations Cost

If you're extending your home, you're likely to need to apply for planning permission to ensure what you have in mind meets local criteria. This is also the case if you plan on changing your home's external appearance - such as the height of the roof - as this will also require permission. Planning permission costs £206 per application.

Once you've built your extension, you must pay a fee to get it certified to signify it meets building control regulations. Regulation approval applies to almost all extensions, but it is particularly important for those needing fireproofing, insulation, ventilation, plumbing, and drainage to ensure everything meets the guidelines and is compliant.

Building regulation costs usually cover several site visits and paperwork, totalling £500 to £1,000.

Insurance Cost

You should take out an insurance policy on your home if you're planning on moving out while the work is taking place. This is because your existing home insurance might not cover damage or theft while not actively living at the property.

The best solution in these cases is to take out an insurance bundle to cover the employer’s liability, public liability, and construction, which usually costs around £500 to £800.

What Impacts Rear Extension Costs?

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

Size of Extension

Unsurprisingly, the bigger your extension is, the more it will 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 wrap-around extension.

If you're considering an extension in a different part of your house - garage or loft being popular choices - your costs will differ. Loft conversions cost around £40,000, while garage conversions can be anywhere from £7,000 to £20,000 for a single garage conversion, up to £15,000 to £55,000 for a double garage conversion.

To put that into context, let's compare the cost per m² for these different conversion and extension styles:

• Single-storey rear extenstion costs are around £1,000 to £1,700 per m² for a basic finish.
• Double-storey rear extension costs range from £950 to £1,500 per m²
• Loft conversion costs per m² range from £1,000 to £3,000, depending on the conversion style.
• Garage conversion costs range from £400 to £700 per m² for an average-sized integral garage, while detached garages are closer to £1,000 to £1,600 per m².

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 those 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 you're building, the plan will vary. You can expect the work to go as detailed below for a single-storey extension - but for a two-storey extension, you should add an extra 3 to 5 weeks.

Weeks 1 to 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 to 4

Your builders will install your damp proof course and your concrete floor and will fit your drains.

Week 5

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

Week 6

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

Weeks 7 to 8

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

Week 9

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

Week 10

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

Week 11

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 12

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

Week 13

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

Week 14

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 15

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 contractors 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 of 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 wrap-around 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 to the house. The average cost for this type of extension is £30,000 to £40,000.

Pros

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

Cons

• ✖ 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 levels. This usually allows for an extra bedroom and/or bathroom, along with space for a new kitchen. The average cost for this is £40,000 to £50,000.

Pros

• ✔ Gives the most extra space

Cons

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

Wrap-Around Rear Extension Cost

A wrap-around 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 to £100,000.

Pros

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

Cons

• ✖ 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. 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 arranged. 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 brighter.

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 restrictions on the work you can get done. If you have any doubts, 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 before 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 planning 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 property.
• 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 to £150 for this.

These limits may be smaller if you live in a conservation area or a national park.

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.

Once your application is submitted, you will receive confirmation of the application. 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 to £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 comes to flooring, depending on your budget and tastes:

• Tile flooring
• Wood flooring
• Carpet flooring
• Laminate flooring
• Vinyl flooring

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 materials as the rest of the house. If you apply for planning permission, you may be able to have more choices.

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 expansive the footprint, the higher the base cost will be. Try to keep the new footprint as simple as possible. You can maximise your investment in the groundwork by building more storeys. 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 in making a decision that will keep you on track.

Alternatives to 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.

Pros

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

Cons

• ✖ 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.

Pros

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

Cons

• ✖ 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 and £70,000.

Pros

• ✔ Great views
• ✔ Contemporary style
• ✔ Bespoke

Cons

• ✖ 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 conversion 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.

Pros

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

Cons

• ✖ 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.

Pros

• ✔ Less disruption at home
• ✔ Quick installation process

Cons

• ✖ Limited design options

Checklist: Hiring Rear Extension Contractors in the UK

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 completed.
• They should be happy to let you speak to previous clients.
• Your contractor should be able to give you a breakdown 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 to do. It means that things are less likely to get forgotten about.
• Check who will be responsible for organising building control visits. Most contractors will do this, but make sure it is clearly agreed as it is your responsibility.

FAQs

Sources

https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission