How Much Does a Rear Extension Cost in the UK?

The average rear extension cost in the UK is between £37,600 and £86,300. Prices vary the most depending on whether the rear extension has one or two floors.

However, expect to pay over £100,000 for a large rear extension which is also built to the highest level of specification. This is when the extension isn’t just about gaining extra space, but also elevating the property as a whole through bespoke architectural features.

Whether you’re looking to extend modestly on a budget, or will be undertaking an ambitious redesign of your home, the WhatPrice team have covered all of the expenses involved in a rear extension in the UK to guide you.

What is a rear extension?

In the UK, various types of property extensions exist, making it possible to extend your home from multiple angles.

However, as the name suggests, a rear extension involves adding extra living space towards the back of your property.

Traditionally, this area of the home contains the kitchen, dining or living room space. Therefore, a rear extension is usually chosen when there is a need to extend these specific rooms. Often, a rear extension is also built in a way to create freedom of movement from the kitchen into the garden.

Speaking of the garden, and the main point to note with a rear extension is that it will inevitably reduce the size of your garden. But, with the choice of a single and double storey rear extension, you also have the option to build up as well as out to get the extra space you need.

Rear vs. side and wraparound extensions

Extension type	Defined as	Typical price range	Typical uses	Benefits	Considerations
Rear extension	An extension at the back of the property, overlooking or leading out into the rear garden.	£37,600-£86,300	Creating a larger kitchen and dining room. Also, a living room or sunroom. Two storey rear extensions can be used to create additional bedrooms upstairs.	Ideal for solving the challenge of how kitchens were traditionally built to be smaller. Often a great way of flooding your home with more natural light.	Will reduce your garden space, which may be an issue for smaller gardens.
Side extension	Building between the existing wall and the side boundary.	£5,000-£55,000	Extending narrow hallways or making space for a downstairs WC.	Removes awkward nooks to create more freedom of movement. Can be used to bring in more natural light at the side of your property.	The amount of space gained is dependent on how much room there is at the side of your property to build on.
Wraparound extension	An ‘L-shaped’ extension which runs along two sides of your property.	£27,000-£100,000	Vastly extending the footprint of a property, especially if opting for a double height wraparound extension.	Offers the most potential to gain living space in multiple areas of the property. As a result, there is a significant amount of value that can be added to your home.	Often the most expensive extension type owing to the amount of living space that will be gained. Usually requires planning permission.

When you want to extend your home, there are lots of ways to do so. In terms of price, a rear extension is one of the more expensive options. Though, wraparound extensions have the potential to be most expensive, as large or high specification wraparound extensions can cost in excess of £100,000.

Side extensions, especially those which merely extend the hallway can be completed for as little as £5,000. Though, for significant space gains, this extension type can cost up to £55,000.

As a homeowner or developer, you also need to look at the available space to build on and consider what will add the most value.

For instance, a rear extension is popular as it allows for the kitchen and dining areas to be extended. The space then leads out into the garden, creating a more hybrid living experience, especially in summer.

Single storey vs. double storey rear extensions

With a rear extension, you have the option to build a single ground floor extension, or add additional space upstairs with a double storey extension.

The general price of a single storey extension in the UK ranges between £25,640 and £54,400.

However, the cost of a two storey extension in the UK ranges between £41,400 and £86,300.

Beyond the price, it’s important to consider where the additional space is needed most in your property. If this is upstairs and downstairs, then a double storey extension could be suitable. But, if you just want to increase the size of your kitchen or living room, then a single storey rear extension would suffice.

Common uses: kitchen, office, living room, etc.

Most people build a rear extension to create a larger kitchen, including having the room for a dining room table. Typically, the extension forms one large and inviting open plan space which then transitions seamlessly into the garden via bi-fold doors.

But this isn’t a set rule! Depending on the layout of your property, if you work from home or if you have a multigenerational home, you might want to use a rear extension for something else.

Some alternative ideas for single or double storey rear extensions include:

• A home office
• Accessible downstairs bedroom
• Additional upstairs bedrooms
• Additional bathroom
• Playroom
• Home gym
• Garden facing living room
• Glass box extension

If you do stick with the traditional option of extending your kitchen, our research has shown that a kitchen extension can add up to 15% to the value of your home.

How much does a rear extension cost in the UK?

Extension size	Single Storey average costs	Double storey average costs
Small 3m x 4m (18m2 )	£25,640 (£2,850 per sq/m)	£41,400 (£2,300 per sq/m)
Medium 4m x 6m (40m2)	£40,500 (£2,025 per sq/m)	£64,590 (£1,615 per sq/m)
Large 5m x 7m (60m2)	£54,400 (£1,810 per sq/m)	£86,300 (£1,440 per sq/m)

The general cost of extending your home ranges from £25,640 to £86,300 on average.

But in terms of average rear extension costs, this works out to be £1,800 to £3,000 per m² for a medium sized extension of 20-30m². Therefore, most UK homeowners pay between £37,600 and £86,300 to extend towards the back of their property.

However, expect to pay over £100,000 if a rear extension has more than one floor, known as a double storey extension.

Throughout this guide, we’ll get into exactly what impacts the costs of a rear extension.

Want a specific price based on your extension type and size?: Try our free rear extension cost calculator tool.

Rear extension cost breakdown (by component)

Rear extension feature	What might this include?	Typical price	Expected portion of budget (excluding any contingency)
Foundations and groundworks	Excavation, concrete, drainage and any piling or underpinning if required.	£5,600-£17,000	15%-20%
Structure(walls, roofing, insulation)	Brick or blockwork, timber or steel framing, roofing structure and insulation.	£9,400-£25,900	25%-30%
Windows and doors	Bi-folds, skylights, rear or side windows.	£3,700-£12,900	10%-15%
Internal fit-out (plastering,flooring and decoration)	Plasterboarding, skim coat and painting. Basic flooring such as laminate or vinyl.	£3,700-£12,900	10%-15%
Plumbing and electrics	Sockets, lighting, basic plumbing, underfloor heating (if any).	£3,000-£10,000	8%-12%
Heating and ventilation	Radiators, underfloor heating and extractor fans.	£1,900-£6,900	5%-8%
Structural elements	Steel supports for open-plan designs or knocking through to the existing house	£1,900-£8,600	5%-10%
Professional fees and surveys	Architect, planning, building regs, structural engineer and soil surveys.	£2,000-£8,000	5%-10%

Based on the average £37,600 to £86,300 rear extension cost, the above is a guide as to how the budget might be broken down.

This would give an approximate cost of £1,800 to £3,000 per m² for an extension that was 20-30m² in size.

Of course, these costs depend on many factors, including whether you opt for a single or double storey extension.

Since rear extensions can also be used to add or extend rooms, the purpose of the room will also impact the price. Our research has found that adding a kitchen (basic specification) to a rear extension can cost £5,000 to £12,000. For a new bathroom, this can cost £4,000 to £10,000.

Factors that influence rear extension costs

Curious as to why there’s a huge fluctuation in the average £37,600 to £86,300 cost of a rear extension? Or why some projects cost over £100,000?

As with every building project, there are key parameters which can either make the extension cheaper and easier to build, or more complicated and costlier. We’ll get into these now.

Location and labour costs

It’s no secret that the cost of hiring tradespeople varies significantly depending on where you are based in the UK. With London and the South East having the highest costs of living, this normally results in higher day rates for tradespeople.

When collecting quotes, research all firms that operate near you, including those willing to travel. Also, see if the cost of the job would work out cheaper or more expensive if you were to hire one firm vs lots of independent tradespeople.

Materials and structural design

Building materials, along with the overall approach to the design can be selected towards providing value for money. Or, you might decide to opt for a high specification for your rear extension, which would involve using a finer grade of materials as well as a more complex structure.

Consider not only what you’re comfortable spending, but also what makes financial sense based on property prices in your area.

Site access and ground conditions

In an ideal world, your property will provide easy access to tradespeople, including all of the equipment and materials needed to build an extension. For instance, if your property is detached and has accessible space around all four sides.

A semi detached house rear extension is the next best option. At the very least, side access will be provided for a semi detached rear extension.

However, things become more difficult with terraced properties. Ultimately, poor or limited access will slow your project down and also increase labour costs. Potential site access difficulties should be discussed early on with your builder when they come to quote your job.

Similarly, the ground conditions will also impact how easy it is to build a rear extension on your land. To avoid any nasty surprises, it’s essential to get a site survey including soil investigation early in the planning stages. Doing so can cost between £500 and £1,500, although it is an excellent investment if there are any concerns.

The presence of soil that is clay, sandy or loose can mean your extension requires deeper foundations. Also, contaminated land or tree roots can also create issues with the foundations. While basic foundations can cost £100 to £150/m², the cost can increase to £250 to £400/m² if deeper foundations are required to mitigate any issues with the ground or soil quality.

Custom vs. standard design choices

Every building project has a level of specification which is determined by how much you want to spend.

As you might expect, the more customised the design, the higher the costs will be. On a small scale, this could involve opting for luxury finishes here and there within the interior decor. But when it comes to those architecturally ambitious projects, this is where you’ll need very deep pockets to realise your vision.

Don’t begin your project until you’ve decided on a feasible budget. Remember to include a contingency for unexpected costs.

Another thing that can help here is to research the companies, including the types of extensions they create. Some companies will lean towards cheaper specifications, whereas others will specialise in high end home transformations.

Timeline: how long does a rear extension take to build?

Most single storey extensions take 2 to 4 months to build. However if you’ll be making that rear extension two storeys, the project timeline will be closer to 4 to 6 months.

Planning and design phase

Pre-build stage	How long does it usually take?
Design & drawings	2-4 weeks
Planning permission (if required)	8 weeks
Building Regulations approval	2-4 weeks
Party wall agreements (if applicable)	2-8 weeks
Contractor selection & tendering	2-4 weeks

As soon as you’ve decided you want to build a rear extension, it’s time to get straight onto any required permissions or plans. While you’re waiting for these to come through, you can also make a start on narrowing down your contractors.

Getting organised with the admin means your project won’t experience lengthy delays. In contrast, forgetting to notify the applicable people will be both frustrating and costly to sort out.

Build phase (step-by-step timeline)

Building stage	How long does it usually take?
Setting up the site & digging the foundations	1-2 weeks
Drainage & subfloor	1 week
Walls (brick/blockwork)	2-4 weeks
Steel installation (RSJs)	1 week
Roofing	1-2 weeks
Windows & doors	1 week
First fix (electrics & plumbing)	1-2 weeks
Plastering & insulation	1-2 weeks
Second fix (kitchen, bathroom & sockets)	1-2 weeks
Decoration & flooring	1-2 weeks
Final snagging & inspection	1 week

As we just touched upon, a rear extension can take between 2 to 6 months to build depending on its size and specification. However, it’s often the case that tradespeople will work around each other on site, rather than one trade necessarily having to wait for the other to finish.

Delays to watch out for

• Planning or party wall disputes
• The great British weather
• Material delays
• Design changes mid-build
• Unforeseen site challenges

Do you need planning permission or building regs?

Across the UK, planning permission can add between £230 and £624 to the cost of a rear extension.

For building regulations, applications can cost anywhere from £250 to £500. Additionally, a building notice for a single storey extension (such as a rear extension) can cost £400 to £800. If your extension also requires an inspection when complete, this can add a further £400 to the total cost.

Thankfully, planning permissions and building regulations are not always required. But, you’ll need to make your own enquiries to determine what applies in your specific case.

Permitted development rules (2025 update)

Keeping your rear extension plans within permitted development rules means planning permission won’t be required.

For a single storey rear extension, the rules state:

• The extension can be up to 3 metres deep for terraced or semi detached homes (up to 4 metres for detached homes)
• The height of the extension cannot be greater than 4 metres
• If the eaves are within 2 metres of the boundary, they must be under 3 metres
• The extension cannot cover more than 50% of the garden.

Planning permission is almost certainly required if you’ll be building a double storey rear extension. Additional rules include the following:

• The extension must not extend beyond the rear wall of the original house by more than 3 metres.
• The extension also cannot be within 7 metres of any boundary which is opposite the rear wall of the property
• The pitch of the roof must match the existing structure
• Any upper floor windows added to the side of the structure must have obscure glazing. The windows must also be non-opening unless the opening part is 1.7 metres above the floor

When you’ll need full planning permission

There are lots of scenarios when planning permission is required for an extension. But to summarise, this would be when any part of your plans does not fall under permitted development rights. If your extension will be built in a way which exceeds these limits, then yes, you will need to apply for planning permission.

Building regulations compliance

Is it your responsibility to check whether your rear extension requires building regulations compliance. In the UK, rear extensions may need to comply with Part A to Part S, depending on the scope of your plans.

Building regulations can apply to various aspects of an extension, including the structural safety, fire safety, electrical safety, ventilation, drainage and thermal performance.

Legal and neighbour considerations

When extending your home, the last thing you want is to create friction with your neighbours. By consulting your neighbours early in the process, you are less likely to experience any issues.

Likewise, while ensuring your plans receive all the required legal permissions might seem like a hassle, it is essential to protect your interests.

So make sure you’ve researched all of your legal and moral obligations before going ahead with a rear extension.

Party wall agreements explained

Party wall agreements relate to the Party Wall Act 1996. If you will be building on or close to a boundary, you will likely require an agreement. This is a formal process where you need to write for permission. It is designed to protect both parties from disputes by agreeing to any work in writing.

What’s a party wall?

• A wall that is shared by two properties (i.e. the party wall which separates a semi detached or terraced home from the neighbouring property).
• A floor or ceiling which separates two properties
• A garden wall which separates two properties (not a fence)

When do you need to serve a party wall notice?

If you are building a rear extension and any of the following apply, you’ll need to serve a party wall notice:

• Building up to or within 3 metres of the boundary wall
• Digging below the foundations near to a neighbouring property
• Building a new wall or cutting into a party wall
• Adjusting the dimensions of the party wall
• Removing chimneys from a party wall
• Demolishing or rebuilding a party wall

Unless you live on a large detached plot (i.e. do not have any neighbours nearby), the Party Wall Act 1996 is likely to apply, especially for a rear extension.

Ensure you give notice to your neighbours within the required 2-12 month timeframe prior to any work starting. You must give a full explanation of your plans in writing.

What if a party wall notice isn’t given?: Our research has found the average costs range from £5,000 to £10,000. This is because if the neighbour stops the work, the building owner is liable for all of their legal expenses. You may also face difficulties selling your property.

Boundary and overlooking issues

Your neighbours cannot stop you from building an extension. But they are likely to raise objections if your rear extension will result in a loss of light or privacy. Also, if the scale of the extension is considered overbearing.

For instance, if the windows you initially planned to add would create a privacy issue, it may be the case that you’ll need to adapt your design, such as adding rooflights rather than glazed windows.

In terms of the boundary, you can build up to the boundary, but not over it. Also, if you are doing any foundation work within 3 metres of a boundary, the Party Wall Act 1996 applies. This means you’ll need to apply for permission in writing before going ahead.

Insurance and liability concerns

You’ll need to inform your home insurance provider before going ahead with a rear extension. They may increase your premiums, both during the build itself (home renovations insurance cover), and also once the work is complete to reflect the increased build value. However, if you don’t do this, you may void your policy, especially if you plan on doing any structural work.

Also, you’ll need to ask your builder for proof of:

Builders insurance – Your builder must have public liability insurance, ideally between £2M-£5M of cover. This covers damage to third parties, including damage to your neighbour’s property or if someone is injured while on-site.

Employers’ liability insurance – If your tradespeople have any employees working for them, this insurance type is a legal requirement.

Party wall insurance – If your rear extension will be built close to a neighbouring property, you may need a party wall agreement in addition to an insurance policy which covers party wall risk.

How to protect yourself:

• Avoid traders who insist on cash-in-hand
• Ensure written contracts are provided detailing any insurance coverage
• Get a completion certificate from building control
• Keep records of all contractor details, project plans, permissions and insurance documents
• Consult your home insurance provider if you are unsure about anything

Who you’ll need: professionals for rear extensions

Rear extensions don’t build themselves! Instead, you’ll need to dedicate a sizable portion of your budget to hiring the right skills and expertise to literally get your extension off the ground.

Architect, builder, structural engineer and more

Unless opting for a very basic design, most rear extensions will need the input of professionals to design the extension and ensure its structural integrity. Architects and structural engineers are involved in the early stages to draw up plans and decide what will be feasible for your particular property.

Once any plans are approved, you will then need to appoint a builder, followed by various tradespeople to construct the rear extension and complete any interior works to make the space liveable.

Average costs for each professional

Professional you might need for a rear extension	Typical cost
Architect	£1,000-£8,000
Structural engineer	£500-£1,000
Builder + additional tradespeople	£10,000-£30,000

With the average rear extension costing between £37,600 and £86,300 to build, you should expect the majority of the costs to cover the labour.

Some tradespeople may charge by the hour, although it’s typical to be quoted a fixed fee for the work.

The tradespeople you may need include an architect, designer, structural engineer, bricklayer, joiner, electrician, plumber, plasterer, glazing specialist, kitchen fitter, tiler, floor fitter, roofer and window glazing specialist.

If you’re hiring one firm, they will have all of the specialists you need or may subcontract the work. Ultimately, the size and scope of your rear extension will determine the types of trades needed to complete the job.

DIY vs. hiring a contractor

We know that some of our WhatPrice users will be highly skilled at DIY, to the point where you could be considering building at least some of your rear extension yourself. A huge advantage of the DIY route for extensions is that you can shave some serious cash off of the total cost.

Some questions to ask yourself before attempting a DIY rear extension:

• Do you have the skills and time to take this on?
• How will you manage any legal or planning requirements?
• Can you project manage everything yourself?

Even if you’re highly skilled, you’ll likely still need to enlist the help of some professionals along the way. For instance, Part P of the building regulations requires electrician installations to meet specific standards. This is just one aspect of your extension that will need to not only be inspected, but also have certain certifications in place.

Therefore, get clear about the areas you can handle yourself, and where professional input will ultimately work out cheaper in the long run, especially where avoiding costly mistakes is concerned.

Rear extension design ideas and layout tips

In many ways, a rear extension offers you a blank canvas to use the space exactly as you wish. For some homeowners, their priority is adding usable space to areas which are currently too small to use as desired.

Alternatively, some use a rear extension as a way to add a real design statement that is just about looks as it is extending the footprint of their home.

Depending on the usage intention of your extension, you have some decisions to make about the design and layout. Also, whether you will opt for basic and functional features or something truly bespoke.

Modern vs. traditional designs

Building aspect	Traditional design	Contemporary design
Walls	Concrete blocks	Class 1 bricks or crisp rendering to the external walls
Roof	Flat or pitched roof	Flat roof with skylights or roof lanterns
Windows	uPVC double glazed	Aluminium or timber frames with double or triple glazing
Doors	French or bi-fold doors	Crittall-style steel doors. Bi-fold doors that are remote controlled. Or, corner bi-fold doors which open up most of the wall space to the outside.
Interiors	Cosy/warm	Holistic design with an abundance of natural light. Architecturally-led design choices.

Something which will impact the typical rear extension cost is its design. Traditionally, rear extensions have a square or rectangular shape and have a set of doors leading into the garden.

However, some homeowners decide to work with an architect to create something far more ambitious.

As you might expect, your budget, coupled with any local planning restrictions will determine how far you push the design of your rear extension. But, it’s good to be aware of the design possibilities, especially for those of you who aren’t looking to build something run of the mill.

Space optimisation and natural light

Quick question: Do you know the sunlight direction which applies to the rear of your property?

Knowing the answer will be crucial to designing and planning a rear extension effectively. Unless the rear of your property benefits from south facing sunlight (the strongest natural light), you may need to take extra steps to ensure your extension gets adequate natural light all year round.

Ways to increase natural light for a rear extension include:

• Clerestory windows
• Crittall-style windows
• Double-height spaces
• Large glazing
• Light reflective internal finishes
• Open plan layout
• Pale decor
• Roof lanterns
• Side return windows
• Skylights
• Velux windows

Naturally, your budget will dictate what measures you can actually implement. But for those north facing extensions especially, you’ll need to do all you can to bring in as much daylight as possible. During the winter months when the sun is at its weakest, it’s crucial that the space still feels light and inviting.

How to save money on your rear extension

While you might be looking to gain lots of extra space with a rear extension, your budget might need to remain more contained. Making smart decisions at every stage can help you save money. So here’s what to look out for if you’re looking to build frugally.

Smart material choices

Building element	More expensive	Less expensive
Walls	Traditional bricks	Concrete blocks or pre-fabricated structures
Roofing	Pitched roofing with roof tiles	Flat roof, i.e. EPDM
Windows	Aluminium or timber frames. Custom shaped windows, especially curves or arches. Triple glazed windows.	uPVC frames with double glazing. Standardised window shapes.
Doors	Bi-fold doors	French doors
Utilities	Relocating utilities	Keeping the existing locations wherever possible
Internal floors	Solid wood or luxury tiles	Laminate or vinyl flooring
Interior decoration	Venetian plaster. Custom made/high end fixtures and fittings.	Traditional plaster and paint, off-the-shelf materials used throughout.

When pricing up a rear extension, every feature you need will have a cheap and more expensive option. It’s important to decide what to splurge on or save on.

The budget-conscious among you might want to go for the less expensive option every time, or choose a hybrid approach. Just remember that your decisions will impact the specification of the rear extension, and this will ultimately be reflected in the value added to your property.

Energy-efficient upgrades that pay off

Many people think of the cost of a rear extension in terms of the upfront expense to build it. However, it’s equally important to consider the running costs of your new space. Crucially, there may be opportunities to lower your energy bills throughout the extended area and even elsewhere in your property.

For instance, many properties in the UK qualify for up to £7,500 of government funding towards the cost of a heat pump. Since heat pumps are normally installed at the rear of your property, a rear extension could be the ideal time to invest in one. Compared with a gas boiler, a heat pump can drastically lower your energy bills, especially if paired with solar panels.

In terms of the extension itself, the walls, windows and roof should also be thermally efficient.

Using larger windows on south facing walls can also help capture more sunlight during the colder winter months. In contrast, window treatments, roof overhangs or pergolas can also prevent too much sunlight from getting in during summer.

Modern LED lighting is also a more environmentally friendly option, and underfloor heating can also work to make your property more energy efficient.

Getting and comparing quotes

As we’ve touched upon above, your actual rear extension cost will very much depend on the scope of your plans. That’s why you’ll need to collect several quotes to see if your budget aligns, and also to get an idea of the current market rates.

Collecting quotes can take time. It’s not uncommon for a quote to take 2-3 weeks to come through, especially if the tradesperson has a lot of work on.

Unfortunately, you may also find that some tradespeople don’t respond to your request to come out and quote the job. This can lead to frustrating delays if this is typical for tradespeople in your area.

Overall, we recommend getting quotes early in the process so that you can narrow down the most reputable firms in your area.

FAQs about rear extensions

Still have questions about all things ‘extension back of house’? We’ve answered some of the top queries below.

For anything else related to home improvements, be sure to check out our other free guides elsewhere on the WhatPrice website!