COST GUIDES
How much does a house extension cost?
Updated 06/26
Need a bit more space in your home but don't want to move? An extension could be the answer to your problems. Find out the average costs of a house extension.
Average costs by size and quality
Additional factors to consider
Different types of house extension
Hidden costs that could occur
Furnishings and fixtures that you may need
Comparison against DIY attempts
Checklist for project plans
Frequently asked questions
How much does a house extension cost in 2026?
There’s no 'one size fits all' option when it comes to expanding your home.
However, the average cost of a house extension ranges between £30,000–£140,000 for a single storey, and £108,000–£180,000 for two storeys.
| Extension quality | Size | Price range - Low (excl. VAT) | Price range - High (excl. VAT) | Average UK cost (excl. VAT) |
|---|---|---|---|---|
| Basic extension cost | 20m² | £40,000 | £56,000 | £48,000 |
| Standard cost for small extension | 20m² | £40,000 | £56,000 | £48,000 |
| Standard cost for medium extension | 30m² | £60,000 | £84,000 | £72,000 |
| Standard cost for large extension | 50m² | £100,000 | £140,000 | £120,000 |
| Premium extension cost | 30m² | £75,000 | £110,000 | £92,500 |
| Last updated: May 2024 Our costs are ballpark averages - get a local tradesperson to quote now | ||||
There’s a huge range of options, whether you want to build above, below, to the side, or behind your property.
So that you can understand what a house extension will cost:
You can initially request an estimate
If you’re happy with that, then your estimator will draw up a final quote
Make sure you get a quote that covers the full bill of materials
Consider the value it will add to your home when deciding whether to go ahead, as well as any potential extra costs should the project overrun.
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Overall price
Average cost for a 20 m2 extension is £48,000
Extra value
A house extension may add 10–20% onto your home's value
Things to consider
You may not need planning permission if your plans fall under permitted development rights
Cost of a house extension by type
| Extension type | Unit | Cost range low - high (excl. VAT) | Average cost |
|---|---|---|---|
| Shell only extension cost | Per m² | £1,200–£1,700 | £1,450 |
| Extension cost per square metre | Per m² | £1,800–£3,000 | £2,400 |
| Two storey extension cost | Based on 60m² | £108,000–£180,000 | £144,000 |
| First floor extension cost (e.g. over a garage) | Based on 25m² | £45,000–£75,000 | £60,000 |
| Last updated: June 2026 Our costs are ballpark averages - get a local tradesperson to quote now | |||
Cost of extension shell only
A shell only extension consists of the external build only giving you a watertight foundation for future work.
Foundations
Brick or block walls
Roof
That means you’ll have no internal walls (no plastering or stud work), no utilities, no glazing and no flooring (beyond a concrete floor).
This is useful if you’d like to carry out the non-structural work (plastering, painting etc) yourself. In this case, a shell only extension is a good way to save money.
Shell extensions cost around £1,200–£1,700 per square metre (m²).
Check you have the correct legal permissions and planning permissions before starting any work. These may vary depending on your local authority.
Cost of full house extension per square metre
Unlike a shell only extension, a full extension means you’ll have a complete living space once the work has been completed.
Things that can be included as part of this work include:
Internal walls
Utilities
Flooring
Glazing
Doors
Roof
Decorations are unlikely to be included, but you’ll have a functional room all the same.
A good rule of thumb for calculating the cost of an extension per square metre is to budget £1,800–£3,000 per m².
This covers standard rooms, whilst lofts, basements, kitchens, and bathrooms cost more.
Looking for some ideas for your extension? Want to know how much value one could add? Wherever you are in the planning process or looking to hire a builder, we're here to help.
House extension prices per project
There are many different ways you can extend your home - and which you choose will have a big impact on your overall cost.
For instance, a garage conversion is likely to cost somewhere between £8.5k–£20k. Meanwhile, the average cost of a basement extension is between £100k–£160k, with some projects even rising beyond that.
Here, we'll cover some of the most common extension projects, including:
Single storey extensions
Multi-storey extensions
Semi-detached extensions
Kitchen extension
Bathroom extension
Garage conversion
Loft conversion
Basement extensions
Single storey house extension cost
Ground floor extensions are one of the easiest to build, although they still come with their fair share of challenges. These extensions are usually done at the side or rear of a building.
At £1,800–£3,000 per m², we’ve roughly calculated the costs to be:
A 20m² extension can cost £40,000–£56,000
A 30m² extension ranges from £60,000–£84,000
A 50m² extension could set you back between £100,000–£140,000
As rooms get larger there can be some cost reductions in single storey house extension costs per m², which we’ve reflected at the upper end of this scale.
Because of permitted development rights, you won't usually need planning permission for single storey or conservatory extensions.
Cost of two storey house extension
Thankfully double the storeys doesn’t mean double the price. As you are already adding the foundations and roof, you just need:
New walls
Floor joists
Interior fixtures
Other additional costs are mainly for safe working at height – such as scaffolding and some additional structural support.
If your long-term plan is to build two storeys it can be cheaper in the long run to save up the budget and build them at the same time.
A typical 60m² two-storey house extension costs £108,000–£180,000 .
First-floor extension cost
A first-floor extension - over an existing room, e.g. a garage - is typically around 25m².
The cost of a 25m² first-floor extension is normally in the region of £45,000–£75,000.
Semi-detached house extension costs
Semi-detached extensions are some of the most commonly completed extensions and carry similar costs to other extension types.
The cost of a semi-detached extension is around £1,800–£3,000 per m².
You may need to secure a party wall agreement if the work will be conducted on or near a shared wall.
What does a mid terrace house extension to the rear cost?
Due to the nature of terraced houses, the property is likely to involve a small or medium extension.
The average rear extension terraced house cost will vary, but typically, you can expect to pay in the region of £30,000–£42,000.
Typical costs you can expect to pay:
Small (15m²) rear house extension, expect £30,000–£42,000
Medium (30m²) rear house extension, expect £60,000–£84,000
Large (50m²) rear house extension, expect £100,000–£140,000
House side extension costs
Side extensions are a great way to expand an existing room, such as a living room or kitchen, to give you more space. If you have limited space, they can be used for adding smaller rooms, such as:
Home gyms
Offices
Conservatories
Orangeries
Wet rooms
You may even choose to have a wraparound extension, which consists of an L-shaped space, covering the rear and the side of the property. This is a great solution if you don’t want to take up much of your garden but want to maximise the new space.
House side extensions cost between £1,800–£3,000 per m².
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Cost of bathroom extension
They are two main types of extensions for bathrooms:
Adding a new bathroom to your home: Such as adding an en suite, or a downstairs toilet
Expanding a bathroom: Increasing the space of an existing bathroom that could be used to install a walk-in shower, extra sinks or more
Whilst costs for the space are similar to those listed above, by the time the project is complete the cost of your bathroom extension will be higher.
These are some of the most expensive extension types, due to the added complexity.
Additional plumbing
Fixtures
Furnishings
As such you should budget an extra £5,500–£9,000 to the total cost of bathroom renovations (average) and extension costs.
Kitchen extension costs
Kitchens are even more specialist than bathrooms, so the cost of your kitchen extension will be higher than a standard extension.
Extra costs to consider include:
Plumbing
Gas
Electrics
Planning to purchase a new boiler
Moving an existing boiler
As such you should add an extra £15,000–£35,000 to the average cost of your kitchen extension.
We’ve also compiled a useful guide filled with kitchen extension ideas and planning tips to help you get started.
| Room | Unit | Cost + VAT (Range low - high) |
|---|---|---|
| Bathroom extension cost | Price per bathroom | Add £5.5k–£9k to the total cost (average) |
| Kitchen extension cost | Price per kitchen | Add £15k-£35k to total cost (average) |
| Last updated: June 2026 Our costs are ballpark averages - get a local tradesperson to quote now | ||
Find a local house extension expert
Cost of garage conversion
One of the best value ways to have more practical space in your home is to convert an existing garage into a brand-new room.
You can even have a partial garage conversion, so you’ll still keep some storage space, with a stud wall in place to give you living space too.
Including materials and labour, the cost of a 15m² garage conversion is between £8.5k–£20k.
Costs will increase further if you wish to turn the garage into a bathroom or kitchen, and if the garage is not attached directly to the house – as additional plumbing and electrical work will be needed.
| Room location | Size | Cost + VAT (Range low - high) | Average cost |
|---|---|---|---|
| Garage conversion cost | 15m2 | £8.5k–£20k | £14.25k |
| Loft conversion cost | 30m2 | £50k–£95k | £72.5k |
| Basement extension cost | 40m2 | £100k–£160k | £130k |
| Last updated: June 2026 Our costs are ballpark averages - get a local tradesperson to quote now | |||
Looking for some ideas for your extension? Want to know how much value one could add? Wherever you are in the planning process or looking to hire a builder, we're here to help.
Cost of loft conversion
Wouldn’t it be great to turn the unused space in your loft into something more than a place for boxes and a water tank? With loft conversions, you can do just that.
Building a children's playroom
New bedroom
Media room
Creating a home office
Better storage space
Whilst you won’t be extending the space, the largest cost with loft conversions is making them fit for purpose and temperature-controlled living space.
There are further additional costs from working at height such as scaffolding. As such, loft conversions can cost around £50k–£95k for a 30m² space.
Check out our guide for more information on loft conversion costs.
30 inspiring loft conversion ideas to maximise your attic space
We guide you through a gallery of stunning loft conversion ideas to inspire your home renovation and make better use of that wasted attic space.
Cost to add a basement extension
Basement extensions are by far the most expensive type of extension. But they're growing in popularity in the UK, as they are an excellent way to increase property values in space-restrictive areas.
If space is at a premium, or other above-ground extension options have been exhausted, basement extensions are a valuable option.
Cost is driven by the considerable groundwork
Pipes, cables and tree roots could need moving
There may be issues with damp or water tables
These are highly specialised extensions
It is important to maintain structural integrity
There is little opportunity for DIY solutions to help reduce costs
Based on a typical 40m² space, the cost to add a basement extension is around £100k–£160k.
Rear house extension cost
The average cost of a rear house extension varies depending on the size and complexity of the design.
As a rough guide, you're looking at a house rear extension cost of £1,800–£3,000 per m².
For unusual extension build projects, complex designs or very large rear extensions, the costs will most likely be higher.
The total cost will also be affected by:
Quantity and quality of materials
Rates of individual tradespeople
Factors affecting the cost of a house extension
Constructing home extensions is a very complex task.
From building the structure (foundations, walls, roof) to making it fit for living (electrics, plumbing, plastering) there is lots to consider.
For the house extension prices in this guide, we have assumed the following:
These costs are based on average market costs, average specifications and normal construction methods
Costs are based on an extension to a normal double-storey house
No allowance for specialist equipment
The extension cost is based on average electrical and plumbing costs
Unless otherwise stated prices are based on average quality extension
No allowance for refurbishment to the existing area (this will be additional)
The costs are not to be used as pro-rata for different areas (smaller areas may represent a higher cost)
Additional assumptions are mentioned below
With these considerations in mind, let’s look at some of the biggest changes that can affect home extension costs:
The biggest cost of an extension is usually the build cost (materials and labour). Smaller, less complex extensions will cost less than those with grander aspirations.
Builders in London and the South East of the UK typically charge more than those in other locations. The costs in this guide are not location-based, so you should consider this if you live in one of these areas.
Builders will need easy access to the work site for tools and machinery. For this guide, we’ve assumed a good level of access.
Not all extensions are made equal and depending on the materials used, the complexity of the build, and the skill of your contractor you can expect to pay more or less than the market rate.
From beautiful lighting to bespoke flooring and joinery, how you’d like the room to look should be factored into the total cost of your extension. In this guide, we have mainly focused on simple designs, painting and lighting.
In addition to the structure you also need to think about the windows and doors you’ll need. If you want double glazing, bay windows or window seats then you’ll have to budget more than for standard windows.
Do I need planning permission for an extension?
Need help figuring out what planning permission is needed for your extension? Our complete guide answers all your questions.
Hidden home extension costs to watch out for
When you're budgeting for your house extension, it's important to budget for every eventuality.
Here are some hidden costs you might not have accounted for.
Planning permission
In our 2019 HomeOwners Survey, 27% of homeowners said one of the biggest barriers to undertaking home improvement projects is problems related to planning permission.
Thankfully, not all extensions need planning permission as they fall under permitted development rights.
NB. For more information on what’s required view the official Government site. You can also learn more in our planning permission cost guide.
House extension drawings
To get the extension of your dreams, or to make sure you’re completely happy with the new design before any work starts, it’s recommended you use an architect or architectural technician to draw up plans.
You and your builders will get great insights into:
Specifications
Timescales
Calculations
Prices vary based on the size and complexity of an extension but house extension drawings typically cost roughly £350–£950.
Additional costs will apply if you want them to submit a planning application.
How much does an architect cost to hire?
If you have a design in mind, a skilled architect can help to turn your idea into a detailed technical drawing complete with calculations, specifications and timescales. Naturally, work of this nature comes at a cost. If you have the budget, it’s well worth instructing a professional for large-scale projects. However, even small extensions can benefit from an architect’s touch.
Groundworks
This essential task ensures the structural integrity of your extension.
There can be additional costs depending on:
Soil type (as some types require specific approaches to building)
If drains need moving
If any tree roots need removing.
For this cost guide, we’ve assumed foundation depths are normal and there are no complications.
Insurance
An easy thing to forget, but make sure to tell your home insurance company of any planned development work before it starts.
This will likely increase your premium but will be necessary.
You want to ensure you’re covered during the development work and that your new extension is included in the policy once complete.
Clearing the space
There’s often a lot to be done to make the space fit for purpose before any work can begin.
Removing trees and shrubbery
Dismantling awnings and patios
There may even be contaminated materials that need to be removed
Depending on the extent of work here, the cost can increase. No allowance has been made for the removal of contaminated materials in this guide.
Decorating
Finally, (and we appreciate that’s a lot of things!) once you have your magnificent new structure in place, you’ll need to decorate it.
Be sure to save some of your budget for making the new space look as great on the inside as it does on the outside.
Extension painted by Sean Saunders Painting & Decorating
Cost of hiring a project manager vs DIY
Typically, a builder will add around 15–25% to their labour and materials cost if you ask them to manage your project.
You can save this sizeable cost by doing it yourself, but you'll need to be prepared to:
Direct all work on-site
Liaise with your architect
Deal with your local planning authority
Project managing also means you'll need to source the right tradespeople for the job and all of the materials needed for your house extension.
It's important to choose the right builder to work on your home, as it can make or break the success of your extension project.
How to save money on an extension
Aside from project managing the build yourself, there are a number of ways you can cut the cost of your extension without compromising on your vision.
Do some prep work for your architect
Using a professional architect is essential for any house extension, but you can reduce the amount of billable time you need from them by preparing a plan for them to work from.
There is a range of low-cost, online architecture services that enable you to add your desired extension measurements and use the software to create an accurate drawing.
This enables you to see your extension design early on in the project so that you can go to your architect with confidence about what you want your extension to look like.
Submit your planning application yourself
While the traditional route is to use an expert, you can save money on their fees by submitting your planning application yourself.
Bear in mind that there's a reason experts are often used, so be prepared to take your time and research.
Plus, one of the costliest problems faced with most planning applications is objections from neighbours, so it can be a big money-saver to spend time keeping yours fully informed and on board at all stages.
Be square
When planning your new extension, remember that a square or rectangular footprint is the cheapest.
Looking upwards, a straight-forward pitched roof is again the lowest cost option.
Reuse, recycle and repair
Before you blow your budget on brand new fixtures and fittings, think if there's anything you can reuse from your existing building, such as doors, furniture and floorboards.
Consider going to salvage yards or using online auction sites to pick up second-hand items that could save you a packet.
Looking for some ideas for your extension? Want to know how much value one could add? Wherever you are in the planning process or looking to hire a builder, we're here to help.
House extension cost checklist
Extensions can be built in any direction, meaning every structurally sound home has the potential to be expanded
The size, materials, quality and location of your extension play a big role in the cost of an extension
You can spread the cost by building an extension shell, only if you don't mind completing the work in stages
Careful planning can save you money later, such as by hiring an architect or building two storeys in one project
House extensions are highly complex and whilst money can be saved by project managing the work, this can lead to extra costs later on
House extension cost checklist
See available house extension builders covering your local area.
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FAQs
Depending on the size of the build, any difficulties with planning permission, sourcing materials and any challenging weather, it can take around 3-6 months for an extension to be built.
Delays are very common with this type of project, especially when multiple contractors are involved, so you should be prepared for it to take slightly longer than planned.
As these tend to be long projects, often you can spread the cost as key milestones are reached, rather than being lumbered with the entire cost in one go.
House extensions are complex projects, requiring a range of different disciplines and skills to be done to standard and safely. As such, it’s highly recommended the work be completed by a qualified professional.
If you can handle some of the work yourself, or project manage the work, then you’ll be able to save money on the house extension cost. But you also run the risk of a substandard extension that will cost far more to fix down the line.
It's really important to get a completion certificate from building regs to make sure the work is safe. That way when you go to sell, you won't have any issues.
Whether you should go up or out will depend on the room you want to create, the limits presented by your loft and garden, and your budget. Both have their benefits.
Going up
Going up allows you to keep valuable outdoor space and gain an extra room or two, depending on your loft space, without major building work.
Going into your loft space also opens up potentially attractive views of your surrounding area
However, it also requires you to find space for a set of stairs internally; potentially eating into landing space or even the need to use an existing bedroom for access.
Extending out
Extending out into your garden presents more freedom in what you can create and gain, enabling you to add serious square footage and impressive open-plan spaces.
It also offers the opportunity to create a real wow factor by connecting your interior to your exterior.
This varies from council to council and it's worth ringing your local planning department to get an idea. But typically a decision is made within the statutory 8 weeks for household and minor developments.
Generally, it's best to move out in order to ensure the health and comfort of members of your household. But if you don't have an option to move out, then that's okay too, tradespeople can work around you.
Moving out, however, will help the project flow smoothly as builders won't need to check in with you before they do certain tasks, like turning off the water, electricity or gas. Instead, they can just get on with the work without asking and affecting you.
That said, if it's not possible to move out, or if the extension is in an isolated area of the property, then you can ask your builders to set up a sealed work area to prevent dust and debris from contaminating the rest of the house.
Our members will be more than happy to make sure you're comfortable while they work.
In the UK, homeowners are able to extend their property without formal planning under Permitted Development (PD). Under these rights you can do the following, without the need for planning permission:
Add a single story extension at the rear of 8m on a detached property and 6m on a semi-detached or terraced property
Add a two story extension of 3m max, at least 7m from the rear boundary
Add single story side extensions, 4m high and up to half the width of the original building
More restrictions apply, be sure to check out the Government Planning Portal for a full list.
An extension will almost certainly add value to your home, although the amount will depend on what room you add.
Research by Nationwide Building Society found that an additional bathroom can add 6%, a double bedroom 12% and a double bedroom with an en-suite 23%. These figures are based on extensions on the average three-bed house in the UK.
To find a suitable tradesperson in your area and get a personalised cost to build a home extension, use our free search feature.
Whether you want to increase the size of a current room, add a new room, or even an extra floor, adding on a home extension comes with a range of benefits.
Increase your living space
Cheaper than moving to a bigger home (in most cases) – especially when considering legal fees, stamp duty and other costs from moving
That depends on what kind of extension you're hoping to undertake. A budget of 30k will generally be enough for a garage conversion, which costs on average between £8.5k–£20k.
As a rough guide, you can expect to pay around £1,800–£3,000 per m² for a house extension in the UK.
The average cost of a 20m² single storey extension is £40,000–£56,000.
From a newly qualified builder up to being a builder business owner, you can earn anywhere from £18,000 to £81,000. If you're a sole trader or limited business owner, your earning potential as a builder is much higher.
Find out what you could earn as a builder.
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