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House extension cost guide
by
Checkatrade

Why add on an extension?

Congratulations! You’re about to embark on one of the most exciting building projects there is – adding an extension to your home. Whether you want to increase the size of a current room, add a new room, or even an extra floor, there are loads of great ways to increase your living space. And, very few projects are as rewarding as the result of seeing a brand-new structural addition to your home.

In most cases, extending a house is cheaper than moving to a bigger home – especially when considering legal fees, stamp duty and over costs from moving. So, if you love where you live, want to avoid the hassle or moving, or increase the value of your home, read on.

How much does it cost to build an extension on a house?

The cost of an extension can vary from £7,500 for a garage conversion to £160,000 for a basement extension, with some projects even rising beyond that. 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 per square metre you can initially request an estimate. If you’re happy with that, then your estimator will draw up a final 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.

For a breakdown of each cost we spoke to the online estimators at My Build Estimate – a professional estimating company monitored by the Royal Institute of Chartered Surveyors (RICS). This guide features the estimated home extension cost examples they provided.

Before committing to a project please seek independent costing advice.

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types of extensionsTypes of extensions

There are many different ways you can extend your home. We’ve covered some of the most common types in this guide, including:

  • Single storey extensions
  • Multi-storey extensions
  • Semi-detached extensions
  • Kitchen extension
  • Bathroom extension
  • Garage conversion
  • Loft conversion
  • Basement extensions

Factors affecting the cost of home extensions

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’s lots to consider.

For the prices in this guide, we have assumed the following:

  • These costs are based on average market costs, average specification and normal construction methods.
  • Costs are based on an extension to normal double storey house.
  • No allowance for specialist equipment.
  • The cost is based on average electrical and plumbing costs.
  • Unless otherwise stated prices are based on average quality.
  • No allowance for refurbishment to 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:

Size of extension: 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.

Location of home: 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.

Ease of access: Builders will need easy access to the work site for tools and machinery. For this guide, we’ve assumed a good level of access.

Extension qualitySizeCost + VAT (Range low - high)Average cost
Basic extension cost20m2 £25k - £50k£37.5k
Standard cost for small extension20m2 £25k - £50k£37.5k
Standard cost for medium extension30m2 £37.5k - £67.5k£52.5k
Standard cost for large extension50m2 £62.5k - £112.5k£87.5k
Premium extension cost30m2 £52.5k - £100k£76.5k

Quality: Not all extensions are made equal and depending on the materials used, complexity of the build, and the skill of your contractor you can expect to pay more or less than the market rate.

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. For more information on what’s required view the official Government site.

Architects: Using an architect helps streamline building an extension in many ways. They can come up with the perfect design that suits your requirements (saving you costs to fix it later), secure accurate quotes and aid with the planning permission process. They work on either a percentage rate (based on the total build cost) or flat fee.

Finishings and fittings: From beautiful lighting, to bespoke flooring and joinery, how you’d like the room to look should be factored into your costs. In this guide we have mainly focused on simple designs, painting and lighting.

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 and 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 to ensure you’re covered during the development work and that your new extension is included in the policy once complete.

Clearing the space: From removing tress and shrubbery to dismantling awnings and patios, there’s often a lot to be done to make the space fit for purpose before any work can begin. Depending on the extent of work here, the cost can increase. There may even be contaminated materials that need to be removed – no allowance had been made for the removal of contaminated materials in this guide.

Windows and doors: 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.

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.

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Extension typeUnitCost + VAT (Range low - high)Average cost
Extension planning drawings costN/A£350 - £950£750
Shell only extension costPer m2£750-£1,500£1,250
Extension cost per square metrePer m2£1,250 - £2,500£2,000
Two story extension costBased on 60m2£75k - £135k £105k
Semi-detached extension costPer m2£1,250 - £2,500£2,000
Side extension costPer m2£1,250 - £2,500£2,000

How much do house extension drawings cost?

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 and calculations, making everything easier once work starts.

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. For more information, including planning application costs, view our guide to architect fees.

Cost per extension type

Cost of extension shell only

A shell only extension consists of the external build only – foundations, brick or block walls and a roof – giving you a watertight foundation for future work. 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 therefore a shell only extension is a good way to save money.

This is also a good option if you’d like to do the work in stages as you get the funds available, or wish to use a specialist for the internal design.

Shell extensions cost around £750-£1,500 per square metre (m2).

Cost of full 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. So, the internal walls, utilities, flooring, glazing, doors, roof and more can all be completed as part of this work. 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 £1,250 – £2,500 per m2. This covers standard rooms, whilst lofts, basements, kitchens, and bathrooms cost more, as we explain below.

Back extensionSingle storey house extension cost

Ground floor extensions are one of the easiest to be built, although they still come with their fair share of challenges. These extensions are usually done at the side or rear of a building.

As per our £1,250 – £2,500 per m2 rule above, we’ve roughly calculated the costs per room in the following house extension cost examples:

A 20m2 extension can cost £25,000 – £50,000.

A 30m2 extension ranges from £37,500 – £67,500.

A 50m2 extension could set you back between £62,500 – £112,500.

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.

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 and 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 £75,000 – £135,000.

If new legislation comes to pass, then double-storey rear extensions could be allowed without planning permission under permitted development rights. Whilst building regulations will still need to be complied with, this could massively speed up the time to get an extension. View our guide to this new legislation.

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 biggest thing to consider is that you may need to secure a party wall agreement if the work will be conducted on or near to a shared wall.

The cost of a semi-detached extension is around £1,250 – £2,500 per m2.

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. Or, if you have limited space at the side of your home, they can be used for adding smaller rooms, such as home gyms, offices or conservatories.

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. If you don’t want to take up much of your garden but want to maximise the new space, this is a great solution.

House side extensions cost between £1,250 – £2,500 per m2.

RoomUnitCost + VAT (Range low - high)
Bathroom extension costPrice per m2 plus cost of bathroomAdd £2.5k - £5 to the total cost (average)
Kitchen extension costPrice per m2 plus cost of kitchenAdd £10k-£25k to total cost (average)

Cost of bathroom extension

They are two main types of extension 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 of additional plumbing, fixtures and furnishings. As such you should budget an extra £2,500 – £5,000 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. In addition to plumbing, you’ll need gas and electrics, plus extra costs will be incurred if you plan to purchase a new boiler or move a boiler.

As such you should add an extra £10,000-£25,000 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 locationSizeCost + VAT (Range low - high)Average cost
Garage conversion cost15m2£7.5k - £20k£13.75k
Loft conversion cost30m2£37.5k - £60k£48k
Basement extension cost40m2£100k - £160k£130k

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.

For those looking for a quicker, cheaper option that adds value to a home you can’t do better than converting a garage.

Including materials and labour, the cost of a 15m2 garage conversion is between £7,500 – £20,000. Costs will increase further if you wish to turn the garage in to a bathroom or kitchen, and if the garage is not attached directly to the house – as additional plumbing and electrical work will be needed.

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. From building a room for children to play, to a new bedroom or even an office, there are lots of ways to make the most of the space.

Whilst you won’t be extending the space, the largest cost with loft conversions is making them a fit for purpose and temperature-controlled living space. There are further additional costs from working at heights such as scaffolding. As such, loft conversions can still cost around £37,500 – £60,000 for a 30m2 space.

Check out guide for more information on loft conversion costs.

Cost to add a basement extension

By far the most expensive extension type, basement extensions are growing in popularity in the UK, as 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.

Much of their cost is driven by the considerable groundworks (including moving pipes and tree roots) and creating a watertight shell. As these are highly specialised tasks, it means there’s very little room for DIY works to bring the cost down.

Based on a typical 40m2 space, the cost to add a basement extension is around £100,000 – £160,000.

Average cost of house extensions

We’ve covered a lot of different extension types in this guide, with a range of sizes, shapes, functions and locations. As such, there’s no one size fits all option when it comes to expanding your home.

Taking all the options into consideration the average cost of house extensions ranges from £25,000 – £100,000 for single storey and £75,000-£135,000 for two storey.

How long will my extension take?

Dependant 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.

Can I add on a house extension myself?

House extensions are complex projects, requiring a range of different disciples 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 overall cost, but you also run the risk of a substandard extension that will cost far more to fix down the line.

To find a local, reputable professional in your area and get a personalised cost to build a home extension, use our free search feature.

Key takeaways

  • 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.

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