How much do house extensions cost?

The home extension costs in this article are correct as of 2019

Building an extension is a popular way for families to get the most out of their homes, giving them extra space without having to move. It’s easy to imagine all of the benefits an extension can provide - more room, added versatility for your home, a potential bump for the value of your home - but it’s hard to know before you get started what kind of costs you might be facing. We’ve spoken to some of the expert extension builders on MyBuilder.com​ to get an accurate price guide for an extension job. Here’s what you need to know.

House extension cost calculator

Extensions are complex projects that encompass a variety of different elements and trades, from laying foundations and building walls, to plastering, electrics and plumbing - and that’s before getting to decoration, or features like adding a new kitchen. A large variety of things can affect the costs from ease of access to your choice of kitchen counter. With our house extension price guide we hope to give you the basic knowledge to start your planning your project.

Below, we have broken down different elements of an extension build and the different kind of costs involved, but the key underlying cost relates to the size of the extension being built. Speaking to our experts, we have established that the basic price of building an extension is £1,100 to £1,400 per square metre. So, for a 20msq extension (4m x 5m), the cost will range from £22,000 to £28,000. These prices don’t include VAT, charged at 20%. The price bracket reflects the variance of materials used, the complexity of design, quality of finish and where in the UK you live.

This price covers the essential elements of the build, constructing the foundations, building the walls, insulating the new structure, fitting out with plasterboard, adding a subfloor, and installing a basic provision for plumbing and electrics. It will not include any of the finishing elements, such as decoration or kitchen installation, which we cover later in this article.

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Stages of a house extension project

An extension building project goes through several stages of building, which are broken out below. Establishing individual prices for each section is difficult, as many builders will not separate costs for elements such as groundworks, which are rarely done in isolation from the rest of the project. However, if you are having an extension built, many homeowners will agree to pay their tradesmen in stages, typically after some of the following elements are competed.

1- Design & planning permission costs

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You may choose to have an architect or architectural technician draw up plans for the builders to follow, rather than hire a firm to design and build your extension. Architectural drawings may come in around £500 to £1,000 depending on the scale and complexity of the work - for more information, read our guide on how much an architect costs.

Planning permission is not always necessary for an extension project, but in some cases you may need permission from your local authority, which can be around £200 depending on your circumstances, such as if you live in a conservation area. Check out our guide on planning permission.

Building regulations are the standards to which all construction projects must adhere and are essential for all building projects in the UK. Building Regulations are designed to ensure buildings are safe, structurally sound, and water and energy efficient. The cost will vary depending on the size of the build and the type of work being carried out, but will typically be between £300 and £500.

2- Groundworks

The preparing of the site and digging of foundations is essential to making sure the extension is built safely and with structural integrity. Though costs for this phase are difficult to separate out, if there are complications at this stage, such as having to move drains or deal with tree roots, it can slow down the process and increase the cost.

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3- The Shell

This is where the main building takes place, building the basic structure with either brick and block or a timber frame. A roof is built, and interior elements such as plasterboard and a subfloor are added to ensure a watertight structure. Costs here are typically related to the size of the build. Nowadays, both brick and blockwork or timber frame extensions are at a similar price point, so while they may have various advantages and drawbacks in certain situations, the main building material choice should not be too much of a cost consideration for the average extension job.

4- Fitting

Fitting out the extension with features like skirting boards, doors, electrical sockets and switches, as well as necessities like radiators. The costs of all of these will depend on whether you want to purchase high-end materials or not. Sliding or bifold doors can cost thousands of pounds per metre, while simple French doors will cost much less.

5- Finishing

There is no upper limit to what you can spend when finishing the extension - every individual element will have a wildly different cost, from cheap carpet that is a few pounds per square metre, to imported marble tiles that will costs hundreds of pounds to cover the same space. Costs will also increase significantly if you are adding a new kitchen or bathroom to the extended space. As a benchmark, a new kitchen could cost between £7,000 and £25,000, while a bathroom could cost between £3,000 and £10,000.

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How do different home extension types vary in cost?

While a price per metre is the best way to roughly work out the cost of an extension, there are a number of different types of extension which can have some bearing on the final cost.

Side extension vs rear extension costs

As the main calculation for pricing an extension is based on floorspace, whether an extension is to the rear or side of a property will not have a large impact on the overall cost of the build. There will be other factors that affect pricing depending on the design of the extension though, such as how integrated with the original building it is and what structural implications this has, how many windows and doors it has, if a new kitchen or bathroom will be in the extension, or how difficult access is.

Double vs single storey extension costs

Many sources claim that a second storey will be priced at only half or two thirds of the cost of the first floor (so if the single storey extension is 20msq and costs £22,000, the second floor could be an additional £11,000, for a total of £33,000). However, tradespeople we have spoken with tend to disagree, and say that the majority of extension builders will price a job based on the total square meterage - so a two-storey extension with both floors of 20msq floor space will be £44,000.

While it may seem intuitive that the ground floor is the more labour intensive element because of the groundwork and foundations that must be completed, there are a number of additional costs involved in a second storey that are not applicable to a single storey project. Using scaffolding, working at height, increased structural integration with the original building, changing the roof - all of these factors serve to ultimately balance out the cost.

Masonry vs timber frame extension costs

Masonry, or “brick and block” construction has been the most common way of home-building in the UK for nearly a century, but timber frame projects are becoming more and more common. Masonry construction sees the structural element built from blocks, with brick or another form of exterior cladding added around it, whereas timber frame uses wood to construct core panels that are assembled on site. Both have benefits and drawbacks in terms of speed of construction and their structural benefits, but in terms of cost, as stated above, the two methods are fairly equivalent.

Bathroom & kitchen extension costs

The two biggest additional factors that can impact the overall cost of an extension is if you plan to install a kitchen or bathroom in the new space. The price of new kitchens and bathrooms vary widely depending on your requirements and the quality of the products you choose to buy. As mentioned earlier, the typical price range for fitting a bathroom may range from £3,000 to £10,000, while a kitchen could cost anywhere from £7,000 to more than £25,000. For more detailed pricing for kitchens and bathrooms, take a look at our pricing guide for kitchens and bathrooms.

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Garden room extension costs

Garden room style extensions tend to be very design-focussed, typically single storey with an emphasis on large windows or bifold doors that allows the house to flow out into the garden. Although you can still use the basic calculation of price per square metre, there are a number of complicating factors that will impact the final price of a garden room project, such as the materials used and the desired features, such as what kind of doors will be used.

Bungalow extensions costs

As above, the main calculation for extension pricing is based on cost per square metre of floor space, so the cost for a bungalow extension can simply be considered along these lines.

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