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How Much Does an Extension Cost? Here's What you can Expect to Pay

how much does an extension cost?
(Image credit: IQ Glass)

If you're thinking about extending then you'll be asking 'how much does an extension cost?' 

The answer you get will come down to many factors and variables. So much will depend on the type of extension you want to build - single storey or two storey, loft conversion or basement - the size, the materials, the amount of glazing that'll be featured and so on. There's no one size fits all when it comes to building an extension.

Add to that the additional costs you'll need to factor in such as architects' fees, surveys, planning permission, structural engineer fees, decor and furnishings that will soon add up, too.

So how do you cost up the price of your extension? Below, we've pulled together some ballpark estimates on how much the build element for different types of extensions might cost you so that you can gauge how much to budget.

Once you’ve read our advice and got a general view of extension costs, use our extension cost calculator to get a more in-depth estimate of how much your extension will cost. 

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Before getting too wrapped up in the specifics of 'how much does an extension cost?', take time to consider whether an extension is financially viable and right for your property. 

Is this your forever home? If so, an extension is a no-brainer compared to the hassle of moving house. In a financial sense, opting for an extension means you'll save on moving costs, solicitors fees and stamp duty — costs which aren't recouped by adding value in the same way as your extension. However, consider that increasing the size of your home may move you into a higher council tax bracket.

If you are planning on selling up, whether soon or in the future, you'll need to ensure that you add more value to your home than the extension costs. Speak to local estate agents and research ceiling prices in the area before you decide to go ahead.

A Builder Can Help Identify Costs

One really important thing to do when considering an extension project is to talk to a builder (rather than a designer). The builders will be able to give you a sense of the typical solutions for houses of your type and almost certainly have carried out similar projects in the local neighbourhood. They will be able to guide you on some of the practical issues you might be worried about (e.g. drains) and help you get an initial sense of what the project might cost. 

Designers will be essential at some part of the project, but not yet. These are not individual self build projects and bar the very few totally unique extension schemes, most are simple, practical and largely templated solutions to common house types and common spatial needs. At this stage you’re trying to get a sense of the feasibility of extending from an engineering and construction perspective, and no one is better placed than a builder for that.

Factors Affecting Costs

how much does an extension cost - kitchen extension

Open up an extension, connecting inside and out, with bifold doors like these SUNFLEX SF55 Aluminium bifold doors by IDSystems. (Image credit: IDSystems)

Extension Costs to Consider

If you do decide that building an extension is the route for you, there are multiple factors that can affect your project's costs.

How much your extension costs can be affected by things like:

  • How many storeys you're going to build 
  • The size and shape of the extension
  • The quality of the build: standard, good, excellent
  • The build route you'll take —  how involved in the project you'll be
  • The amount of glazing you'd like
  • Whether the extension contains a kitchen or bathroom 

In short, most extension projects cost around £1,350-£2,250/m² of new internal space. So a 30m² kitchen extension could be estimated at somewhere between £40,500-£67,500, plus, VAT at 20%. 

If that sounds suitably general — that’s because it is. And that’s all you can expect when you have no detail or idea of what the extension looks like, or what materials you’re going to use, or how it is going to be built (and who’s going to do it). The £1,350-£2,250 price, however, is a very good range of pricing based on averages.

Single Storey Extensions

How Much Does a Single Storey Extension Cost?

a rear extension by a biophilic design architect

This glazed kitchen extension has transformed how this house works for its owners. (Image credit: Juliet Murphy )

If you're planning on building a fairly straightforward, box-shaped, single storey extension then allow for around £1,350-£1,750/m² . Do bear in mind that this price will fluctuate depending on where you are in the UK and the standard of build you opt for.

In certain parts of the South and high value areas of the UK – affected by higher cost of living – this you could easily be paying £1,700-£2,500/m², particularly on smaller schemes. That makes an 8x4m kitchen extension a £43,000-£80,000 project.

  • Basic quality: £1,350 to £1,650/m²
  • Good quality: £1,700 to £2,000/m²
  • Excellent quality: £1,800 to £2,500/m² or more.

For an excellent finish you can typically expect to pay 40% more than a standard finish.

(MORE: Single Storey Extension Ideas)

Double Storey Extensions

How Much Does a Double Storey Extension Cost?

how much does a double storey extension cost?

(Image credit: French + Tye for Bradley Van Der Straeten Architects)

A double storey extension won't cost much more per m² than a single storey extension, at around £1,250-£1,650.

That's because, aside from the extra interior fixtures and finishes, you are only adding walls and floor joists. A roof and foundations are required whether you build a single or double storey extension.

(MORE: Planning Rules for Two Storey Extensions)

  • Basic quality: £1,250 to £1,650/m²
  • Good quality: £1,650 to £2,250/m²
  • Excellent quality: £2,250 to £3,000/m² or more.

Side Extensions

How Much Does a Side Extension Cost?

a glass box side extension from IQ glass

This clever extension created using IQ Glass's structural glazing made use of the existing party wall to create an indoor-outdoors terrace in the previously unused side return.  (Image credit: IQ Glass)

Extending to the side of your house is a smart way to boost living space. Taking this route offers the chance to add width to your home’s floorplan, without necessarily eating into precious garden space at the back. 

The cost of a side extension depends largely on the size of the structure, the location of the build and the quality of the materials and finishes. 

“A basic range of costs might be £1,500 - £2,000 per m2, with smaller extensions generally having a higher per m2 cost than larger ones,” says Nicola Chambers from Pardon Chambers Architects.

The likely cost of the works will also vary depending on the scope of the internal work and any reconfiguration that’s required. For example, the cost of simply extending your living space will be very different to the cost of re-locating and plumbing in a new kitchen. 

“On average, homeowners should budget between £75,000 and £300,000 for a side extension. The upper limit for the project cost could be as much as you’re willing to pay for your dream extension. In any instance, it’s important to set a maximum budget you’re comfortable with and to allow for a contingency fund,” says Adam Knibb from Adam Knibb Architects.

(MORE: Side Extension Ideas)

Conservatories

How Much Does a Conservatory Cost to Build?

a large conservatory on a traditional home

(Image credit: Mozolowski & Murray)

A lean-to uPVC structure sits at the lower end of the price spectrum and could be achieved for around £5,000. More complex structures, such as Victorian, Edwardian, L-, P- and T-shape conservatories are more likely to cost between £10,000 and £15,000 (in PVCu)

For an oak frame conservatory, expect to cost to be somewhere between £30,000 and £40,000. “A realistic cost for a 20m2 oak frame conservatory would be £2,500 per m2 (+VAT),” says James Underwood, a regional design consultant at Oakwrights. “This will deliver an above average build. It’s worth bearing in mind, too, that this average cost will fall with larger builds as the computer design and CNC machining costs are spread over a larger footprint.”

(MORE: Conservatories)

Basements

How Much Does a Basement Cost?

Small basement extension idea

(Image credit: French + Tye c/o Paper House Project)

“The cost of the basement can depend on numerous factors, including location, soil type, party walls, type of the property, site access, local authority restrictions and, of course, the size of the existing cellar," says Stuart Braid, director at MoreSpace Basements

"The minimum cost to waterproof and prepare a cellar for decoration and finishes would be within the region of £20 - 25,000 upwards. For projects where excavation and underpinning are required to create more head height or enlarge the space, prices would start from around £3 - 4,000 per m2. Typically, costs would increase depending on the desired purpose of the new area. For instance, if a shower or utility room is required costs will go up due to the additional pumping systems required.” 

Loft Conversions

How Much Does a Loft Conversion Cost?

dormer loft conversion ideas

(Image credit: Simon Maxwell)

“An average 40m3 dormer conversion is around £35,000 for one room and this can rise to around £50,000, depending on the size of the conversion and whether or not an additional bathroom is required," says Rebecca Tibbert, director of Econoloft.  

"A substantial conversion can cost more than £80,000 and the homeowner should also factor in the costs required to decorate and furnish it. It’s worth putting away a small contingency fund in case unsuspected issues crop up. Don’t underestimate how much money you will need for décor, flooring and furnishings.”

(MORE: Loft Conversions)

Construction Systems

Construction System Versus Costs

The construction system you choose to build your extension will of course have an impact on budget. 

If you want to build an extension fairly cheaply then opt for concrete blockwork. It's a system most builders know well, too. Alternative modern methods of construction, like structural insulated panels, might cost more to begin with, but cost-savings might come from less labour time needed on site, especially when it comes to insulating your extension. 

Timber frame is another construction method you could choose. The cost of the frame, associated design work, delivery and assembly on site is charged as a package. On the plus side, this provides cost certainty.

(MORE: How to Choose a Construction System)

Exterior Finishes

Factor in the Cost of Cladding your Extension

timber cladding contemporary extension

This contemporary extension to a bungalow has been clad in timber. (Image credit: Simon Maxwell)

The way your new extension looks from the outside is just as important as the space and light you create inside. While you’ll be seeing more of the internal finishes than the exterior ones, a well-specified, well-designed and well-built extension will not only add more value but give you more pleasure than one built without beauty.

When it comes to the finish of the extension externally, it is important to understand in the first instance that the external wall of the extension is not the same as the internal supporting structure. This means, for instance, that a home can have blockwork structural walls but be finished in, say, timber or stone, as well as brick. The brick is not a structural element — it’s house cladding in the same way as everything else. So the choice is wide, and important and must be factored into the cost of your extension.

Additional Costs

Additional Costs to Factor in to Your Extension Budget

As we've discussed above, the spec of your build, the construction costs (which can be dependent on things like soil type and glazing, for example), the build route and the trades, there are other costs to consider outside of the build itself. 

Here are some example costs you might need to factor in:

  • architect fees (around 7% of construction cost)
  • structural engineer (£500-£1,250) 
  • surveys (between £700-£1,800)
  • planning permission (if needed — currently £206 in England for a two-storey extension, but check with your local authority).
  • Building Regulations applications
  • Lawful Development Certificate (currently £103)
  • fitting out the extension (costs will vary depending on how you plan on using the new space)
  • Party Wall Agreement (if needed — £1,000-£2,000 per affected neighbour for a straightforward agreement outside London).
  • insurance

(MORE: Building Regulations for Extensions)

Build Route

Your Choice of Build Route Will Affect How Much Your Extension Costs

single storey rear extension ideas

(Image credit: Simon Burt)

The build route determines who will build and manage your extension project. How the extension is managed will have an impact on the budget so it's an area most people agonise over for a while.

Taking on the project DIY-style could see a 40% variation between what is possibly the cheapest build route compared to taking on and paying for a main contractor.

There are four main build route options:

  • DIY: building on a largely DIY basis, substituting around 30% of the labour costs with DIY and employing help with the rest of the building work. Materials purchased directly
  • Self managed/subcontractor: building using tradespeople hired directly. Minimal DIY involvement. Most materials are purchased directly
  • Hiring a main contractor and subcontractors: building using a main contractor or package supplier to complete the extension to a weathertight stage, with the remaining work being undertaken by subcontractors with most materials purchased by self builder direct from suppliers
  • Hiring a main contractor: building using a main contractor. Building in this way requires the least involvement from the self builder.

Quotes vs Estimates

Be Clear on the Difference Between a Quote and an Estimate

How much should your builder be quoting for an extension project? Firstly, be very wary about anyone giving too detailed an estimate at this early stage. An estimate is no more than a guess based on limited information and effectively worthless until you go to quote stage, which is a fully-fledged cost offer based on the facts of the construction project in detail. All of the decisions you will end up making, from the kitchen to the provision (or not) of underfloor heating, from the sliding doors to the lighting, will affect costs. And if your builder doesn’t know this, the estimate is no more than a finger in the air.

Engaging with a quantity surveyor on a larger extension project could really help with costs. Quantity surveyors provide an expert view on the costs of a construction project, which could help reduce the stress of budgeting for a build.

Put simply:

An estimate is normally a contractor’s guess as to what your extension will cost. Whether given verbally, or in writing, is not legally binding and the final bill may exceed it.

A quotation is a definite price. The written quotes should itemise the work to be done, provide a breakdown of costs and a total, and state whether VAT is included.

Tips for Comparing Quotes

  • When you receive the bids, check whether there are any caveats that might involve extra expense
  • Compare provisional sums for work such as foundations to make sure you are comparing like with like

VAT

Most Extensions will be Subject to VAT

Most extensions will be subject to VAT on labour and materials at the standard rate of 20%, especially if you use a contractor to undertake the work.

If you use local tradespeople who are not VAT registered you can save the 20% VAT on their labour, but you will still have to pay VAT on materials at the standard rate.

Some extension projects are eligible for VAT relief, such as:

  • the conversion of an existing dwelling that changes the number of units
  • work to a building that has been unoccupied for at least two years

To benefit from VAT relief from the above, you must use a VAT registered builder — you can’t reclaim the VAT yourself.