‘How much does an extension cost?’ is a big question that should be asked before you consider adding space to your home. The answer is rather tricky…
Before you can get a clear idea of the costs involved in building an extension, you’ll need to consider a number of variables. How big your extension is, for example, the materials you’re using, the area of glazing — every choice will affect the final price when extending, and that’s before you get into ‘soft’ costs such as surveys, architects’ and structural engineers’ fees, planning application fees and more.
However, it’s important to be able to have a grasp on how much an extension costs to decide whether this is a viable project for you. With that in mind, we’ve created an easy reference which offers some ballpark figures you can use to begin forming your budget.
While these figures may not match up perfectly with your builders’ quotes or estimates, and may differ depending on where you live in the UK, they should give you a good ‘high level’ indication of the costs involved.
How much do extensions cost?
In short, building an extension is likely to cost around £1,500-£2,250/m² of new internal space (excluding VAT). So a 30m² kitchen extension could be estimated at somewhere between £45,500-£67,500, plus, VAT at 20%.
If that sounds rather 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 £1,500-£2,250 price, however, is a good range of pricing based on averages.
And the good news is that there is huge scope to alter the overall cost significantly for the same-sized extension, depending on the choices you make for every element of the design, build and finish. Remember to factor in premiums for those living in higher-cost areas, such as Yorkshire & Humber, which is currently higher than London, and where trades can attract a premium.
These figures do exclude VAT, so unless your tradesperson turns over less than the VAT threshold, there will be a VAT charge at 20% on all labour and work carried out on an existing home. Materials will also attract a 20% VAT charge, too.
How do you calculate extension costs?
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Use the estimated costs per square metre, plus the extra fees to calculate how much an extension will cost. You can also use our extension cost calculator to get a more detailed estimate of how much your extension will cost.
Once you have this figure, more specific budgeting will assist to achieve a more accurate price, such as price per square metre of the interior fit out materials — like underfloor heating or flooring type, or windows (the price of timber windows is higher than uPVC, say). To give you an idea of how costs might breakdown, using an example of a 30m2 single storey extension for someone with a budget of £50,000 , the key costs might be:
- Structure (foundations, walls and roof) £20,000
- Plumbing and heating £3,500
- Electrics and lighting £3,500
- Flooring £1,500
- Kitchen units and worktops £14,000
- Decorating £2,000
- Bifold doors £3,500
- Landscaping works/making good £2,000.
For all extensions, you also need to allow for the cost of knocking through into the existing house and making good the connection, plus any other alterations or updates to improve the existing property, from rewiring and redecorating, to adding insulation or new doors and windows.
Also, consider if you will be living inside the house during the building work, or if you need to rent a house elsewhere. These costs should all be carefully considered from the start.
Use an extension planner to work out if living on site is for you, and to follow a plan to work out what materials you will need to budget for.
Single storey extension costs
If you’re planning on building a fairly straightforward, box-shaped, then allow for around £1,500-£2,500/m² (excluding VAT). Do bear in mind that this price will fluctuate depending on where you are in the UK, your build route (a main contractor or self-managed subcontractors, or DIY), the standard of build you opt for and the complexity of the design.
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 single storey extension ideas. That makes an 8x4m kitchen extension a £43,000-£80,000 project.
- Standard quality: £1,500 to £1,750/m²
- Good quality: £1,700 to £2,100/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.
Two storey extension costs
A two storey extension will often cost slightly less per m² than a single storey extension at around £1,500-£2,450 (excluding VAT). 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, but these elements will cost largely the same no matter which you choose. In short, the bigger the project, the lower the price per metre squared, in theory.
- Standard quality: £1,500 to £1,700/m²
- Good quality: £1,650 to £2,050/m²
- Excellent quality: £1,750 to £2,450/m² or more.
Side extension costs
The cost of your side extension depends largely on the size of the structure, the location of the build and the quality of the materials and finishes. An approximate figure should be around £2,250-2,500/m2 (excluding VAT).
However, Adam Knibb, director of Adam Knibb Architects suggests: “On average homeowners, should budget in the region of £3,000/m2 for a side extension.” With this in mind, alongside fluctuating figures in trades and materials, you may want to over estimate when budgeting for an extension to cushion further price rises.
The likely cost of the works will also vary depending on the type of side return extension ideas you have in mind, 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.
Conservatory extension costs
A lean-to uPVC structure sits at the lower end of the price spectrum when it comes to conservatory ideas 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 £3,500 per m2 (+VAT),” says James Underwood, a regional design consultant at Oakwrights.
He added: “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.”
Cheapest extension types
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. If you have the skills and time, a blockwork extension on a DIY basis will be the cheapest way of adding an extension.
Timber frame extensions are a popular choice, too. This might be stick built (typically cheaper as the frame is made on site, but the build time on site is longer) or prefabricated (which may or may not be an option for your home). With the latter, 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 for the superstructure element of the build (excluding foundations).
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.
Oak frame extensions are amongst the most costly design options, but their inherent beauty more than makes up the price for those that undertake them.
How the extension is managed will have an impact on the budget so it’s an area most people agonise over for a while. The build route determines who will build and manage your extension project.
Taking on the project DIY-style could see a 40% variation between what has the potential to be 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 homeowner.
FAQs
Do you pay VAT on extensions?
Building work to extend an existing house is usually subject to VAT at the standard rate, currently 20%.
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.
The construction work for some extension projects are eligible for the reduced rate of VAT, currently 5%. These include extensions as part of:
- 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.
How do builders price extensions?
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. 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. Written builders’ quotes should itemise the work to be done, provide a breakdown of costs and a total, list provisional sums, prime costs sums and exclusions, and state whether VAT is included.
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.
Quantity surveyors can produce a BQ (Bill of Quantities) for the builders to quote. This means that all of the builders are pricing the same control document in the form of a BQ. The BQ can then be used as the contract sum when you’ve selected your builder and added as an appendix into the ‘Build Contract’.
One really important thing to do when considering an extension project is to talk to a builder early on. 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.
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.
How much will interior fit-out cost?
“We would suggest you allocate around 30–40% of the budget for the interior for an average specification. It could be much higher than this if the finish is very high end and there is lots of built in technology,” advises Simon Graham.
How much will an architect cost?
Architects tend to charge fees of between 3% – 7% of the build costs, but this can be as much as 15% if they are engaged to fully project manage your extension. However, it’s worth considering using a member of CIAT (Chartered Institute of Architectural Technologists) for your plans, as they will be cheaper than a fully qualified architect, and possibly carry out lots more extensions, so they will be familiar with the dos and don’ts of your planning authority.
What other hidden costs might there be?
It’s always important to factor in a contingency to your extension budget for hidden extension costs you might not have initially considered.
Some of these can rack up before building work even starts, but hidden costs can include:
- Listed building consent
- Survey fees
- Building regulations approval fees
- Structural engineer fees
- Party wall
- Insurance
- Waste disposal
- Electrical upgrades
- Council tax
- External requirements
What will make my extension cost more?
If you are looking a how to build a budget extension, there are several other things you need to know that can affect your project’s costs. These are:
1) External finishes Just as with internal finishes, your choice of external materials will impact cost, too. A render finish will be the cheapest, with cedar and stone cladding increasing your budget. Think about ongoing maintenance costs of external finishes, brick will be maintenance free but render and timber will require periodic decoration.
2) Electrical work Every extension space will require new sockets and lights, but if your extension includes a kitchen, allow for more of both, particularly with today’s endless range of electrical appliances and potential lighting solutions. In addition, you will need a supply for a cooker and the hob, extraction fan, fridge freezer, microwave, dishwasher, coffee maker etc. You will also need a separate 32-amp circuit for the cooking facilities, either installed if you don’t already have one, or extended.
3) Plumbing Whether your new extension is for a kitchen, utility room or shower room, these areas will require a water supply and drainage at the very least. Most modern kitchens require more than this, of course.
Once you start adding in a dishwasher, possibly a separate utility section with a further sink and washing machine, or an American-style fridge freezer with an ice and water dispenser, a boiling-water tap or even a plumbed-in coffee machine, the plumbing and drainage costs rise accordingly says Mike Phillips of Darlun Design Studio.
4) Glazing Glazing costs will be impacted by the amount of glazing you opt for in the design, size of each glazed area and the materials you choose, ranging from standard uPVC windows at the lower end of the scale to huge bifold aluminium doors at the other end. You may also be considering installing Velux-type windows.
5) Flooring At the most affordable end of the scale you have the options of vinyl or laminate from around £15/m2, but opting for higher-end choices, such as hardwood, stone or polished concrete, can take the cost to over £100/m2.
6) Bespoke joinery vs off the shelf A completely bespoke finish will push your costs to the highest levels. Items that come in standard sizes will have much lower costs as they will probably be mass manufactured. Bespoke items also take time to procure and delay alone will add to your overall costs.
7) Kitchen specifications It’s true for every extension space that costs will be impacted by your choice of specification and finish. However, kitchens in general have the greatest potential for cost variations in the specification and finish element of the budget. The kitchen itself can range from under £5,000 to £60,000 and way beyond if your budget is limitless.
8) Worktops and splashbacks Worktop choices can range from a very basic laminate at around £20 – £40/m2 to granite at £150 – £400/m2, quartz at £225 – 475/m2 and Corian, which costs around £500/m2.
Splashback options again have considerably different price options, from uPVC or glass panels, stainless steel or tiling, through to using quartz etc, which at the above prices can significantly impact the overall cost of the kitchen.
9) Site-specific issues Your costs may also be impacted by things specific to your property. For example, nearby trees, which may require specialist attention.
Check for any protected trees right at the outset. Or you may need to build over a public sewer, which means you’ll incur the additional costs (utility company plus legal fees) and time delays of a ‘build over’ agreement or the cost of a redirection. You may also need to underpin existing foundations.
Will an extension add value to my home?
Before getting too wrapped up in the specifics of extension costs, 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 to add even small house extension ideas 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. You can also take a look at our guide on how to work out ‘how much is my house worth?’
For more information on building extensions, take a look at our extensions guide as well as our article covering house extension ideas.