Planning a house extension is a big job that can seem daunting, especially with so many things to think about.
There are so many decisions to make before you even start building your extension, and so many rules and regulations to follow. That’s why it’s a good idea to get a proper handle on extension costs before you dive in.
In this guide, we’ll discuss:
- How much an extension costs
- What affects the cost of building an extension
- How to save money on an extension
- What’s involved in fitting an extension
- How to find and hire a builder.
If you want to extend your home and add to your living space, but aren’t quite sure where to begin, keep reading to find out everything you need to know.
How Much Does an Extension Cost?
Extension costs vary widely depending on factors such as the size and how you intend to use it. Smaller, less complex extensions tend to be the most affordable.
Here are some common extension types and their associated costs, from budget to mid-range and to luxury, as well as the time required to complete them:
|Extension type||Budget (per sqm)||Mid-range (per sqm)||Luxury (per sqm)||Time required|
|Single-storey extension||£1,000 to £1,600||£1,700 to £2,000||£2,200 to £4,000 and up||8 to 10 weeks|
|Two-storey extension||£1,200 to £1,900||£2,000 to £2,200||£2,300 to £4,000 and up||12 to 16 weeks|
|Side return extension||£1,500 to £1,900||£1,900 to £2,200||£2,200 to £2,500||10 to 12 weeks|
|Glass extension||£1,350 to £1,950||£1,800 to £2,300||£3,000 and up||10 to 12 weeks|
|Flat pack extension||£750 to £1,440||£1,275 to £1,800||£1,650 to £3,600||1 to 2 weeks|
Please note that the estimates given above are for building the shell of the extension only. The estimates do not include the cost of any fittings and fixtures, since these are entirely of your own choosing.
However, our estimates do include VAT. This is because the vast majority of builders who undertake extensions are VAT registered. Otherwise, they would only be able to complete one or two projects per year before hitting the government’s VAT registration threshold.
Single and Double Storey Extension Costs
On average, a 30 square metre single storey extension built on a budget costs between £30,000 to £48,000. If your finances can stretch a bit more, a mid-range extension of this size usually costs between £51,000 and £60,000. These prices exclude fitting costs.
If you want to build a two-storey extension, the costs per square metre aren’t significantly higher. Builders usually quote by the square metre, not on the number of storeys.
Both single storey and two-storey extensions require foundations and a roof, so the main additional costs you’ll need to consider are scaffolding, extra material for the floor joists and walls, heating, plumbing, and electrics, as well as whatever fittings you choose.
On average, a 40 square metre two-storey extension can cost anywhere from £48,000 to £76,000 if you’re working on a budget or between £80,000 and £88,000 if your budget is more flexible.
A two-storey extension measuring 60 square metres typically costs in the range of £72,000 to £114,000 for a budget project or £120,000 to £132,000 for a mid-range build.
Where you live plays an important factor, too. If you live anywhere near London, it’s likely your extension will cost more.
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The cost of a glass extension per square metre come in at around £1,350 to £1,950 per square metre for a budget finish; mid-range will be around £1,800 to £2,300, and a luxury spec will be in excess of £3,000 per square metre.
For a typical extension, budget for around 10 to 12 weeks of labour.
Flat Pack Extension
A flat pack extension is a cost-effective route, with prices for a budget option starting at £750 to £1,140 per square metre, mid-range £1,275 to £1,800, and luxury at £1,650 to £3,600. As you’d imagine from the name, the time to build is a lot shorter, too, at just 1 to 2 weeks.
Kitchen and Bathroom Extension Costs
Planning permission needs to account for plumbing, and then the plumbing actually needs to be installed. To ensure your extension is safe and stands the test of time, it’s a good idea to get this done properly.
A kitchen will cost you more than a bathroom, adding on around £10,000 to your fee for a low- to mid-range kitchen.
In addition to the costs of building the shell of your extension, you’ll need to consider the cost of any fittings.
If you’re building a 20 square metre kitchen extension, you should allow an extra £2,600 to £6,200 on top of the building costs if you’re on a strict budget (including labour, but excluding appliances).
If your budget is higher, you could pay anywhere from £5,600 to £12,000 on top of the cost of the extension (including labour, but excluding appliances).
Bathroom extensions tend to be smaller. The average costs of fitting a new bathroom suite range from £2,750 to £7,000 for a high specification finish.
What Affects the Cost of Building an Extension?
Quite a few factors come into play when looking at the overall cost of an extension. With the price per square metre of construction so high, it’s good to know what else might ramp the costs up for your project.
Design and Planning
The cost of hiring a surveyor, structural engineer and architect will all need to be considered within your project. Getting hold of the relevant planning permission (if required), building regs approval and party wall agreements will also factor in, costing both time and money.
Extension Size, Shape, and Height
Typically, the size, shape and height of your extension will bring your costs up. The larger you go, and with more premium materials, the higher your cost will be.
Similarly, the type of building materials – brick face, timber clad, glass – will affect your price.
The groundworks, such as digging a foundation, improving drainage, or underpinning will all play into your final cost.
If you’re looking to build upon an area of uneven terrain, expect your prices to be higher than if you were building onto something existing and flat.
Trees can sometimes be even trickier than the extension itself. Many trees are protected by Tree Preservation Orders (TPOs), so make sure you take any tree into account when you are requesting planning permission.
If you remove or alter a tree without the correct permission, you could end up in big trouble.
Building Site Constraints
Do not forget to factor in additional costs for your site. If you have a complex site, you’ll need to ask your builder to factor in any extra costs.
For this, issues may include:
- Soil type that demands a specific building technique or material
- Changes to existing structure – any changes to steelwork or walls or will see additional costs
- Ease of access to the actual site (your house!) – if it’s difficult to manoeuvre tools and materials, you should plan ahead so work isn’t delayed
- If you’re moving drainage, pipework, gas meters and so on this will require extra planning and cost.
Windows and Doors
Windows and doors can be pricey and easily increase the cost of even a modest extension.
If you want to bring light but keep costs down, ask your builder whether a small bit of bespoke glazing could work in combination with standard-sized windows and doors.
Fittings and Fixtures
As briefly mentioned above with kitchen and bathroom extensions, it’s all the little extras that can sometimes add up and lead to a hefty bill.
If you’re happy with simply painted walls, carpet or engineered wood floors, and standard lightings and electronics then your costs will be relatively low.
More luxurious touches such as bespoke flooring or tiling and fitted joinery can add a unique look to your extension but also increase the costs.
Depending on the type of work you’re having done, you may need different types of tradespeople involved in your project. Builders, plumbers, electricians, heating engineers, painters and decorators – they all come with their own skill sets, and their own pricing strategies.
It’s good to note that small vs large contractors will offer very different prices. Where possible, opt for the smaller companies as they won’t have large overheads to recoup with their pricing, and will generally cost you less than the bigger national companies.
An unfortunately inescapable fact is that your location will dictate your price – living in London will ramp up your project costs considerably.
Also, remember that if you don’t have off-road parking available at your home or site, you will be expected to pay a parking permit for your contractors over the duration of their stay.
You should inform your home insurance provider of the building work, as your insurer may need to adjust your cover and premiums accordingly.
How Can I Save Money on an Extension?
You can see how costs can spiral when getting an extension added to your home, so it’s important to also know the ways in which you can bring those costs down.
The devil is in the details—so plan ahead to avoid any costly last-minute changes. If you have your architect drawings complete and finalised, it can be expensive to have them changed, so make sure you really assess things at each stage of the sign off process to make changes when it’s most easy to do so.
Similarly, if you know you want to use your extension for a certain purpose – like as a kitchen or a bathroom – it’s important everyone knows this beforehand so that certain considerations can be taken into account when building to ensure electricity, gas and water can be accessed in the space.
Don’t cut costs when it comes to the structure. Hire a surveyor and structural engineer or architect to draw up plans – you can look to save on fitting costs if you’re on a tight budget, but you shouldn’t try to scrimp on the structural integrity of your extension.
Consider a Conversion Over an Extension
Permitting this is within your allowable home development, it might be more cost-effective to undertake a conversion over an extension when looking to add space to your home.
Basements, lofts and garages are all perfect spaces to do this – so make sure you explore your options before going down one route.
Avoid Moving Gas Pipes, Plumbing, or Electrical Cables
Where possible, it’s best to avoid the need to move gas, electric or water pipes. If you’re planning a kitchen extension, it’s best to do it as sympathetically as possible, and have your fittings put in place where they make the most sense for your existing pipework.
Source Second-hand Fittings
Take advantage of online marketplaces, or choose off-the-shelf products from high street retailers over bespoke options. This will help you to save considerably on your fixtures and fittings.
Similarly, you can buy ahead during sales or percentage-off periods at shops, which will also save you money in the long-run.
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What’s Involved in Adding an Extension?
The steps will differ depending on the scope of your extension, but you can expect some, if not all, of the below to be involved when adding an extension to your home:
- Obtaining Building Regulations or Planning Permission (if required), while also checking your leasehold agreement (if applicable) to make sure you can do what you want to do
- Enlisting an architect to draw up plans, considering your intended use (kitchen, bathroom, study)
- Contacting your insurance provider to let them know of the planned work ahead of it beginning
- Obtaining parking permits if required for your contractors to ensure they have easy access to your property
- Making sure all planned work falls within permitted allowables
- Clearing the space and excavating if necessary, including the removal of trees (if not held under TPOs or in protected areas)
- Building work can begin.
How Do I Find and Hire a Builder?
When seeking out a contractor for your work, you should look to get 3 to 5 quotes for any extension to your home. That way you can get a feel for what the market rate is, as well as see who you get on with the most or feel is most capable of the job.
It’s important you don’t confuse an estimate with a quote. It’s fairly common for tradespeople, surveyors, and architects to offer an estimate the first time they visit your property. This estimate is a good starting point, but if you think they might be a good fit for the job, be sure to ask for a detailed quote so you can get a more precise idea of the costs.
It’s also worth making sure you have someone else with you when you get quotes. It can be helpful to have another opinion on price, timeframes, and personalities. What’s more, asking a family member or trusted friend to join you might help you feel less pressured to make a commitment on the spot.
When looking for contractors, you can ask your friends and relatives – even neighbours – if they’d had work done recently of a similar type and whether or not they’d recommend their professional to you.
This can help you in your search by speeding up the process if someone you trust has used someone who can do exactly what you need doing in your home.
But, if this isn’t the case, you can always look to the internet to aid your search. Using HouseholdQuotes means you can find a verified, reputable trader quickly, while being able to compare multiple quotes against each other which will save you time.
What’s more, comparing quotes using HouseholdQuotes can help to save you up to 40% off your project fee – which, when considering the expense of an extension, can really help in the long term.
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What Questions Should You Ask Them?
First things first, a written quote is an essential part of any job. You should never accept a verbal agreement as your binding terms when undertaking any work, let alone an expensive home renovation.
With a written quote, you have something firm to refer to if needs be, simply stating what is and isn’t included in your quote, as well as for what fee and under what time frames.
Just like with a job interview, it’s good to find out your contractor’s experience, and whether or not they’ve built something like the structure you’re wanting for your home extension. Even if you have a brilliant verbatim recommendation from a friend or neighbour, if the contractor isn’t familiar with what you want, they mightn’t be the best fit for you.
Similarly, anyone can write something that sounds good on a website – but the proof is in the finished project. Asking for photos or videos of the contractor’s past work can further help to ensure you’re getting exactly what you want from your trader.
You should finally always double-check your professional has insurance to cover both themselves and you in the event of any trouble while constructing. This will also help you sidestep any cowboy traders, as they likely won’t bother with insurance and will get tetchy if you ask to see proof of it.
Final Checklist and Conclusion
If the answer to your lack of space at home lies in an extension, here’s our final checklist to make sure you’ve considered everything before embarking on your project.
- Is an extension the right solution? Alternative space-making options can be loft conversions, basement/cellar conversions, as well as making use of an unloved garage. Make sure you’ve considered these avenues before settling on an extension first
- Single or double storey? Think about the functionality of the room and what you want from it. If it’s a kitchen, make sure you consider the cost of new appliances and fixtures when looking at your budget
- Make sure you have the correct building regulations before you get started on any work to ensure there are no hitches along the way – or wasted time and money
- Find a contractor using HouseholdQuotes to help you save up to 40% off your project’s fee
- Make sure you get a written quote from your trader before starting any work to safeguard you and your money
- Enjoy your newly-created space!
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Frequently Asked Questions
Do I Need Planning Permission or Building Regs Approval for My Extension?
Adding an extension to your home falls within permitted development, so it doesn’t require planning permission—as long as it meets certain criteria.
You will require building regulations to get started on your extension project, however, with rulings on doors/windows, drainage, electrics, walls and roofing, to name a new factors.
You can read more detailed advice on the Planning Portal website.
What Are Some Alternatives to Building an Extension?
If an extension is a step too far for you right now, there are alternative ways of adding some extra square footage into your property.
Instead of adding space outside, you can venture into the basement with a cellar conversion, or upwards with a loft conversion. If you have a garage that is more of a dumping ground than it is somewhere to store a vehicle, then turning that space into a useful room may be an option, too.
Could an Extension Add Value to My Home?
In short – yes. According to the insurer Hiscox, adding a bedroom could add up to 11.8% to the value of your home.
Adding a kitchen or dining room extension could increase the value of your home by up to 10.8%, while adding a bathroom could increase the value by up to 5.7% (all figures based on the average UK home value of £226,071 as priced in 2017).
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