Extension Costs 2023: What Can I Build For My Budget?

Extension costs will vary on a case-by-case basis and will depend on a whole host of factors, from the size of the project to your home’s location and how complex the works may be. However, it’ll be extremely helpful to have rough overall outline before you decide if it’s for you and whether your project goals and extension ideas can match up to the cash you have available.

Extending your home is a great option for those wanting to update and adapt their space to suit any growing family needs, without the hassle and cost of moving house. It can also add real value, potentially netting you a profit if you ever do decide to move – provided you devise a well thought-out budget and stick to it.

Factors such as the size and how open-plan you want your space to be (which will dictate any requirement for supporting steels etc) are among the biggest contributors to the total cost. According to Tamsin Bryant of ABL3 Architects, the average extension her firm works on costs around £2,500-£3,500 per m², subject to complexity and finishes. 

“Structural complexity, the types of foundations required, any existing services and whether you’re building over a public sewer will also impact on your total budget,” says Tamsin. But perhaps the biggest variable will be the specification: high-quality bespoke kitchens or top-of-the-range glazed doors don’t come cheap, but they will add real wow factor to your finished project.

Where in the country you’re building will also weigh in on the cost of your extension as labour rates differ significantly across the UK. “A side return or wraparound extension on a terraced property in London would typically cost between £80,000-£100,000 to take it to a builder’s finish,” says James Bernard, director of Plus Rooms.

Getting an Idea of Your Extension Costs

Before setting a budget, you need to know what you’re spending your money on. “It’s vital to establish your priorities, as only then can you get advice from your designer on what it could cost,” says David Blaikie of David Blaikie Architects.

A good place to start is to discuss what type of extension you want – side-return, basement, loft conversion etc – as well the types of materials, roofing, glazing and internal fixtures you prefer. From here, a ballpark price will start to emerge. It’s likely that a simple extension design will make it easier to stick to your budget once the works begin. “A good architect will identify cost-effective ways of maximising the existing building’s features,” says Leigh Bowen, principal at 50 Degrees North Architects.

This contemporary, single-storey extension was designed by Paul Archer Design in 2017 and came in at £1,671 per m². A main steel beam spans 14.5m across the entire width of the zone, supporting a spacious kitchen-diner with fabulous garden views

You’ll need to think beyond the extension, too. “Take into account the basics, including any repairs or upgrades you might need, such as rewiring or installing loft insulation,” says David. As you’ll be adding to your overall heating and hot water demand, you may need to make some changes to these systems, too.

Research key products such as rooflights or kitchen worktop materials early, so you can understand how they’ll influence your budget. “High-quality finishes always cost more than anticipated, so find out how much extra this could be in comparison to the more economic options,” says Tamsin.

One of the biggest reasons for overspending on an extension project is changing your mind during the works. It’s important to be realistic, as multiple one-off upgrades could quickly add up, potentially leading to compromises elsewhere.

Thinking about extending your home to create a brand new open-plan kitchen space? Take a look at these Kitchen Extension Ideas: 20 Inspiring Designs

Nigel and Sue Evans spent an estimated £3,600 per m² on their new steel frame extension. Designed by Capital A Architecture, the bright open-plan kitchen-diner addition replaces an old conservatory and has helped create a better connection between the home and garden area

Securing Estimates & Quotes for Your House Extension

Once you’ve got an idea of what you want from a home extension, arrange a preliminary chat with a local architect. “Architects have a good idea of construction prices as well as the extension costs for the different types of project,” says David. Whoever you end up choosing to design your scheme, make sure they understand your project’s budget and timeline.

“We’d advise you get the project cost before you put in a planning application,” says Leigh. “You’ll have the chance to value engineer your extension as you haven’t got planning permission for a fixed design yet.” The Build It Estimating Service is a great option for this – providing a detailed breakdown of materials prices, labour costs and a suggested project schedule.

In this project by James Dale Architects, the owners of the London home budgeted £300,000 for a side and rear extension to the ground floor to create an open-plan kitchen and living area, as well as a loft extension, giving them an additional guest room, home office and bathroom. Photo: Neil Perry

Alternatively, it could be worth engaging a Quantity Surveyor (QS). “Many people think this will be too expensive, but a QS can price your extension in detail,” says Leigh. The cost of this service might be difficult to justify on a standard project; but on complex extensions, a Quantity Surveyor’s insight could save you money later down the line.

As you start to agree quotes, be sure to stay organised. “I recommend setting up a spreadsheet that lists all elements of your project and the corresponding fees for specialists,” says David. Pay attention to how long the quote is valid for, too, as materials and labour prices can change quickly.

Financing an Extension Project & VAT

Having adequate funding in place is, of course, crucial to a successful project. There are numerous ways you can finance a new house extension. Some of you might be fortunate enough to have a pot of savings to rely on; others might remortgage or seek specialist home remodelling finance. “You want to make sure you can finish what you start, as there’s nothing worse than embarking on a major project, disrupting your living conditions, just to get to a point where you’ve run out of funds so you can’t actually complete it,” says James.

Another aspect to be mindful of is VAT. Quotes for materials and labour don’t always specifically set this out; sometimes there will simply be a line saying the price is subject to VAT at the relevant rate. On an extension, this will be 20% – so you can imagine how misinterpreting this could lead to inuring a budget-busting figure if you’re spending £50,000 with a main contractor!

The total contract price of this refurbishment and replacement extension in London by ABL3 Architects was £1,515 per m². Leftover materials, such as timber wall cladding, were used across various areas of the home to minimise costs. Photo: Rachel Ferriman

To Summarise: Our Tips For Staying on Top of Your Extension Costs

Identify your needs: Think carefully about your reasons for extending – are you looking to add more general living space or do you want a room with a specific purpose, such as an extra bedroom? Thoroughly considering what you want from the finished addition will help to form the foundations for coming up with a suitable brief and design.

Set a brief: Once you’ve determined your motivations and how you intend to use the new space, consider how your property will best respond to the addition. Key areas to focus on are establishing the potential ways that you could maximise space and natural light.

Work with skilled designers: While you may choose to come up with plans yourself and work with a structural engineer and a good builder to fine tune the details, an architect or experienced designer’s flair will help you get the most out of your home improvement project.

Learn More: Who Should Design My House Extension?

This contemporary extension is a real contrast to the original Georgian farmhouse. The 60m2 steel frame addition features plenty of wow factor, with floor-to-ceiling glazing and zinc cladding. The renovation and extension came in at £429,600

Assess the opportunities: When you talk it through with your designer, you’ll probably have a vision for how you think your home can be tailored. Keep an open mind and be flexible – it may even work in your favour.

Consider the rest of the house: A home extension project will add valuable new living space; but the results can be maximised if you go back to basics and consider how you really want to live in your home. The most successful schemes tend to include layout changes and renovation work elsewhere in the house, too.

Cost versus value: It can be easy to get carried away with all the exciting extension possibilities, but take a moment to step back and have a look at how much value it will add to your house. You might have different priorities if this is your forever home, but as a rule, try not to spend more on a property update than its finished value.

Investigate local house prices to get an idea of what uplift you can expect; and check out the Office for National Statistics’ handy extension value calculator to get a quick idea of the potential increase in value.

Create a project schedule: While it’s inevitable for there to be a few bumps in the road, by and large, a well-organised scheme should come together smoothly with minimal changes needed as you go along.

Protect your project: Ensuring you have the right insurance in place for your extension scheme will protect the new works and the original building from anything that could go wrong during the build.

Engage skilled builders: When you put the scheme out to tender, be sure to provide enough detail to compare quotes like-for-like. Given a basic outline, one contractor may over-specify to cover all margins, while another might quote cheaply knowing they’ll charge extras – so provide a clear and comprehensive brief. Busy builders may quote higher, but remember they’re busy for a reason.

Efficient management: Whether doing it yourself or bringing in a professional, it’s important for the project manager to keep a tight rein. It’s their job to make sure things are done to a high standard at the pace you need.

Understand the planning rules: You may not actually need to submit a formal planning application thanks to permitted development (PD) rights. But if your house is in a designated zone (such as a conservation area or an area of outstanding natural beauty) or if the property you’re proposing to extend is listed, your PD rights are restricted.

Did you know? Not all projects require formal planning permission – some can be achieved under what’s known as Permitted Development. Take a look at these 23 Projects You Can Do Without Planning Permission