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Extension budgets 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.
Building an extension 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 extension 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 extension cost and 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.
Here, we look at the key factors that will influence your extension budget, plus a collection of great real-life projects, so you can figure out whether your design ideas match up with how much you have to spend.
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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.
There are practical considerations that may seem simple, but could hold up a project significantly and add on costs if not carefully thought out. “Drainage is a constraint that can catch people out, particularly if you have a main drain running along the back of your house where you plan to put the extension, or a manhole,” says Caroline Shortt, director of RIBA-chartered practice Barc Architects.
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. Photo: David Barbour
If you do have a drain where your extension is, find out if it’s shared with other houses and whether it’s privately owned or by the local water authority. “Do your research early on to find whether you’ll need to relocate the manhole or if you can potentially build over it. Make sure you carry out a decent initial survey to highlight potential issues.”
The height of your extension may also affect the first-floor windows if you live in a property with more than one storey. “Can these be moved, and if not, will you get enough ceiling height in your new space? This can be surprisingly difficult to achieve,” says David. Getting a design professional on board can help you address these potential obstacles well in advance.
A standard-spec single-storey extension might start at around £2,100 to £2,700 per m² – based on a main contractor route, where a general building firm manages the project to completion on your behalf, using a standard contract. You can bring prices down by taking on more of the responsibility yourself. Here are typical indicative savings for other popular build routes:
You could potentially reduce build costs by circa 10% by hiring a main contractor to complete the structure to watertight stage. At this point you take over from the main contractor as a project manager and the remaining work is undertaken by subcontractors (individual trades), whom you manage through to project completion.
Rhodri and Meinir Jones took a DIY approach to transforming their tiny one-bedroom barn conversion into a light-filled family home with a sympathetic extension. Rhodri chose structural insulated panels (SIPs) as the build method for the extension, opting for a package from SIPS@Clays. Photo: Pete Helme
By project managing the entire scheme yourself, including the main structural phase, you could knock up to 20% off total build costs. This route doesn’t involve undertaking any construction works yourself, but rather fully managing the subcontractors on a DIY basis. So you are both client and building contractor, hiring trades and supplying plant, machinery, tools and most of the materials. You will need to be confident that you can keep the works on schedule to meet your budget.
Many will ask themselves ‘can I build my own extension?’ and the answer is yes, of course. Undertaking a large proportion of the build on a DIY basis could enable you to reduce project costs by as much as a quarter. This route assumes you’ll use trades for the key structural and infrastructure works, but will carry out much of the second fix tasks, landscaping, general labouring, decorating, tiling etc yourself. You will also be project manager, buying most of the materials and supplying all tools, plant, scaffold etc.
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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.
Each type of house extension comes with its own advantages, design considerations and price range. Understanding the intrinsic benefits of the different extension types can help you shape a project that balances practicality, ambition and budget. Here’s what you can expect to pay for the most popular types of home extension, plus what they tend to involve (indicative figures are based on core construction costs plus a standard fit-out).
Often built at the rear of a property (though they can also be positioned at the side or front), a single-storey addition works well if your goal is to create a generous open-plan kitchen-diner or family room with strong visual and physical links to the garden. Single-storey extensions are also extremely flexible from a design perspective.
A standard-spec single-storey extension might start at around £2,100 to £2,700 per m², rising to £3,200+ per m² if you choose higher-end materials, such as premium glazing systems and roofing finishes.
Extending into the side-return is one of the most popular solutions for period terraced and semi-detached homes. It involves infilling the narrow strip of land alongside the original rear projection of the house – a space that is often underused or simply acts as an access path to the garden. By widening an existing kitchen or dining room, it becomes possible to create space for features such as a kitchen island, dedicated dining zone or relaxed seating area.
Usually built as single-storey structures, side-returns fall within similar price bands – but the compact footprint may add a small uplift. Budget from £2,300 to £2,900 per m² for a standard spec.
The large glazed pivot door in this side return extension by Bradley Van Der Straeten Architects is just one of the focal points bringing design flair to this home. Inside, a curved edge reaches up from the original ceiling to meet a large rooflight over the side addition, while playfully laid recycled bricks bring pattern and interest to the external elevation. Photo: French + Tye
A wraparound extension combines a rear addition with a side infill to form an L-shaped volume, which could create a large open area or be divided, depending on your requirements. Because the structure turns around the corner of the house, it can add substantial floor area without pushing too far into the main garden.
Construction costs per m² are broadly similar to those for other types of single-storey extension, generally starting from around £2,100 to £2,700 per m², depending on specification.
By stacking additional accommodation above a ground-floor extension, you effectively double the amount of floor area created without increasing the building footprint – and with efficiencies in project costs, too.
A two-storey extension can transform how the entire house functions. For instance, the ground floor might accommodate a new kitchen-diner or family space, while the upper level provides extra bedrooms, bathrooms or a home office. Careful glazing design, plus attention to circulation routes and ceiling heights, can help to avoid creating awkward spaces.
Because the same foundations and roof structure serve two levels, two-storey extensions can offer better value per m² of space added, with prices typically starting at around £1,800 to £2,400 per m², rising to £2,600+ per m² for higher-quality finishes.
A clever angled extension has brought a new, open feeling to this townhouse in London. Fraher were behind the design, which involved a full internal restructure. The staircase has been relocated to the centre of the building to allow for maximum floor space and improved, dynamic living areas. Photo: Adam Scott
If you are completing your extension under permitted development, applying to the council for a certificate of lawful development (£264 – 50% of the planning application fee) can be a good move, as it confirms in writing that your proposal meets the rules.
If planning permission is required for your extension, you will need to pay £528 for a full householder application plus £70.83 + VAT for the Planning Portal’s service fee.
Where outside space is limited or planning constraints restrict above-ground additions – such as in dense urban areas – a basement conversion or extension can provide an alternative route to gaining extra living area. Construction methods vary depending on the project. Some basements are created by underpinning the existing structure and excavating beneath it, while others extend beyond the original footprint beneath the garden, often incorporating rooflights or sunken courtyards to bring in natural light.
A basement conversion might start from around £1,000 per m², while a full new basement extension typically begins at around £3,000 per m². These indicative figures are to waterproofed shell and floor screed only. You’ll need to add fit-out costs on top.
A loft conversion can be one of the quickest and least disruptive ways to increase living space – homeowners can typically add around 30% more usable floor area. The simplest projects involve converting an existing roof space where there is sufficient headroom – ideally at least 2.3m from floor to ridge.
A straightforward rooflight loft conversion might start from around £1,800 per m²; with dormer loft conversions beginning at £2,000 per m². More complex alterations such as hip-to-gable or mansard extensions start at approximately £2,550 per m².
John and Susan Yates have completely transformed this traditional-style bungalow in Scotland with a modern, light-filled zinc-clad extension and brand new interior scheme. The full renovation and extension cost £227,500 (£1,507 per m²). Photo: David Barbour
Case study Detached 1930s house upgraded with a single-storey rear extension
Krystelle and Ed Floyd-Wass have sympathetically renovated and extended their 1930s detached house, located in south east London breathing life back into it with bold colour and texture.
The couple engaged YellowDoor Architecture to help them develop a scheme for the extension, which also involved a full-house renovation. “We realised that, if we were going to add an extension, it made sense to complete everything at once,” says Ed. “A large, sociable kitchen diner with an island was important to us, as we enjoy cooking and entertaining, so knocking through just didn’t give us the type of space we wanted.”
Photos: Adelina Iliev
Krystelle and Ed funded the project privately and were determined to stick to their budget. As with most renovation and extension projects, there were unforeseen issues and additional expenses, including introducing a new water main and drainage to the site. Ed and Krystelle ordered in many of the materials themselves, from patio slabs to light switches, with Helen keen that the couple should be consulted on every detail.
Photos: Adelina Iliev
The newly extended kitchen is now the most well-used space in the house, and features yellow-painted structural steel beams, which make a bold statement. “We were thinking about boxing in the steels, but Helen convinced us to take a chance and we’re so glad we did – everyone comments on the colour,” says Krystelle.
So, how much did the house extension cost? The renovation and single-storey rear extension cost the couple £260,000 (£1,444 per m²).
| Demolition & alterations | £45,000 (£250 per m²) |
| Windows & doors | £34,000 (£189 per m²) |
| Electrics | £12,000 (£67 per m²) |
| Drainage & external works | £6,000 (£33 per m²) |
| Kitchen | £20,000 (£111 per m²) |
| Extension steel frame | £14,000 (£78 per m²) |
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.
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
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.
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.
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.
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.
Emil Eve Architects upgraded this five-storey Edwardian terraced house with a breathtaking, double-height kitchen-diner extension. The fixed glazing and sliding doors were supplied by IDSystems. Photo: Taran Wilkhu
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.
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.
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.
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.
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.
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.
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.
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.
To help you get a sense of what size and quality of extension you could achieve for your money, take a look at this collection of real-life home extension projects and how much they cost:
John and Melanie Stott searched for some time for the perfect long-term home in Edinburgh for their young family. They eventually found it in the form of a three-storey, six-bed late Victorian terraced house. The couple worked with Somner Macdonald Architects to develop the open-plan, energy-efficient scheme they sought.
Photo: David Barbour
The final scheme added 20m² of space to the back of the house by replacing an existing stone annexe and connecting it to the house. The stone from the existing structure was carefully removed and reused so that it resembles the original annexe, except better insulated and waterproofed. The renovation and extension cost £320,000 (£1,245 per m²) and took 36 weeks to complete.
Interior designer Alice Constable Maxwell unleashed her keen eye for colour on a dilapidated Victorian home, transforming it into the perfect family dwelling. The home has been upgraded with a dormer loft conversion and a light-filled rear kitchen-diner extension with colourful glazed features and a charming interior scheme. The renovation and extension project cost £320,000 in total (£1,916 per m²).
Photo: Simon Maxwell
Typical for houses of the era, this beautiful 1920s Arts & Crafts home was a little dark inside. So when owners Paul and Karen Ayton needed to replace the old boiler room adjoining the back of the property, they took the opportunity to extend their kitchen by creating a garden room that was perfect for indoor/outdoor entertaining. Oakwrights‘ green oak frame extension was chosen for its instant charm. The project in full cost them £140,000.
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Frank and Paloma Gilks viewed around 30 properties before they found the perfect fit for their next renovation project; this dilapidated Victorian house in west London. The couple set out to transform the property with a side return extension and loft conversion, maximising space and light throughout.
Photo: Simon Maxwell
The newly formed attic space is now an elegant office, complete with an exposed brick chimney breast and white ceiling beams. To the rear, the side return extension houses an open-plan kitchen-diner with a simple timber kitchen and lean-to rooflights that fill the space with natural light. The whole renovation and extension cost £212,446 (£1,650 per m²) for the structural work and internal fit-out.
Ed and Nicola Dolman lived in a Victorian mid-terrace for many years, but were desperate for more space. So, when the couple found the perfect do-up project in the form of a 1970s brick home, they transformed it with a contemporary and spacious timber-clad extension.
The couple worked with LA Hally Architect on the scheme. The redesign included turning the old balcony into a double-storey extension, creating a garage and office on the ground floor and a new master bedroom above. The initial scheme was much larger, so it was scaled back to be more affordable, focusing on function and aesthetics, rather than floorspace.
Photo: Simon Maxwell
It took just over a month to secure planning permission for the extension project. With the green light, the project was put out to tender, with Lesley taking on a contract administration role, guiding Ed and Nicola through the process. The full project (including extension and renovation works) cost them a total of £371,800 (£1,754 per m²).
Jamie Adams and his wife Madeline chose to add an oak frame sunroom to expand their kitchen and dining space. Welsh Oak Frame created the wow-factor addition, which is complete with charming oak trusses and a striking glazed gable that looks out over the garden. The 30m² extension cost £45,000.
Before buying their current house, Alex and Mireia Gregor-Smith had had no plans to move. They had extended and renovated their previous home, where their daughters Marina and Sofia were born, and were very happy with it. But one day, friends, who lived around the corner, mentioned that a property just a few doors along from them might be coming on the market soon. The couple bought the home with plans to redesign the layout and expand the floorplan.
Photo: Gilda Cevasco
After tackling various planning and construction obstacles, the couple finally finished the transformation, turning a Victorian property into a family home by reconfiguring the interior and adding a light-filled rear extension. The whole project cost the couple £229,177 (£1,549 per m²) for the renovation and extension works.
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