How Much Does a House Extension Cost? - UK Homeowner Guide
- Harper Latter Architects
- 4 days ago
- 15 min read
So, you’re thinking about expanding your home. It’s an exciting prospect, but let’s be honest, the first question on everyone's mind is always the same: how much does a house extension cost?
In the UK, a good starting point for a basic single-storey extension is somewhere between £2,000 and £2,800 per square metre. For a small 20m² addition, you could be looking at a starting figure of around £40,000. But that’s just the beginning of the story.
Your Quick Guide to House Extension Costs
Getting a handle on the potential cost of your house extension is the first real step in turning your ideas into a project you can actually move forward with. While the price per square metre is a handy benchmark, it’s really just a ballpark figure. The final bill will be shaped by your specific choices, the complexity of the build, and even your postcode.
Think of that "cost per square metre" figure as the price for the essential structure—the walls, floor, roof, and standard windows. Everything else, from those sleek bifold doors you’ve been eyeing up to the high-end kitchen you plan to install, will add to this base cost.
Breaking Down the Averages
To give you a clearer picture, it helps to look at a few typical project sizes. For a standard 20m² single-storey extension, most homeowners can expect to pay between £40,000 and £56,000. As you’d imagine, this figure climbs with size; a 30m² extension typically ranges from £60,000 to £84,000, while a more substantial 50m² build can set you back anywhere from £100,000 to £140,000.
It's also crucial to remember that these estimates mainly cover the construction or 'shell' cost. The final interior fit-out—plastering, flooring, electrics, and decoration—will be an additional expense. For a more tailored estimate, you can start mapping out your budget with our detailed UK home extension cost calculator to get a clearer idea of where your project might land.
A common rule of thumb is to set aside a contingency fund of at least 10-15% of your total budget. This financial cushion is essential for covering unforeseen issues, such as tricky ground conditions or unexpected structural problems, ensuring your project stays on track without causing any financial stress.
To make things even clearer, here’s a quick overview of what you might expect for a standard single-storey extension.
Average UK House Extension Costs at a Glance
Remember, these are just averages to help you get started. The final cost will depend on the specifics of your project, including the quality of the finish and where you are in the country.
Comparing Costs for Different Extension Types
Once you get past the broad cost-per-square-metre figures, the single biggest factor shaping your budget is the type of extension you choose. Not all extensions are created equal. Some are relatively straightforward, while others demand complex structural work that can send costs soaring.
Getting your head around these differences is the key to matching your dream space with a realistic budget.
The most common choice, a single-storey rear extension, is often the simplest and most cost-effective way to gain more room. It usually involves building out from the back of your property on one level, making it perfect for creating that much-needed larger kitchen-diner or a new family room. Its relative simplicity means fewer structural headaches and a more predictable spend.
Single-Storey Versus Double-Storey Extensions
When you need more than just one new room, a double-storey extension often represents fantastic value for money. Yes, the total project cost is higher than for a single-storey build, but the cost per square metre is usually lower. Why? Because you're building two floors on a single foundation and roof structure.
You're effectively doubling your new space without doubling the cost of the most expensive parts of the build. It’s an efficient way to turn a significant investment into a truly transformative amount of space—perhaps adding a new master suite directly above that new ground-floor kitchen.
More Complex and Bespoke Extension Styles
If you’re looking for a more dramatic transformation, a wrap-around extension can be a superb choice. This style combines a rear and a side extension to create a large, L-shaped space that completely reconfigures the ground floor. Be warned, though: this added complexity comes at a price.
It requires more intricate structural work, plenty of steel supports, and careful integration with the existing house, pushing the overall cost higher than a standard rear extension of the same size.
Similarly, other projects come with their own unique cost profiles. Basement extensions, for example, are among the priciest, often costing £100,000–£160,000 or more due to the specialist work involved. At the other end of the scale, garage conversions are far more affordable, averaging around £14,250.
Double-storey extensions, providing a huge amount of usable area, typically land somewhere between £108,000–£180,000, reflecting the added complexity of building upwards. For more cost insights, it's always worth browsing resources like Checkatrade.com.
This infographic gives a simple visual breakdown of how extension costs tend to scale with the project size.
As you can see, the budget jumps significantly as the ambition and square meterage increase, moving from smaller projects under £60,000 to larger ones that can easily top £100,000.
Finding the Right Fit for Your Budget
The real key is to balance your ambitions with what’s practical. If your budget is tight, a garage conversion is often the most economical route to adding a new room, as the main structure is already in place. It’s perfect for creating a home office, gym, or playroom without the expense of major groundworks.
Ultimately, the best type of extension is one that meets your family's needs while respecting your financial boundaries. A well-designed but modest single-storey extension can be far more successful than an over-ambitious project that stretches your finances to breaking point.
To help you decide, here’s a quick rundown of the most popular extension types and their general cost implications:
Single-Storey Rear Extension: The baseline for cost and complexity, ideal for extending a kitchen or living area.
Double-Storey Extension: Offers better value per square metre by adding two floors on one foundation.
Wrap-Around Extension: A more expensive and complex option that dramatically reconfigures the ground floor layout.
Basement Conversion: One of the most expensive options due to the specialist excavation and waterproofing required.
Garage Conversion: A highly budget-friendly way to gain an extra room by using an existing structure.
By carefully weighing the pros and cons of each type, you can make an informed decision that adds maximum value and enjoyment to your home.
How a Kitchen Extension Reshapes Your Budget
The purpose of your new space is a huge cost driver, and nowhere is this clearer than with a kitchen extension. A simple shell for a home office or living room is one thing, but creating the heart of the home is a different beast altogether, involving far more complexity and expense.
A new kitchen isn't just about building four walls and a roof; it's about fitting out a highly functional, service-heavy room. This is where the concept of the 'fit-out' cost becomes so important. The fit-out covers everything inside the structural shell: plumbing, electrics, flooring, plastering, cabinetry, appliances, and worktops. For a kitchen, these costs can easily match, or even eclipse, the cost of the structure itself.
The Specialist Trades Your Kitchen Demands
Unlike a simple living room, a kitchen extension calls for a coordinated team of specialist tradespeople, each adding to the final bill. The intricate network of services needed to make a modern kitchen tick is a major reason how much a house extension costs shoots up when it’s for this purpose.
Just think about the essential services involved:
Plumbing and Gas Work: You’ll need certified plumbers and Gas Safe engineers to run new water supply and waste pipes for sinks and dishwashers, not to mention gas lines for hobs.
Complex Electrics: Modern kitchens are power-hungry. An electrician will be busy installing a dedicated circuit for ovens and induction hobs, plus countless sockets for small appliances and sophisticated lighting schemes.
Ventilation: Proper extraction isn't just nice to have; it's a building regulation requirement and is essential for managing steam and cooking odours, adding another layer of cost.
Specialist Fitting: Kitchen fitters, tilers, and worktop specialists are all needed to achieve that high-quality, durable finish you're after.
This is a world away from fitting out a living room, which might only need a few new sockets, a radiator, and plastering before you can start decorating.
Cabinetry, Appliances, and Finishes
Beyond the hidden services, the visible elements of a kitchen carry a significant cost. The choices you make here will have a massive impact on your budget.
The "kitchen triangle" of sink, hob, and fridge is just the beginning. The real cost lies in the quality of cabinetry, the durability of worktops, and the sophistication of the appliances you choose to fill the space.
Bespoke, handmade cabinetry will cost many times more than standard units from a high-street supplier. In the same way, a durable quartz or granite worktop is a far greater investment than a laminate alternative. These are the choices that define the final character and cost of your new kitchen.
Because of this complexity, kitchen extensions are among the most popular and potentially costly home improvements. Prices typically range from £50,000 to £120,000. The structural work for a 25m² extension alone could cost £62,500–£87,500. On a per-square-metre basis, you can expect to pay £2,200–£3,300, but this can easily push towards £3,500 per m² or more with premium finishes. This just goes to show why you must budget for the room you actually want, not just the space you're building.
Unpacking the Factors That Drive Your Final Cost
Beyond the headline figures for size and type, the real answer to "how much does a house extension cost?" is always found in the details. A dozen smaller decisions and circumstances can dramatically inflate your final budget, turning a manageable project into a seriously costly one. Getting your head around these variables is the best way to guard against any nasty surprises.
Think of it like buying a car. The base model always has an attractive price tag, but as soon as you start adding the extras—a better engine, a luxury interior, advanced tech—the cost climbs. Your extension is exactly the same; the final specification is what truly shapes the price.
Material Choices and Specification Levels
Your choice of materials is one of the biggest cost drivers, without a doubt. This goes far beyond just picking a brick and touches every part of the build, from the windows and doors to the roof tiles and flooring. Every choice you make is a trade-off between cost, aesthetics, and long-term performance.
For instance, large glazed doors are a hugely popular feature in modern extensions. But the cost difference between standard uPVC French doors and a set of premium, slim-profile aluminium bifold or sliding doors can be immense—we’re often talking thousands of pounds. This same principle applies right across the board.
Here are a few common choices and what they mean for your budget:
Windows and Doors: Basic uPVC is the most budget-friendly option. Timber and aluminium, on the other hand, command a premium for their superior look and durability.
Roofing: Standard concrete tiles are cost-effective and do the job perfectly well. Natural slate or a contemporary zinc roof, however, will be considerably more expensive.
Cladding: Simple render is an affordable finish, but if you're looking at options like timber or composite cladding, expect the costs to jump significantly.
Flooring: Laminate or vinyl flooring offers a budget solution for a high-traffic area. In contrast, engineered wood, polished concrete, or high-end tiles represent a much greater investment.
Each of these decisions contributes to the final "fit and finish" quality, which can range from a basic, functional space to a high-end, architect-designed showcase.
Site Conditions and Access Issues
Let's be clear: not all building sites are created equal. The specific conditions of your property can throw up challenges that add time, complexity, and, ultimately, cost to the project. These factors are often hidden until work begins, which is precisely why a contingency fund is so vital.
Site access is a major one. If your property is a terrace with no way to get to the back garden, every single bag of cement, every brick, and all the waste material has to be carried through your house. This is slower, more labour-intensive, and much riskier than having a driveway where a skip and materials can be easily stored. Poor access alone can add 5-10% to your labour costs.
The ground beneath your extension holds the biggest potential for surprise costs. What lies unseen can have a major impact on the foundations, which are the most critical structural element of your build.
Unexpected ground conditions are a classic source of budget overruns. If your builders start digging and discover soft clay, a high water table, or even old drains where they planned to put the footings, standard trench foundations might not be good enough. This could force you into a more complex and expensive solution, like a piled or raft foundation, adding thousands to your bill before you’ve even built anything above ground.
The Impact of Your Location
Finally, where you live in the UK plays a massive role in what your extension will cost. Labour rates, material supply costs, and professional fees vary significantly from one region to another. The "London premium" is a real phenomenon, with construction costs in the capital and the South East often being up to 40% higher than in other parts of the country, such as the North of England or Wales.
This regional difference affects every quote you get, from your builder and architect to your structural engineer. It’s absolutely essential to research local rates and make sure any average costings you see are adjusted for your postcode. If you don't, you could end up with a budget that's completely unrealistic and a significant financial shortfall.
Budgeting for Professional Fees and Other Costs
When you first start thinking about your extension, it’s easy to get fixated on the builder's quote. That’s the big number, right? But focusing only on the construction cost is one of the most common mistakes homeowners make. Think of it like buying a car – the sticker price is just the beginning. You still have insurance, road tax, and fuel to consider.
In reality, a successful extension relies on a whole team of professionals, plus various statutory fees and surveys. These aren't optional extras; they're essential for making sure your project is well-designed, structurally sound, and legally compliant.
Ignoring these costs is a recipe for budget shock later on. To build a realistic, stress-proof budget, you need to account for these "hidden" expenses right from the start.
The Key Professionals You Will Need
Several key experts will guide your project from a scribble on a napkin to a finished, liveable space. Their fees are a crucial investment in the quality and safety of your build.
Architect or Architectural Designer: This is your creative lead. They’re the ones who translate your vision into workable plans, expertly navigate the planning system, and produce the highly detailed drawings your builders need to work from. Fees are often a percentage of the total build cost, typically 5-12%, or a fixed sum. To get a better handle on this, check out our complete guide to the cost of an architect in the UK.
Structural Engineer: Once the design is settled, a structural engineer does the critical number-crunching. They calculate the size of steel beams (RSJs), the required depth of the foundations, and all the other elements that keep the building standing strong. This is a non-negotiable step for safety and building control approval. Budget between £500 and £1,500 for their drawings and calculations.
Party Wall Surveyor: If any of your extension work happens on or near the boundary with a neighbour, the Party Wall Act comes into play. If your neighbour doesn't consent to the works or wants their own surveyor to oversee things, you’ll have to cover the fees. This can range from £1,000 to £2,000 per neighbour.
Essential Administrative and Survey Costs
On top of your professional team, there are several statutory charges and preliminary costs you must factor in. These are all about making sure your project is legal and gets started on solid ground.
Think of these as the project's administrative backbone. They aren’t as exciting as choosing your new kitchen, but they’re every bit as important. For instance, if your garden has a steep slope or other complex features, you might need a topographical survey, which could add another £400 to £800 to the initial outlay.
To help you get organised, here’s a breakdown of the typical professional and statutory fees you should plan for.
Estimated Professional and Statutory Fees for a House Extension
This table outlines the common costs you'll encounter beyond the main construction quote. Budgeting for them from day one is key to keeping your project on track financially.
By itemising these costs right at the beginning, you transform a vague estimate into a robust, realistic budget. This approach doesn't just prepare you financially; it empowers you to manage the entire process with confidence, knowing all your bases are covered.
Smart Ways to Save on Your House Extension
Building an extension is a major investment, there's no doubt about it. But that doesn't mean the final bill has to be eye-watering. With some clever planning and savvy decisions along the way, you can bring the overall cost down significantly without ever compromising on the quality of the space you create. It’s all about making your budget work harder by making smart choices, not just cheaper ones.
The real key is to figure out where the big costs are hiding and then find practical ways to simplify things. Most budget overruns come from three main areas: structural complexity, bespoke features, and high-end finishes. By focusing on more straightforward designs and standard materials, you can take back control of your spending.
Simplify the Structure and Design
One of the single most effective ways to save money is to keep the building's form simple. Think about the roof – a complex design with multiple hips, valleys, and dormers demands more materials, specialist labour, and a lot more time to build than a simple pitched or flat roof. Opting for a simpler roof design is a quick and easy win for your budget.
In the same way, the actual shape of the extension matters hugely. A simple rectangular footprint is far cheaper to build than an L-shaped or multi-faceted design. Every extra corner you add creates complexity in the foundations, brickwork, and roofing, which all translates directly into higher costs.
"We realised we could finish the whole house with the money we were about to splash out on just one room... We're happier now. Knowing that we can get our kitchen diner and the rest of the house complete on budget is a huge weight off us financially."
This is a perfect example of how re-evaluating priorities can pay off. Sometimes, a more modest but beautifully executed plan delivers far more satisfaction than an over-ambitious and financially draining one.
Be Strategic with Sourcing and Finishes
You can find some serious savings in how you source your materials, especially when it comes to the 'second fix' stage. We're talking about things like kitchen units, bathroom suites, flooring, and tiles.
Instead of just going with your builder's suppliers, think about sourcing these items yourself. You can often find much better deals online, during sales, or from specialist retailers. Sure, this approach means you’ll need to be more organised, but it can genuinely save you thousands.
Here are a few strategies you can put into action:
Choose Standard Sizes: Bespoke windows and doors look fantastic, but they carry a premium price tag. Designing your extension around standard, off-the-shelf window and door sizes can save you a small fortune without really affecting the final look.
Source Your Own Kitchen: Let your builder get the main structure built and the first-fix electrics and plumbing in place. Then, you can buy your kitchen directly from a supplier and either fit it yourself or hire a specialist fitter. This is often far cheaper than having the main contractor manage the whole thing.
Phase the Project: If the budget is feeling tight, concentrate on getting the structural shell built and watertight first. You can always come back and tackle the interior fit-out and decoration in stages as and when your finances allow.
Common Questions About House Extensions
Even with a solid plan, embarking on a house extension project can feel like navigating a maze of rules, costs, and timelines. It's only natural to have questions.
To help you find your way, we've gathered and answered some of the most common queries homeowners have when they're thinking about expanding their living space. Consider this your straightforward guide to moving forward with confidence.
Do I Need Planning Permission for My Extension?
Not always. Many single-storey rear extensions can actually be built under Permitted Development rights, which is great news as it allows you to bypass a full planning application, saving both time and money. This usually applies as long as your plans meet specific size and design criteria.
However, the rules get much stricter for double-storey extensions, or if your home is in a conservation area or is a listed building. The best first step is always to check with your local planning authority or a qualified architect before any work begins. This ensures you’re fully compliant and can avoid any costly mistakes down the line.
How Much Value Does an Extension Add to a House?
A well-designed extension can add some serious value, often increasing your property's worth by 10% to 20%. A new kitchen-diner or an extra bedroom almost always provides the best return on your investment.
The key, though, is not to overdevelop for your street's ceiling price. It's a good idea to research local property values to make sure your investment is financially sound. You don't want to spend more than you could realistically get back when it's time to sell.
Is It Cheaper to Extend Up or Out?
Generally speaking, extending upwards with a loft conversion or adding a second storey can be more cost-effective per square metre. Why? Mainly because you aren't paying for new foundations, which is a major part of any ground-floor build.
That said, the total cost really depends on your home's existing structure and how complex the project is. A double-storey extension often provides the best value for money, as you're effectively doubling the new space on a single foundation footprint.
How Long Does a Typical House Extension Take to Build?
Once the builders are on-site, the construction phase for a standard single-storey extension typically takes between 10 to 18 weeks.
But remember, that’s just the build itself. The entire journey—from initial designs and securing planning permission to the final lick of paint—can easily take six months to a year. Things like bad weather, supply chain issues, or unforeseen structural problems can cause delays, so it’s always wise to build a bit of flexibility into your timeline.
At Harper Latter Architects, we specialise in creating bespoke extensions that perfectly blend with your home and lifestyle. If you're planning a project in South West London, we can guide you through every stage, from initial concept to final completion. Explore our architectural services to see how we can bring your vision to life.
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