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Cost of House Extension UK: A Practical Guide
- Dan Hall
- Nov 13
- 17 min read
So, you’re thinking about an extension. It’s an exciting step, but let's be honest, the first question everyone asks is: "How much is this actually going to cost?"
Getting a handle on the budget from day one is the single most important part of planning. The last thing anyone wants is a nasty surprise halfway through the build. In the UK, a typical house extension will land somewhere between £30,000 and £80,000+. Of course, that’s a pretty wide range. A simple single-storey addition will sit at the lower end, while a grand two-storey project with all the high-end trimmings will naturally push the budget upwards. This is why our in-house service is designed to give you a clear and accurate understanding of the cost of house extension uk from the very beginning.
Your Guide to UK House Extension Costs
Think of any figures you see as a solid starting point, not a fixed-price guarantee. The final number on your quote is shaped by a whole host of factors. For instance, a project in a sought-after area like Bournemouth or Poole might have different labour rates compared to other parts of the country.
The level of finish you opt for makes a huge difference, too. Standard-issue fittings and finishes will keep your costs down, whereas bespoke joinery, high-end appliances, and premium flooring will have a significant impact on the final invoice. Our in-house design service helps you navigate these choices to balance your dream extension with your budget.
Typical Extension Cost Ranges
To give you a clearer picture, let's break down some common project sizes. Remember, every single build is unique, and these estimates are for a standard-quality finish. A tricky site with limited access or a particularly complex design will always add to the cost of a house extension in the UK.
Small Single-Storey Extension (15-20m²): Perfect for a new utility room or a compact kitchen diner. You should budget somewhere in the region of £25,000 to £40,000.
Medium Single-Storey Extension (30-40m²): This gives you enough space for a proper open-plan kitchen and living area. Expect costs to range from £45,000 to £70,000.
Two-Storey Extension (50m²+): Adding a second level is a much bigger job. The complexity jumps, and so does the price, with costs typically starting from £75,000 and climbing depending on the scope.
At Hallmoore Developments, our in-house service team handles everything—from the foundations and brickwork right through to the final lick of paint. This means we can give you a precise, itemised quote for your project in Highcliffe or Christchurch that truly reflects what you want, with no hidden extras down the line.
These numbers generally cover the core structural build. To see how they fit into the wider world of home improvements, take a look at our comprehensive guide to UK home renovation costs, which provides some brilliant context.
The table below breaks things down a bit further, showing how the price per square metre shifts depending on the finish you choose.
Estimated UK House Extension Costs by Size
Here’s a quick summary of what you might expect to pay for a standard single-storey extension, based on its size and the quality of the internal finish.
Extension Size (m²) | Average Cost Range (Standard Finish) | Average Cost Range (Premium Finish) |
|---|---|---|
15m² | £25,000 - £35,000 | £35,000 - £45,000+ |
20m² | £32,000 - £42,000 | £42,000 - £55,000+ |
30m² | £45,000 - £60,000 | £60,000 - £75,000+ |
40m² | £60,000 - £75,000 | £75,000 - £90,000+ |
As you can see, opting for higher-spec kitchens, bathrooms, flooring, and bespoke features can add a significant amount to the baseline cost. It’s all about finding that perfect balance between your vision and your budget.
Decoding the Cost Per Square Metre
As you start researching the cost of house extension uk, you'll see one term pop up everywhere: 'price per square metre' (£/m²). It’s the building industry's go-to shorthand for a quick estimate, but it's crucial to understand what that number really means—and more importantly, what it doesn't. Taken out of context, it can be seriously misleading.
Think of it like buying a new car. The manufacturer advertises a starting price for the base model, which will get you from A to B just fine. But if you want the leather seats, the panoramic sunroof, and that premium sound system, the price tag suddenly looks very different. House extensions work in exactly the same way.
The base cost per square metre typically covers the fundamentals: the foundations, walls, roof, and a basic electrical and plumbing setup. Add in premium features like bespoke bi-fold doors, a high-end kitchen, or underfloor heating, and that figure will climb. The final cost is tailored to your unique vision.
Why Your Specification Matters Most
Your choice of materials and finishes is the single biggest factor driving that cost per square metre. A simple, plastered wall is worlds away from the cost of exposed brickwork or custom timber cladding. Likewise, standard laminate flooring is a fraction of the price of engineered oak.
This is precisely why an off-the-cuff estimate can differ so much from a detailed quote. A builder can only give you an accurate price once they understand the exact finish you’re aiming for. It’s where our in-house service at Hallmoore Developments really shines; we work with you from the start to understand your goals and provide a transparent quote that reflects the quality you want for your home in Bournemouth or Christchurch.
Regional Variations and Economies of Scale
It's also worth remembering that the cost of labour and materials isn't the same everywhere in the UK. Building in areas like Southampton or Ringwood will have different cost implications compared to other regions, and that directly impacts the price per square metre. Things like local demand, supplier costs, and even site accessibility all play a part.
Then there’s the matter of economies of scale, especially when comparing single and double-storey extensions. While a two-storey project is obviously more expensive overall, the cost per square metre is often lower. Why? Because you're building on the same foundations and roof footprint—you're getting twice the floor space for a less-than-double increase in cost.
The key takeaway is this: a 'per square metre' cost is a brilliant starting point for a ballpark budget, but it is not a fixed price. It’s a dynamic figure that shifts based on your design, your location, and your personal taste.
Getting a True Picture of Costs
For a standard, single-storey extension, you can expect a range from £1,400 to £2,400 per square metre. If you're aiming for a higher quality finish with more complex features, that figure often moves closer to £2,200–£3,300 per square metre.
As you’d expect, double-storey extensions are more involved. A good rule of thumb is to add around 50–60% to the single-storey rate, putting them in the £2,240–£3,840 per square metre range. You can explore these extension cost figures in more detail to get a feel for the market averages.
These numbers really highlight why a bespoke quote is so essential. At Hallmoore Developments, our in-house service provides detailed, itemised quotes that break down every single element of your project. This means you know exactly where your investment is going, giving you complete clarity and peace of mind before any work begins on your Poole or Highcliffe property.
How Extension Type Shapes Your Budget
When you start pricing up a house extension, one of the first things you'll realise is that not all projects are created equal. The type of extension you choose is one of the biggest factors driving the final cost of house extension uk. A simple, box-like rear extension is a world away from a complex wraparound build that completely reshapes the ground floor of your home.
The design choices you make right at the beginning have a knock-on effect on everything that follows, from the structural engineering and steelwork needed to the amount of roofing and glazing you'll pay for. Each type comes with its own unique set of challenges and, of course, its own price tag.
Comparing Popular Extension Designs
Let's break down the most common extension types you’ll see across the UK and get a feel for how they influence the budget. Getting your head around these differences is the first step to setting a realistic financial goal for your project.
Rear Extension: This is the go-to for a reason. It's often the most straightforward and cost-effective option. By building straight out from the back of your property, you create a simple rectangular space that’s perfect for a new open-plan kitchen or living area. The structural work is generally less complicated, which helps keep the budget in check.
Side-Return Extension: A firm favourite for terraced or semi-detached homes. This clever design fills in that often-neglected alleyway down the side of the house. It’s a brilliant way to add crucial width to a kitchen or dining room without eating into your garden, though it can involve more complex foundation and roofing work where it joins up with your neighbours.
Wraparound Extension: This is the big one. A wraparound combines a side-return and a rear extension into a single, L-shaped structure. The result is a total transformation, creating a huge, light-filled open-plan area. It's also one of the priciest options because of the extensive structural steels, complex foundations, and sheer scale of the build.
Double-Storey Extension: While the initial outlay is obviously much higher than for a single-storey build, a two-storey extension can actually offer better value per square metre. Think about it: you’re getting twice the floor space but only paying for one set of foundations and one roof. It’s a very efficient way to add living space downstairs and new bedrooms upstairs all in one go.
The Special Case of a Kitchen Extension
A new kitchen extension is consistently at the top of homeowners' wish lists, and it's easy to see why. It can completely change how you use and feel about your home. But this popularity and complexity mean they also command a higher cost per square metre than a simple living room extension. A kitchen is what we call a "service-heavy" room.
Your budget needs to stretch to cover:
Complex Plumbing: Rerouting water supplies for sinks and dishwashers and running new waste pipes.
Specialist Electrics: Fitting dedicated circuits for powerful ovens and hobs, extractor fans, and often complex lighting schemes with multiple zones.
High-Value Items: The cost of the kitchen itself—the cabinetry, worktops, appliances, and tiles—can easily run into tens of thousands alone.
Kitchen extensions are one of the most popular home improvement projects in the UK, and their costs reflect this. A typical single-storey kitchen extension will land somewhere between £50,000 and £120,000, though this depends massively on size, spec, and where you are in the country. For a much deeper dive, check out our guide on the average cost of a kitchen extension.
At Hallmoore Developments, we specialise in taking the stress out of these bespoke projects. Our in-house service includes Gas Safe registered plumbers and qualified electricians who work hand-in-glove with our building teams, ensuring every part of your kitchen extension in Bournemouth or Poole is handled with absolute precision.
The materials you clad your extension in will also have a massive say in the final bill. For a great overview of the options, have a look at a comprehensive guide to timber external cladding. This choice between brick, render, or cladding has a huge impact on both the final look and the overall cost of your house extension. Our in-house service team makes sure our clients in Highcliffe and Southampton understand these trade-offs, helping them balance the aesthetics they want with the budget they have.
Where Does the Money Go? A Cost Breakdown
To really get your head around the cost of house extension uk quotes you receive, it’s helpful to see where every pound goes. A quote isn't just a single lump sum; it’s a carefully calculated total of skilled trades, raw materials, and project management hours, all working in harmony.
Think of it like building a custom car. The final price tag includes the engine, the chassis, the electronics, and the interior trim, plus all the labour to assemble it perfectly. An extension is no different. This trade-by-trade overview lifts the lid on the process and shows you how your budget is split across the different stages of the build.
This infographic gives a great visual summary of how costs are typically shared out for different types of extensions, from a simple rear addition to a more complex kitchen project.
As you can see, the slice of the pie for things like plumbing and high-end finishes gets bigger for projects like kitchen extensions where the technical requirements are more demanding.
Let's look at how a typical budget might be allocated across the key stages of a house extension.
Typical Cost Allocation for a House Extension Project
Construction Stage / Trade | Approximate Percentage of Total Budget |
|---|---|
Structural Shell (Foundations to Roof) | 30% - 40% |
Exterior Finishes & Glazing (Windows, Doors) | 15% - 20% |
First & Second Fix (Electrics, Plumbing) | 15% - 25% |
Internal Finishes (Plastering, Flooring, Decorating) | 20% - 25% |
This table provides a bird's-eye view, but the exact percentages will naturally shift based on your specific design choices and the complexity of the build.
The Structural Shell: Foundations to Roof
This is the big one, often eating up 30-40% of your total budget. It’s the heavy lifting that gets your extension out of the ground and makes it a dry, secure box. Our in-house service manages this entire stage seamlessly.
It breaks down into a few critical steps:
Groundworks and Foundations: We're talking digging trenches, pouring concrete footings, and laying all the necessary drainage. This is the bedrock of your entire project.
Superstructure: This is the main frame of your extension. It includes building the blockwork walls and installing any crucial structural steels (RSJs) needed to support new openings and bear the load.
Roofing: Whether you're going for a flat or pitched roof, this stage covers the timber frame construction, adding the insulation, and fitting the final weatherproof layer, like tiles or a modern GRP membrane.
This phase is labour-intensive and uses a serious amount of materials, which is why it commands such a significant chunk of the investment.
Exterior Finishes and Glazing
Once the shell is weathertight, it’s time to make it look like it belongs to your home and let the light flood in. This stage typically accounts for around 15-20% of the project cost.
This covers installing your windows and doors – think stunning bi-folds or sleek sliding doors – as well as applying the external finish. Whether that’s brickwork to perfectly match your existing house, a sharp modern render, or stylish timber cladding, the choices you make here define the final look and have a big say on the budget.
First and Second Fix: Electrics and Plumbing
This is where your extension transforms from a shell into a genuinely liveable space. Representing 15-25% of the total cost, this work is split into two key parts. The "first fix" happens before any plastering, and it’s all about getting the hidden infrastructure right – running cables for sockets and lights, and installing the pipework for radiators, sinks, and appliances.
The "second fix" comes after the plastering is done and dusted. This is when the visible elements are fitted: light switches, sockets, radiators, taps, and sanitaryware. The complexity here, especially for a new kitchen or bathroom, can really influence the cost. For a closer look at the professional standards involved, you can explore our certified Gas Safe plumbing services, an absolute must for any kitchen or utility room project and a key part of our complete in-house service.
Internal Finishes and Decoration
Now for the final stretch! This is all about the details that turn a building site into a beautiful, finished room. Accounting for 20-25% of the budget, this stage covers everything from plastering walls and ceilings to laying flooring, fitting skirting boards, and the final paint job.
Of course, high-end choices like engineered oak flooring, bespoke cabinetry, or designer tiles will nudge the costs in this category upwards. But this is also where you get to inject your personality and add that all-important "wow" factor. Our skilled in-house service team ensures a flawless finish that truly brings your vision to life.
Budgeting For Fees Beyond The Build
The builder's quote for your extension is always the headline figure, the number everyone focuses on. But it's a classic mistake to think that's the final bill. I've seen it time and again: homeowners get laser-focused on the construction price, only to be completely blindsided by the host of professional fees and other costs that are non-negotiable for any legal, properly built project.
To get a real grip on the total cost of a house extension in the UK, you have to account for these non-build expenses right from the start. If you don't, you'll put a massive strain on your finances and could even put the whole project at risk. These aren't just 'nice-to-have' extras; they are the essential costs of getting your project off the ground.
Unpacking The Professional Fees
Long before the first shovel hits the ground, you need a team of experts to design your extension, make sure it's structurally safe, and get it through the council's approval process. Think of these fees as an investment in the quality, safety, and long-term value of your home.
Here are the key players you'll need on your team:
Architect or Architectural Designer: They're the ones who will draw up the detailed plans. Their fees typically run between 5% to 15% of the total construction cost, depending on how involved they are—from just creating the drawings to managing the entire project for you.
Structural Engineer: This is a critical role. They provide the structural calculations and drawings for the absolute essentials, like the foundations and any steel beams (RSJs). You should budget between £500 and £1,500+ for their expertise.
Council Fees: You can't avoid paying the local authority. In England, a standard Planning Application will set you back £206. On top of that, Building Regulations submissions can cost anywhere from £300 to over £1,000, based on the size and complexity of your extension.
The Hidden Costs and Why A Contingency Is Non-Negotiable
Beyond the pros, there are other costs that often fly under the radar until it's too late. It’s absolutely vital to factor these into your initial sums to avoid nasty surprises down the line.
One of the most important lines on your budget sheet should be a contingency fund. We strongly advise setting aside 10-15% of your total build cost for the unexpected. This isn't a 'nice-to-have'; it's an essential safety net for dealing with unforeseen issues, like discovering difficult ground conditions during excavation or needing to move a hidden drain.
Here are a few other costs you need to plan for:
VAT: Always, always ask if your builder's quote includes VAT. It's charged at the standard rate of 20%, and if it's not in the quote, it will be a significant addition to your final invoice.
Increased Insurance: You must tell your home insurer before any work begins. Your premiums will likely go up to make sure your property is covered while it’s effectively a building site.
Financing Costs: When figuring out how to pay for it all, it's crucial to understand your options. You might want to explore whether a mortgage refinancing vs. home equity loan is the better fit for your circumstances.
At Hallmoore Developments, our full in-house service gives homeowners in Southampton and Poole a much clearer financial picture from day one. Because we manage every stage, including all the tricky regulations, we help you plan for these costs proactively. To get a better sense of what's involved, have a read of our guide on UK building regulations for extensions explained.
Just for context, a standard 20-square-metre single-storey extension in the UK costs around £48,000 on average. But that figure almost never includes VAT or the professional fees we've just discussed. As you can see, adding that contingency and the extra costs is the only way to build a truly realistic budget.
Common Questions About Extension Costs
Getting your head around the financial side of a home extension can feel a bit daunting, but trust us, you’re not alone with your questions. Let’s tackle some of the most common queries we hear from homeowners trying to budget for the cost of house extension uk projects. We'll give you straight, practical answers to help you plan with real confidence.
How Much Value Does an Extension Add to My Home?
This is the big one, isn't it? The good news is that a well-designed, professionally built extension almost always adds more value to your property than it costs to build. A beautiful, modern kitchen extension, for instance, is a massive draw for buyers and can bump up your home's value by as much as 15-20%. If you’re adding an extra bedroom and bathroom, that return on investment can be even higher.
The real key to maximising value is the quality of the finish and making sure the new space feels like it's always been there. An extension that flows seamlessly from the original house will always be more valuable than something that feels bolted on as an afterthought.
Here in sought-after areas like Bournemouth and Poole, the return is often fantastic, especially when the build quality is top-notch and the design fits the local style. At Hallmoore Developments, our in-house service always designs with one eye on the local property market, ensuring your investment is a smart one that pays off down the line.
Do I Always Need Planning Permission for an Extension?
Not always, which often comes as a pleasant surprise! Many straightforward extensions fall under 'Permitted Development' rights. This is a scheme that lets you build without a full planning application, as long as your project sticks to a strict set of rules about its size, height, and position on your property.
For example, a single-storey rear extension on a detached house can often push out up to eight metres from the back wall under these rights. But be warned, the rules get much tighter for homes in conservation areas, for listed buildings, or on other types of designated land.
Most double-storey extensions, wraparound extensions, or any project that really changes how your home looks from the street will almost certainly need a full planning application. It's always safest to assume you need permission until you can prove you don't.
Figuring all this out can be a minefield, which is why getting professional advice is so important. For a deep dive into the rules, you can learn more about UK home extension planning permission explained in our dedicated guide. Even better, Hallmoore Developments' in-house service can manage this entire process for you, making sure your project in Christchurch or Ringwood is fully compliant from day one.
How Can I Keep My House Extension Costs Down?
Keeping a tight rein on the budget is everyone’s top priority. The good news is there are several smart ways to control the cost of house extension uk projects without cutting corners on quality. The single most effective strategy? Keep the design simple.
A simple rectangular footprint with a standard pitched roof will always be cheaper to build than a complex L-shaped design with lots of different roof angles. It just requires less complex engineering, less steel, and less tricky roofing work.
Here are a few other practical ways to manage your budget:
Be Smart with Finishes: You don’t need high-end materials everywhere. Splurge on one or two statement features that have a real impact, like fantastic bi-fold doors or a quality kitchen worktop, but then choose durable, standard options for things like internal doors or flooring.
Avoid Moving Utilities: The bill can rocket if you have to start moving main drains, gas pipes, or your boiler. Designing your layout to work with your existing services will save a huge amount of money and disruption.
Plan, Plan, and Plan Again: Making changes once the builders are on-site is the number one cause of budget blowouts. Finalise every last detail—from where the sockets will go to the final paint colours—before a single shovel hits the ground. This will help you avoid costly last-minute decisions.
Honestly, the best way to guarantee cost control is to work with a reputable firm that offers a complete in-house service, like Hallmoore Developments. We give you a detailed, fixed-price quote, so you know exactly where you stand. Our expert project management keeps the build running like clockwork, preventing the budget creep that’s all too common when you’re trying to juggle multiple trades yourself.
How Long Does a Typical Extension Take to Build?
The construction timeline really comes down to the size and complexity of the job. A straightforward, small single-storey extension in a place like Ringwood might take around 3-4 months on-site, from digging the foundations to the final touch-ups.
On the other hand, a larger double-storey or wraparound project could easily take 6-7 months, sometimes longer. And it's crucial to remember that this timeline is just for the build itself.
The pre-construction phase is a project in its own right. This involves creating architectural designs, getting structural calculations sorted, and then securing planning permission (which can take 8-12 weeks from the council alone) and building regulations approval. This prep work can add several months to the overall schedule before any building even begins.
At Hallmoore Developments, our in-house service provides every client in areas like Southampton and Highcliffe with a clear, detailed project schedule from the very start. This gives you a realistic timeline you can actually plan your life around, with clear milestones and open communication every step of the way.
Ready to turn your vision into a reality with a team you can trust? At Hallmoore Developments, our in-house experts manage every aspect of your extension, from initial designs to the final flawless finish. We provide transparent pricing and unparalleled craftsmanship across Bournemouth, Poole, and the wider region.
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