How Much Does Extending a House Cost? how much does extending a house cost

It’s the first question on every homeowner's mind when they start dreaming of a bigger space: how much does extending a house actually cost?

While there’s no magic number that fits every project, a typical single-storey extension in the UK will generally land somewhere between £1,800 and £3,000 per square metre (£/m²). That's just for the build itself, but it gives you a solid starting point before factoring in professional fees, high-end finishes, and VAT.

Establishing Your Initial Extension Budget

Figuring out the cost of an extension is a bit like planning a holiday. The final price tag depends on where you’re going (the type of extension), how you get there (the complexity), and the kind of luxury you want along the way (your finishes). But the journey always starts with a fundamental calculation: the size.

The cost per square metre is the go-to metric for architects, builders, and quantity surveyors to map out a preliminary estimate. This base rate covers the essentials—foundations, walls, roof, and basic plumbing and electrics. Think of it as the foundational price, not the final bill. From here, a few key variables will quickly start to nudge that figure up or down.

Key Factors Influencing Your Base Cost

Before we get into the specifics of rear extensions versus loft conversions, it's worth understanding the main ingredients that shape your budget from day one.

These factors all play off each other, creating a unique cost profile for every single project:

• Location: It’s no secret that building costs are significantly higher in London and the South East. Labour costs and sheer demand mean you’ll pay a premium compared to other parts of the UK.

• Complexity: A simple, clean-lined rectangular extension is always going to be more budget-friendly than one with intricate corners, a unique roof design, or huge structural openings that require serious steelwork.

• Specification Level: This is where you have the most control. The quality of everything from your bi-fold doors and flooring to the kitchen units and light fittings will have a massive impact on the final invoice.

To give you a clearer picture, let’s look at some real-world numbers. A modest 20 m² rear or side extension—perfect for creating that spacious kitchen-diner you’ve been dreaming of—will typically have a build cost between £40,000 and £56,000. If you're planning something more generous, like a 30 m² open-plan family room, you should probably budget between £60,000 and £84,000 for the build.

An accurate budget is the bedrock of a successful project. Starting with a realistic cost per square metre prevents surprises down the line and allows for more informed decision-making during the design phase.

Getting a realistic idea of your potential investment early on is absolutely essential. To get the ball rolling, you can use our UK home extension cost calculator to estimate your project for a personalised baseline figure. It's the perfect first step for setting a solid foundation for all your future plans.

Why Your Postcode Heavily Influences Extension Costs

When you’re planning a house extension, it’s easy to focus on the big-ticket items like glazing, kitchen finishes, and flooring. But one of the biggest factors driving the final cost is something you can’t change: your postcode. This isn’t just about living in a desirable area; it’s a direct reflection of local market forces that create a very real ‘London premium’, felt most sharply across South West London and Surrey.

So, what’s really behind this regional price difference? It boils down to a mix of economics and simple logistics. The cost of doing business is just higher in and around the capital, and that has a direct knock-on effect on what you'll pay to extend your home. Getting your head around these local dynamics is the first step to building a budget that won’t fall apart later.

The London Premium Explained

The main driver is labour. London and the home counties have a huge demand for skilled tradespeople, from excellent bricklayers to specialist joiners. With more projects competing for the best teams, their daily rates naturally go up, and this forms a massive chunk of any builder’s quote.

Beyond that, the practical realities of building in a dense urban area add another layer of expense. Think about the logistical hurdles your builder has to overcome:

• Access and Parking: Getting large delivery vehicles down narrow residential streets, paying for skip permits, and covering parking costs for the team all add up. These small, daily expenses accumulate surprisingly fast over a project that lasts several months.

• Material Storage: Space is tight on most sites in London. Builders often need to coordinate just-in-time deliveries or even pay for off-site storage, adding complexity and cost.

• Proximity to Neighbours: Working in close quarters means careful planning around noise, dust, and party wall agreements. This can stretch timelines and increase associated professional fees.

The regional premium isn't an arbitrary markup; it's a direct consequence of higher operational costs, greater demand for elite craftsmanship, and the logistical challenges inherent in building within a densely populated, high-value area.

This postcode effect has a clear impact on the per-square-metre rates you'll be quoted. Location has an outsized influence on UK extension costs, and nowhere is this more obvious than in London and the South East. While a standard single‑storey extension might average £1,800–£3,000 per m² elsewhere in the UK, the same project in London often lands in the £2,300–£3,400 per m² range.

To put that into perspective, one study found a 60 m² double‑storey rear extension in London had an average build cost of £136,132—that’s around £2,268 per m². The final cost could sit anywhere between £114,540 and £179,281, depending on the final spec. You can learn more about how extension costs vary by region here.

Why Local Expertise Matters

This is exactly why it pays to partner with an architect who knows your specific area inside out. A firm based in South West London, for example, won’t just understand the design possibilities; they’ll be familiar with the local planning departments, have trusted relationships with builders who know how to handle the region’s challenges, and can give you cost estimates that reflect the real market rate.

Their local knowledge helps you sidestep potential pitfalls and ensures your budget is based on reality, not on generic national averages that don't apply here. In a premium market, that kind of expertise is a crucial advantage that protects your investment from the very start.

Calculating Costs for Different Extension Types

While a per-square-metre rate gives you a great starting point, the single biggest factor shaping your final budget is the type of extension you choose. Not all extensions are created equal, and a simple rear extension has a very different cost profile to a complex basement conversion or a two-storey addition.

Getting your head around these differences is the key to aligning your ambitions with a realistic financial plan. Each type brings its own structural challenges, groundwork requirements, and planning hurdles, all of which feed directly into the final build cost.

First, it’s important to understand the regional premium. Building in London and the South East simply costs more, a reality that applies across the board.

As this shows, projects in London start from a significantly higher base cost, and this premium affects every type of extension we’re about to look at.

H3: Single-Storey Rear and Side-Return Extensions

These are by far the most popular types of extension, especially for creating that much-loved open-plan kitchen and dining area that flows beautifully into the garden.

• A single-storey rear extension does what it says on the tin—it extends outwards from the back of your property. It's often the most straightforward and cost-effective option because the structural work is usually less complex.

• A side-return extension is perfect for Victorian and Edwardian terraced houses. It fills in that often-neglected alleyway running alongside the back of the house, and while it adds less raw square metreage, it can completely reorganise your ground floor into a bright, wide, and incredibly functional living space.

Don't be fooled by the smaller footprint, though. Side-return extensions are often more structurally complex than a simple rear extension. They typically demand significant steelwork to support the upper floors of both your house and potentially your neighbour's, which drives up both the build cost and the structural engineer's fees.

H3: More Ambitious Extension Projects

If you need a more substantial amount of space, then wrap-around, two-storey, and basement extensions offer incredible potential but come with a higher price tag to match.

The leap in complexity from a single-storey project to a two-storey or basement extension is huge. These projects involve far more intricate structural engineering, deeper foundations, and more disruption, all of which is reflected in their higher per-square-metre cost.

A wrap-around extension cleverly combines a side-return and a rear extension to form a large L-shaped space, giving you the maximum possible impact on the ground floor. A two-storey extension is a game-changer, adding space on both the ground and first floors—perfect for adding new bedrooms and bathrooms above your new living area.

Basement extensions, however, are in a league of their own. They involve major excavation, underpinning, and waterproofing, making them the most expensive and complex option by a long shot. But in high-value areas like South West London, they can be a brilliant way to add a home gym, cinema, or guest suite without changing the look of your property from the outside. To get a better sense of what’s involved, take a look at our guide to basement conversion costs.

H3: Cost Comparison by Extension Type in London & The South East

To put this all into perspective, the table below gives a clearer picture of the estimated build costs for these different extension types here in London and the South East.

Extension Type	Estimated Cost per m² (£)	Key Cost Drivers
Single-Storey Rear Extension	£2,300 – £3,400+	Foundations, roof structure, level of glazing (e.g., bi-folds).
Side-Return Extension	£2,600 – £3,800+	Extensive steelwork, party wall agreements, complex roofing.
Two-Storey Extension	£3,200 – £4,500+	Deeper foundations, scaffolding, additional structural load.
Basement Extension	£4,500 – £6,500+	Excavation, underpinning, waterproofing, lightwells.

These figures are a guide for the core build cost, but they clearly show how the complexity of the project is the main thing driving the price up. A basement isn’t just digging a hole; it's a major piece of engineering, and the cost reflects that.

Uncovering the Hidden Costs Beyond the Build

When you start asking, "how much does it cost to extend a house?", it’s completely natural to focus on the builder’s quote. That’s the big number, after all. But focusing only on the construction work is a bit like budgeting for a holiday by only pricing up the flights—you're missing a huge part of the picture.

The final cost of your extension is a combination of the build itself and a range of essential professional and statutory fees. These aren’t really ‘hidden costs’; they’re critical investments that ensure your project is legal, safe, and built to last.

Factoring these in from the very beginning is the secret to keeping your budget on track. As a rule of thumb, you should set aside an additional 10% to 20% of the main construction cost to cover them.

Professional Fees: The Team That Brings Your Vision to Life

To get from a rough idea to a beautifully finished reality, you need a team of skilled experts. Their fees aren’t just an expense; they're an investment in getting the project right.

These are the key players you'll need to budget for:

• Architect's Fees: This is probably your most important professional relationship. An architect does so much more than sketch out a floor plan. They design a space that works for you, navigate the maze of planning permission, create the incredibly detailed drawings builders need, and can even manage the project for you. Their fees typically fall between 8-15% of the build cost or can be a fixed sum, reflecting just how central their role is.

• Structural Engineer's Fees: Once you've got a design you love, a structural engineer does the crucial calculations. They figure out the exact specifications for foundations, steel beams (RSJs), and anything else that holds your new extension up. This is a non-negotiable step for safety and building control approval. Expect to pay between £500 to £2,500+, depending on how complex the structure is.

Statutory Fees: The Non-Negotiables

Beyond your design team, there are official fees you have to pay to your local council. These ensure your project is fully legal and meets all national building standards.

Statutory fees are not optional extras; they are legal requirements. Overlooking them can lead to significant delays, fines, or even being forced to demolish a non-compliant structure.

Here are the main statutory costs to plan for:

1. Planning Application: If your extension is too large or complex to fall under Permitted Development rights, you'll need to make a full planning application. The current fee for a householder application in England is £206.

2. Building Regulations Approval: This is completely separate from planning permission. It's a technical review to confirm your plans meet standards for everything from structural integrity and fire safety to ventilation and energy efficiency. The costs can vary, but usually sit between £500 and £1,500+. You can pay this to the council or to an approved private inspector.

3. Lawful Development Certificate (LDC): Even if your project is covered by Permitted Development, we strongly advise getting an LDC. This gives you official proof that your build is lawful, which is incredibly important when you come to sell your home. The fee for this is £103.

Other Potential Costs to Consider

Finally, a few other costs can pop up depending on your home and the scope of your project. In the tightly packed neighbourhoods of South West London and Surrey, one of the most common relates to shared walls.

A Party Wall Agreement is a legal must-have if any of your work affects a wall you share with a neighbour. You have to serve them notice, and if you can't agree directly, you may need to appoint surveyors for both parties. This can cost anywhere from £700 to £2,000 per neighbour. For a full breakdown of what’s involved, our guide to the Party Wall Act provides a clear explanation for homeowners.

And the one that catches everyone out: VAT. It adds 20% to the cost of professional services, labour, and materials. Always, always check with your builder and other professionals whether their quotes include VAT or not. That simple question can save you from a very nasty surprise down the line.

How Finishes and Fittings Shape Your Final Budget

The structural shell of your extension—the walls, roof, and foundations—is where the bulk of the initial build cost goes. But it’s the finishes and fittings, the parts you see and touch every day, that truly give the space its character and can make your budget climb.

This is where your personal taste meets the bottom line. Getting to grips with the cost implications here is absolutely vital for keeping your project on track.

Think of the basic build cost as the price of a standard car. It’s functional, and it gets you from A to B. But the final figure really depends on whether you stick with the standard fabric seats or upgrade to handcrafted leather; opt for the basic stereo or install a premium sound system. In an extension, these choices translate to everything from flooring and lighting to kitchens and glazing.

The difference between a ‘standard’ and a ‘high-end’ or ‘luxury’ specification can add anywhere from £400 to over £1,500 per square metre to your final bill. For a 30m² extension, that’s a potential budget swing of £12,000 to £45,000 or more, purely based on the quality of the internal fit-out.

Standard vs High-End Finishes: A Practical Comparison

So, how do these costs actually stack up? Let’s break down some common choices to see the financial impact. A standard-quality finish is perfectly functional and attractive, often using good-quality, off-the-shelf products. A high-end finish, on the other hand, is all about bespoke craftsmanship, premium materials, and superior performance.

Glazing Choices:

• Standard: A good-quality, off-the-shelf set of uPVC or aluminium bi-fold doors (3-panel, 3m wide) might cost between £3,000 and £5,000.

• High-End: A bespoke, slim-profile architectural glazing system, like large-format sliding doors from a premium brand, could easily cost £10,000 to £20,000+ for the same opening. The superior thermal performance, minimal sightlines, and effortless operation command a significant premium.

Kitchen Selections:

• Standard: An off-the-shelf kitchen from a reputable high-street supplier like Howdens or Wren, with laminate worktops, can be supplied and fitted for £8,000 to £15,000.

• High-End: A bespoke, handcrafted kitchen with custom joinery, stone worktops (like marble or quartzite), and integrated high-end appliances (from brands like Miele or Gaggenau) can range from £30,000 to £80,000+.

The key to budget control is not to cut corners everywhere, but to make informed decisions about where to invest for the greatest impact. A spectacular set of doors or a feature island can elevate the entire space, even if other elements are more modest.

Where to Invest Your Fittings Budget

Making smart choices about your specifications is crucial when you’re figuring out how much extending a house costs. It’s a balancing act between your aspirations and your budget. The good news? You don’t need to specify high-end everything to achieve a luxurious feel.

Instead, focus your investment on key areas that deliver the most value and daily enjoyment.

Here’s where spending a little more can make a big difference:

• Flooring: This is the foundation of your interior design. While cheaper laminates are available, investing in engineered wood, high-quality tiles, or polished concrete provides durability and a timeless aesthetic that elevates the entire room.

• Lighting: A sophisticated, layered lighting scheme designed by a professional can completely transform a space. It moves beyond simple ceiling spotlights to include pendants, wall lights, and task lighting that create mood and highlight architectural features.

• Ironmongery: Details matter. Upgrading from standard door handles, light switches, and socket plates to high-quality brass or matte black fittings is a relatively small cost that adds a disproportionate sense of quality and cohesion.

• Joinery: While a fully bespoke kitchen is a major investment, consider commissioning a custom-built feature, like a window seat with integrated storage or a beautiful media unit. This creates a unique focal point without the cost of a full bespoke fit-out.

Ultimately, the final cost of your finishes is in your hands. By working closely with your architect, you can pinpoint which elements are most important to you and allocate your budget strategically. This approach ensures your new extension not only meets your practical needs but also perfectly reflects your personal vision.

Smart Ways to Manage Your Extension Budget

Keeping a lid on the costs of extending your house doesn't mean you have to cut corners on quality. Far from it. Smart budget management is all about making the right decisions from the very beginning, ensuring every pound you spend adds real, tangible value. The biggest savings are almost always locked in long before the first shovel hits the ground.

Your single most powerful tool for financial control is a meticulously detailed design and specification package. Think of it as the project's rulebook. When every last detail—from the exact brick and the profile of the window frames down to where the light switches will go—is decided and documented upfront, it strips out all the ambiguity. This allows builders to give you a precise, fixed-price quote, massively reducing the risk of expensive changes once construction has started.

Securing the Right Builder at the Right Price

With your detailed plans in hand, the next move is to run a competitive tender. This means inviting a shortlist of vetted, reputable builders to bid for the work. It’s not about automatically picking the lowest number; it’s about comparing like-for-like quotes to find who offers the best value.

An experienced architect is your best ally here. They can help you sift through the tenders, spot any oddities, and make sure you’re comparing apples with apples. Once you've chosen your builder, insisting on a formal contract, like a JCT Minor Works contract, is non-negotiable. This document protects everyone by clearly defining the scope, cost, and timeline, giving the whole project a solid framework.

The most common reason budgets spiral out of control is making significant changes midway through the build. A robust contract and a watertight specification are your best defence against this, demanding clarity and accountability from day one.

The Long-Term Value of Architectural Oversight

It might sound counterintuitive, but paying for professional oversight from your architect is one of the smartest ways to save money. Their job doesn't stop once the drawings are done; their continued involvement protects your investment and generates savings down the line.

Your architect acts as your expert on-site, making sure the work is being done to the agreed standard and that you're only paying for what’s been built correctly. They have the experience to spot potential issues before they balloon into expensive problems and can help keep the project running to schedule.

This oversight brings several key financial benefits:

• Clever Design Solutions: They can often suggest creative, cost-effective ways to get the look you want without the high-end price tag.

• Efficient Project Management: By keeping the project on schedule, they help you sidestep the extra costs that come with delays.

• Quality Control: Their professional eye ensures the quality of the work is up to scratch, preventing the need for costly fixes later on.

Ultimately, managing your extension budget comes down to careful planning, strict processes, and expert guidance. By investing your time and resources in the pre-construction phase, you set the stage for a smooth, financially predictable project that brings your vision to life without any nasty surprises.

A Few Common Questions About Extension Costs

Getting your head around the finances of a home extension can feel a bit daunting. To help you get a clearer picture, we've answered some of the questions we hear most often from homeowners trying to budget for their project and understand the real costs involved.

How Long Does a Typical House Extension Take to Build?

The time it takes to build an extension really depends on its size and how complex it is. A straightforward, single-storey rear extension of around 20-30m² will probably take 3 to 4 months for the on-site construction phase.

If you’re planning something bigger, like a two-storey or a wrap-around extension, you should set aside 5 to 7 months. Basement extensions are in a league of their own and are easily the most time-intensive, often needing 9 to 12 months or even longer because of the major excavation and structural work required. It’s also crucial to remember that these timelines don't include the design and planning phase, which can easily take several months itself before any work starts on site.

Is It Cheaper to Extend Up or Out?

As a general rule, extending upwards with a loft conversion often works out to be more cost-effective per square metre than extending outwards. The main reason for this is that you’re using the existing footprint of the house and avoiding the heavy costs that come with digging new foundations and major groundwork.

That said, the cost of a loft conversion can climb if your roof needs significant structural changes to make it work. Extending outwards might have higher initial costs, but it gives you far more flexibility with the layout—especially if you're dreaming of those large, open-plan spaces that flow right out into the garden. The best choice really comes down to your property, what your family needs, and your overall budget.

Do I Always Need Planning Permission for an Extension?

Not always. Many smaller extensions can be built under what are known as ‘Permitted Development’ (PD) rights, which let you carry out certain types of work without needing to go through a full planning application. These rules are very specific, though, covering the extension’s size, height, and how close it is to your property boundaries.

For example, a single-storey rear extension on a detached house can often push out up to 8 metres under PD rights. However, these rights don't apply in the same way in Conservation Areas or for listed buildings.

Even if your project falls under Permitted Development, it's always a good idea to get a Lawful Development Certificate (LDC) from your local council. This is official proof that your build is legal and will be vital when you eventually sell your home. For anything larger or more ambitious, you'll almost certainly need to submit a full planning application.

How Much Value Does an Extension Add to a House?

A thoughtfully designed and well-built extension will nearly always add more value to your home than it costs. Research from Nationwide Building Society suggests that an extension creating a new double bedroom and an en-suite bathroom can add as much as 23% to a property's value. In the same vein, adding a larger kitchen and family living space could give you a boost of around 15%.

In high-value areas like South West London and Surrey, the return on your investment can be even more significant. The real key to maximising this value is ensuring the design is high-quality, integrates beautifully with the original house, and meets the demands of the local property market.

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At Harper Latter Architects, we specialise in creating bespoke, high-end extensions that not only meet your needs but also significantly enhance the value and character of your home. If you are considering an extension in South West London or Surrey, we invite you to start a conversation with our team. Explore our process and see how we can bring your vision to life at https://harperlatterarchitects.co.uk.