Costs

How Much Does an Extension Cost? Real-Life House Extension Costs, Ideas & Budgets

How much does an extension cost in the UK? Understanding house extension costs and planning your spending is vital to make sure your project runs smoothly. Here we take a look at how much you should set aside for your dream house extension and what you can really build for your budget

by Build It magazine

24th November 2024

If you’re planning a home improvement project, chances are you’ve asked yourself ‘how much does an extension cost?’ Extension costs will vary on a case-by-case basis and will depend on a whole host of factors, from the size of the project to your home’s location and how complex the works may be. However, it’ll be extremely helpful to have rough overall outline before you decide if it’s for you and whether your project goals and extension ideas can match up to the cash you have available.

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. “A side return or wraparound extension on a terraced property in London would typically cost between £80,000-£100,000 to take it to a builder’s finish,” says James Bernard, director of Plus Rooms.

Here, we take a look at how much the different types of house extension cost in the UK, with real-life case studies and advice on how you can effectively budget.

How can you calculate your house extension cost?

Looking for an indicative cost for your extension? The Build It Benchmark Report uses award-winning HBXL software for a quick and cost-effective way of pricing up your project.

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If you are looking for a broad outline of your overall build costs before you decide which route to take, why not send us your plans and request a Benchmark Report? Find out more here.

Getting an Idea of Your House Extension Costs

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

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

How Much Does an Extension Cost? Real-Life House Extension Costs, Ideas & Budgets

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.

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

Securing Estimates & Quotes for Your Home Extension Project

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.

How Much Does it Cost to Get Planning Permission for an Extension?

You won’t always need to submit a formal planning application for an extension, as you can achieve a lot under permitted development rights (PD). As stated by the Planning Portal, this is what you can do under permitted development:

You can build up to 8m out from the rear of a detached house in England, or 4m in Scotland and Wales.
Only half the area of land around the existing house can be covered by an extension.
Extensions cannot be higher than the highest part of the home’s existing roofline.
Where the extension is within two metres of the plot’s boundary, the height at the eaves cannot exceed 3 metres.
Extensions cannot be built forward of the home’s main elevation or where it fronts a highway.

If you do need to make a planning application, this will fall into a Householder planning application. Householder planning applications cover proposals to alter or extend a house, and relate to works undertaken within the boundary or garden. This is the most common type of planning application used for homeowners looking to alter their property or build in the garden as they generally require less detail.

It costs £258 for a householder to make a permission for alterations to a single dwellinghouse, including works within a home’s boundary (this could include adding an extension to your home or creating a garden building). This does not cover works to a flat.

You can cover multiple conditions under a single request, but if you want to submit the documents online via the Planning Portal, there is service charge of £58.33 (+ VAT). Applications with planning fees below £60 will not incur this fee.

Whether your house extension falls under PD or not, it’s important to refer to your local authority’s planning guidance to make sure you’re adhering to the rules that apply to your project and the area in which it’s located.

How Much Does an Extension Cost?

The Build It Estimating Service has developed a series of benchmark costings for extensions, based on a main contractor build route. A typical single-storey house extension could cost £1,900 to £2,400 per m², but this can increase to £2,700+ per m² depending on specifications. This is how much your house extension could cost based on your type of project:

How much does a single-storey extension cost?

A typical starting point for a 25m² single-storey addition built to a standard specification would be around £1,900 to £2,400 per m² of floor space.

This spec would include materials such as brick and block, PVCu windows, standard patio doors and torch-on felt or concrete roof tiles. Expect to pay in the region of £2,700+ per m² if you want to use premium materials (aluminium glazed doors, clay tiles etc).

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

How much does a double-storey extension cost?

A 50m² two-storey extension might cost just 60% more than a 25m² single-storey version using the same footprint – but will give you twice the living space. So, prices per m² are lower.

A standard spec two-storey extension could cost £1,500 to £2,000 per m². Expect to pay £2,200+ per m² for a high-quality specification.

How much does a basement extension cost?

If you have an existing basement with enough ceiling height, a basement conversion could start from around £975 per m² for a fully waterproofed space with floor screed laid, ready for first fix work.

Costs will rise significantly if you need to lower the floor or underpin the existing foundations. The extent of work required to carry out a full basement extension – including properly sequenced underpinning and supporting the existing house – makes this the most expensive extension route. But it can still offer a good return on investment in urban centres and high-value locations.

A typical full basement extension project will start from around £2,875 per m² for a waterproofed basement space with floor screed.

How much does a loft extension cost?

Available head height is a key factor when it comes to establishing the likely costs of creating habitable loft space. These figures are based on building a straightforward loft conversion that creates 40m² of internal floor area. The costs assume that the roof tiles do not need replacing, and are based on a main contractor or loft specialist route.

In general, prices for fully-finished rooflight conversions start from around £1,800 per m². A big factor here will be the number of roof windows inserted: these can cost around £1,200 each.

A typical dormer loft conversion will start at around £2,000 per m², fully-finished and ready to inhabit. If you’re extending the loft to add more space, such as with a hip-to-gable or mansard, expect to pay significantly more – budget from around £2,550 per m².

Paul Archer Design are behind this contemporary loft conversion and extension to an Edwardian flat in North London. The former loft area, located on the flat’s second floor, has been completely upgraded to create a new living space with a pitched roof and large, red dormer windows on either side. The project cost a total of £2,080 per m². Photo: Andy Stagg

How much do other types of extension cost?

According to Resi, and for average costs in the London area, a side return extension could cost £2,200-£2,600 per m² (for the structural shell) and add boost your home’s value by 20-30%.

Structural glazing is a bespoke product, so is individually priced. However, according to Resi, a glazed side-return extension might start from £1,300-£1,700 per m², adding around 10-15% to the property value.

CASE STUDY How much did this London house extension cost?

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.”

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.

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).

Different areas of the extension cost the following:

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)

Photos: Adelina Iliev

How Can You Finance an Extension Project?

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

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

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

Want to learn more about extensions?

Head to Build It Live and visit our dedicated extensions clinic for some one-to-one advice from the professionals!

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To Summarise – Here Are Our Tips For Staying on Top of Your Extension Costs

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

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

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

QUICK GUIDE How will your chosen build route impact extension costs?

Build Route A: Main contractor – standard option

The figures above in our extension costs section are 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:

Build Route B: Builder plus subcontractors – potential saving 10%

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.

Build Route C: Self project managed – potential saving 20%

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.

Build Route D: DIY extension – potential saving 25%

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.

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

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

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

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

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

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

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

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

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

FAQ What issues can impact how much your extension costs?

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.

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.

7 Real Extension Costs – What Can You Build for Your Budget?

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 inspiring projects:

1. Light & Open Victorian Terrace Renovation & Extension in Edinburgh

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.

see the home

2. Period Terrace Home Upgraded with a Colourful Redesign

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 project cost £1,916 per m² for the full renovation and extension works.

Victorian Terrace Home Upgraded with a Colourful Redesign

Photo: Simon Maxwell

see the home

3. Charming Oak Frame Extension to a Period Home

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.

see the home

4. Spacious Victorian Home Loft and Side Return Extension

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 £1,650 per m² for the structural work and internal fit-out.

see the home

5. Modern Timber Clad Extension to a 1970s Brick Home

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.

Photo: Simon Maxwell

The full project (including extension and renovation works) cost them £1,754 per m².

see the home

6. Light-Filled Oak Frame Kitchen-Diner Extension

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.

see the home

7. Victorian Home Transformed with a New Extension & Interior Redesign

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.

Victorian Home Transformed with New Extension & Interior Redesign

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 £1,549 per m² for the renovation and extension works.

see the home

This article was originally published in October 2023 and has been updated in March 2024. Words by Emily Batesmith, Lindsay Blair & Georgina Crothers