The cost of an extension – plus 29 clever ways to stay on budget

Use our guide for a complete breakdown of the cost of an extension in 2020. Complete with 29 clever ways to cut extension costs and stay on budget

Cost of an extension: Architectural plans by Shutterstock
(Image credit: Shutterstock)

Knowing the cost of an extension is crucial before you get started on this home improvement project. After all, what you want from building an extension goes hand in hand with what you can afford, so we have included a breakdown of all the costs to expect in this guide, along with some savvy ways to curb costs – without sacrificing on a great result. 

From tweaking plans at the design stage, to changing your approach to the project altogether, we can help you set a realistic budget and keep house extension costs to a minimum too. 

For more information on how to extend a house be sure to check out our feature which will take you through the planning and design stages. Meanwhile, keep reading for all the extension costs you need to know about, including additional  costs that you might not have considered such as interior fit-out costs, hiring professionals and other design considerations.

Extension costs and how to budget

If you're extending a house, you can get a comprehensive extension cost calculation using our free extension cost calculator, but for a rough idea of what you might have to budget for your house extension costs, see below.

For planning and costing a single storey extension, expect to pay the following: 

Building work costs depend not only on where you live and the team you've hired, but on the results you're after. Expect to pay around £1,500 to £1,900 per square metre for an extension of basic quality; £1,900 to £2,200 per square metre will give you a finish of good quality; and £2,200 to £2,400 per square metre will allow you to achieve excellent quality.

In total, for an extension of 4m x 5m, the build cost would be around £30,000 and upwards for an extension of basic quality; you will pay from around £38,000 for a good quality build; and it will cost you from £44,000 for an extension with an excellent finish. 

These costs do not include professional fees (more on this below), which can cost around 10 to 15 per cent plus VAT. With these extra costs added, the lowest price you will pay for a 4m x 5m single storey extension would be just under £40,000 and up to £58,000 for an extension of excellent quality.

Extending a house: Side return extension exterior

(Image credit: Tim Mitchell)

For planning and costing a double storey extension, expect to pay:

Labour costs do vary around the country; these, along with the design quality, build spec and quality of materials will all affect the final price of your two storey extension. But as a rough idea, expect to pay per square metre for two-storey extension: £1,320 to £1,620 for basic quality; £1,620 to £1,860 for a good quality finish; and £1,860 to £2,100 for excellent quality. 

As with a single storey extension, the cost of fitting out your two storey extension will depend on the rooms within it – for example, a living space or bedroom will be cheaper to furnish than a new kitchen or bathroom.

‘As a very rough rule of thumb, costs should be split into 50 per cent for construction and 50 per cent for the interior,’ says Hugo Tugman, founder of Architect Your Home . If you do not want to compromise on the size of what you are building when extending a house, you may be able to make considerable savings on interior finishes.

It is important that you get quotes from at least four builders as it is almost certain that each of the builders will provide different costs for exactly the same work using the same materials. If you only approach one or two builders, you run the risk of only obtaining high quotes, rather than getting an idea of an average.

Extending a house: Sarah barker Brown and Nicolas Brown extension

(Image credit: Rick Mccullagh)

Additional extension costs to factor in

Architects’ fees for are usually between three and seven per cent of the total construction cost. Expect to pay around £2,700 each for planning drawings and construction drawings. Find architectural technologists on CIAT, and architects via Architects Register or RIBA.

Structural engineers' fees are £500 to £1,000 (if roof joists and foundations are specified).

Surveyors' fees – necessary if a survey of the existing house is required – total £500 to £1,500.

Planning fees in England are £206

A certificate of lawful development (recommended if extending under permitted development rights) costs £103. 

A request for discharging planning conditions costs £34.

Building control charges (vary according to your extension’s size) are between £200 (for an extension of 10m sq) to £900 (for 80 to 100m sq). 

A party wall agreement (if needed) is £700 to £1,000 per neighbour.

Project management costs also need to be factored in. Your building contractor or architect will charge 15 to 20 per cent – this on top of the net cost of labour, materials and overheads. Find out how you can save money if you project manage an extension or renovation yourself.

VAT at 20 per cent of the labour, materials and services. If your individual sub-contractors turn over less than the VAT threshold, they won’t charge you any VAT on labour, which can result in big cost savings.

Extending a house: bright kitchen-diner that opens onto the garden

Plastering or dry-lining and painting is around £85 per square metre.

Bi-fold or sliding doors cost between £1,500 and £2,000 per linear metre.

Additional fees may include a tree report; a flood risk assessment (both £250 upwards); an ecology report (from £400); an archaeological report (perhaps several thousand pounds); a historic building report, likely if your home is listed.

Interior fit-out costs (see below).

What does an extension's interior fit out cost?

The costs for the interior fit out of a single storey extension or two storey extension will largely be dictated by the room types you’re adding – a kitchen will be more expensive than a living room; adding an extra bathroom will be more costly than another bedroom or a home office, for example. You can add these figures to the ones above to give you an estimate.

The cost of adding heating will depend on all sorts of variables. Extending an existing central heating system may only need a few days’ work by a plumber, at around £150 per day (excluding materials). 

Underfloor heating will be more expensive, but is worth considering for a large, open-plan kitchen diner, if you are building an orangery or adding a conservatory. Electric underfloor heating is a cheaper installation choice, some elements of which you can do yourself, but will be more expensive to run than water-fed underfloor heating, which has a more expensive installation cost. You may also need a new boiler; expect to pay around £2,500.

Planning permission fees for extensions

The planning fee for a residential single storey extension in England is £206. If you are extending under permitted development (see more below), you may want to apply for a certificate of lawful development, which will cost you £103; the fee for discharging planning conditions is £34 per request.

Permitted development fees for extensions

If you are planning to build an extension under permitted development rights, study the criteria carefully and apply for a Certificate of Lawful Development from your local authority. This only costs £86 and you will then have the paperwork in place to prove that your extension did not require planning permission.

Real Homes Show | Find out more about the best ways to improve your home within your budget

If you are struggling with your space and wondering what kind of extension will work for you, or just what to do with your budget, check out the Real Homes Show. Every fortnight the Small Space Squad visit a homeowner trying to make their home work for them

What is the cheapest way to build an extension?

If you're on a tight budget, there are many different route you can take to curb extension costs. Here are 29 ways to stay on budget from the onset, through to the finished result.

1. Do your sums then cut the cost of an extension

Cut extension costs: a large white kitchen extension with contemporary finish by mulroy architects

(Image credit: Joakim Boren (Mulroy Architects))
Save money by extending at the best time of year

Timing your extension for good weather is one of the simplest ways to cut the cost of an extension. Why? Because bad weather causes delays, which can mean increased labour costs, rejigged schedules and, if you're renting while you extend, increased rental accommodation costs, too. 

Weather damage from the wind and rain can up your costs, too, and what started as a simple loft extension can become a complete redecoration job for all the bedrooms below if rain penetrates tarpaulins.

When to extend? Try to line yours up for when you know there tend to be prolonged periods of dry weather where you live. Avoid weeks when frost is an issue, too.

Before you consider what you need to trim off your extension budget, work out exactly how much you need to spend to get the extension using the information above, including the interior fit out costs, then when you're happy it can be achieved, go to a trusted local estate agent and ask how much your home might increase in value with your proposed project in place. Ideally, your extension should add more £££s to your home's value than it cost. Bear in mind that larger extensions will be more cost-effective than small extensions but will, of course, cost more, and could push your home past the ceiling value of the street. The importance of balance when it comes to budget versus profit potential can't be exaggerated. 

Find small extension ideas and advice in our guide.

2. Consider building an extension with a simple design

Cut extension costs: conservatory style extension on a victorian house

Curves and corners are costly to build, so keep your extension design as simple as possible. Building a house with a rectangular or square footprint with a simple pitched roof is the best way to reduce the cost of an extension or to extend a house cheaply in the first place – without compromising on the build's quality. 

Use our guide to building a single storey extension for design tips and specific extension costs for this type of project.

3. Keep interior fit outs simple too

Cheap extension ideas: white light-filled kitchen extension with industrial style seating and dining table

(Image credit: James French)

Interior fit out costs can be kept down too. Obviously, you can choose to shop for affordable flooring, lighting and fixtures, such as kitchen units, but planning your project carefully and in great detail before work starts – right down to the position of electrical sockets and lighting controls – and avoiding making changes as you progress will keep costs under control. For more information on kitchen extension costs have a read of our guide. 

Find out more on project managing an extension or house renovation yourself in our guide.

4. Plan ahead in detail to extend a house cheaply

Spending time planning the smallest of details can save you money and limit the cost of an extension further down the line. By specifying everything from light fittings and socket locations to flooring and wall colours at the outset, you’ll save money on the cost of making alterations or snap decisions at a later stage.

Real Homes Show | Find out more about the best ways to improve your home within your budget

If you are struggling with your space and wondering what kind of extension will work for you, or just what to do with your budget, check out the Real Homes Show. Every fortnight the Small Space Squad visit a homeowner trying to make their home work for them

5. Stick to the agreed design to reduce house extension costs

Then, it's important to stick to the agreed design if you want to cut the cost of an extension.

‘It’s important to stick to the plan and design you’ve agreed, as any changes will cost money,’ says surveyor Steven Way. ‘Have a proper building contract with an itemised budget and an order of works in place, which will help save you money in the long run.’

Use our guide to extending for under £50,000 for more budget-friendly tips.

6. Go off-the-shelf not made-to-order to extend cheaply

Design the build around off-the-shelf products, such as standard-size doors and windows. Avoid any products that have to be made to order; instead, choose materials that are readily available and easy to use with all building extensions. So, stick to cast concrete for the sub-floor; concrete blockwork for the walls; brick, render or timber cladding; and a softwood timber roof structure. Try rooflights instead of dormer windows and interlocking concrete tiles for covering the roof.

Related articles: Permitted development rights for extensions | Kitchen extensions: what you need to know | House extensions: for every budget

7. Use affordable cladding to create a contemporary look for less

Cheap extension ideas: panelled extension with large bifold glass doors by IQ glass

(Image credit: IQ Glass)

Yes, you can cut extension costs by being a bit clever with cladding. Because, cladding or rendering the exterior of an extension built with a timber frame or block work will work out much more affordable than facing it with brick. 

Ideal for giving an extension a contemporary finish, it can suit both modern homes and period properties. Bear in mind that some cladding materials will need more upkeep and maintenance than others – read our guide to exterior claddings and renders to pick the right finish for your extension.

8. Build a timber frame extension to cut costs

Cheap extension ideas: timber clad rear extension on a Victorian house

Timber frame extensions can help cut extension costs as although the materials can work out a little more expensive than building with block work, the speed of the build can make up for that in lower labour costs. 

Better still, timber-frame extensions can be finished to look no different to one constructed from block work, so aesthetics don’t have to be a consideration.

Expect to pay from around £1,800 per square metre. Find out more building a timber frame extension in our guide.

9. Cut the cost of an extension by avoiding complicated groundworks

If possible, avoid complicated groundworks, such as building near to trees or drains and sewers, or other buried services, as these will increase groundwork costs. Remember, though, that if you live in a period or character property, your choices will be limited. Find out more about what to consider when extending a house in our ultimate guide.

Read more: Maximise natural light with a glass extension 

Cut extension costs: infographic of ways to cut the cost of your extension

10. Project manage yourself to reduce your building extension cost 

A builder will add 15 to 25 per cent on to the total cost of labour and materials to cover their time for managing the project. So you can save on the cost of an extension by effectively taking on the role of building contractor yourself. This will mean liaising with your designer/architect and your local authority’s building control department, finding and hiring tradespeople, directing the work and supplying all of the necessary materials, plus scaffolding, skips and so on.

Though time-consuming, it can be very rewarding. To do it well, you need time and flexibility, plus confidence, management skills and some knowledge of construction. The job may take longer to complete overall, but the savings can be enormous. 

11. Get the timings right to keep down the cost of an extension

Whether you're project managing or just ordering your own materials to save money, getting the timings of deliveries right to save on wasted labour costs is vital. 'A site with no one working on it is costly, so materials need to arrive in plenty of time,’ advises Steven Way, practice principal at Collier Stevens Chartered Surveyors.

12. Save on VAT to cut back on extension labour costs

Most extension work will attract VAT at 20 per cent on labour and materials, but if you use self-employed tradespeople who each turnover less than the threshold for VAT registration, you will not be charged this tax – saving on labour costs. Second hand materials sold by private individuals on the internet will also be free of VAT.

Some types of work attract reduced-rate VAT at in any case – upgrading insulation and extending a building that’s been empty for two years, for example.

Find out how to reclaim VAT on your building project.

13. Get next door to sign a party wall agreement waiver 

If you are building on or near the boundary of a neighbour, your extension will need to comply with the party wall act. If a party wall settlement is required, it will cost around £700 per neighbour; more if they use their own independent surveyors, which they're within their rights to do, and for which you foot the bill.

You must notify your neighbours in writing about your extension plans eight weeks before you start. If you can get them to write back that they do not object or complete a party wall agreement waiver, you can avoid using a surveyor to arrange a party wall settlement and save on fees.

It pays, therefore, to keep neighbours on board with your project, discussing plans and being considerate about any concerns they have.

14. Save on extension costs by building under permitted development rights

rear extension attached to terraced house by Plus Rooms

(Image credit: Plus Rooms)

This will instantly save you spending on planning fees and costs. While it is a false economy not to invest in design, some designers know how to keep down costs, while others only produce very complicated, if beautiful, plans that are very expensive to build. For a simple, low-cost build, find an architect, architectural technician, chartered surveyor or structural engineer who will produce planning and Building Regulations drawings for a fixed, all-in price. Look for a track record in designing low-cost projects.

If your extension can be designed to fall within your permitted development rights (see www.planningportal.gov.uk), no planning application will be required. This will save the planning fee of £206. Find out if your project needs planning permission in our guide.

The above extension built under permitted development transformed the awkward ground floor layout of the home Louise and Dave Brown share with their two boys. ‘Our house had a weird L-shaped half open-plan design,’ says Louise. ‘You’d come in the front door and there was only one way to access the rest of the ground floor through the living and dining room. It was like a corridor. The house had a conservatory at the back – a lean-to design that was about 40 years old. In summer it was way too hot and in winter it was freezing.'

15. Comply with building regulations to reduce the cost of an extension

Building regulations are often confused with planning permission but they are distinctly different and even if you're building under permitted development, you must comply with them. Doing so means paying building control fees, which you can't avoid. What you can avoid, however, is the cost that will be heaped upon you if you ignore the rules and are made to undo and redo the work correctly at your own expense by your local building control department. A good builder will always be on top of this but do spend time checking yourself – and ensure all the paperwork is signed off at the right stages of the job. The buck stops with you. 

16. Avoid cowboy builders if you're extending a house cheaply

There are plenty of cowboy builders (as in any trade) and they may vastly underestimate costs — through incompetence or, possibly, deliberately to secure a job. They may then ask for more money for changes or extras. They could even demand some or all of the money up front, leaving you with no comeback if they fail to complete the work to satisfactorily.

To avoid being ripped off, always ask for references — and check them. Never, ever pay for building work in advance; pay only for work that has been completed and that you are happy with.

For a small project, pay when the job is finished. For an extension, agree payments at set stages, or interim payments based on a verifiable list of labour and materials used up to date. Never hand over money for materials in advance. If a builder does not want to supply materials, buy them yourself. Find out how to find a reliable builder in our guide.

17. Measure twice, but cut only once to reduce the cost of an extension

Making changes or mistakes that waste labour and materials is a major factor in the final bill for many projects. Measuring everything on site, rather than off your plans will help reduce wastage. Having accurate, scaled plans in the first place will help.

The more time spent at the design stage, visualising the end result, the more problems can be anticipated and headed off. Accurate and complete design will ensure that you have services, such as plumbing and wiring, in the right places, get floor levels right between rooms, and ensure that doors are hung to swing in the most space-efficient direction.

Once you have made up your mind, stick to it; changes always incur extra costs as already mentioned, and often in ways that don’t reveal themselves until much later.

Over-ordering materials can also waste money, but it’s better to have slightly too much than to fall short and have problems making up the difference. This will incur delays and extra delivery charges; worse, you may not be able to find the exact same thing again.

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18. Reuse, recycle and repair to save on extension costs

Existing materials can be reused rather than thrown in a skip. Old floorboards, doors, radiators, towel rails, kitchen units etc. can all be revived and reused when building an extension as all they need is to be cleaned up, and given a new finish. 

Reducing wastage will also reduce costs for skip hire and disposal. Bear in mind, too, that as a private individual, you can dispose of waste in your local council tip for free.

19. Buy secondhand to save on the cost of an extension

Cut extension costs: a kitchen with large pivot door and picture window, concrete-effect worktops and floor, and exposed beams in the pitched extension roof

(Image credit: Chris Snook)

Buying salvaged materials – on the internet or from salvage yards – can be a lot cheaper than buying new. It will also introduce instant character. Second-hand items that offer good value include roof tiles, bricks, internal doors, timber floorboards, fireplaces and roll-top baths. Some people even hunt through skips - although you must ask the householder before removing their waste.

20. Sell what you don't need 

Can’t reuse or recycle existing materials? You'll be surprised what you can sell – from bathroom fittings to old kitchen cabinets. Advertise on local Facebook groups or Gumtree. Remember to comply with Covid-19 Government safety standards at all times.

21. Negotiate trade discounts to extend a house cheaply

rear extension with contemporary feel made from glass attached to period property by iq glass

(Image credit: IQ Glass)

Find out where those in the trade buy their materials and aim to get the same wholesale/trade prices. Always negotiate and see if there is any discount for paying in cash – making sure you get a receipt. Bulk-buy from a single supplier for a further discount and ask about reduced delivery costs. Buying end-of-line deals will save you a fortune, especially on items such as carpets, units and appliances.

Getting the best deal will often mean moving away from the big brand names and finding equivalents without the price premium. If you are clever, however, you can save on price without compromising quality.

To keep down costs, stick with a basic specification – so go for radiator-based central heating, carpets for floors and standard white sanitaryware, for example.

22. Limit the relocation of services

When you're extending a house, consider the position of fittings, for example, a new toilet. If yours can be placed in a convenient position for the existing soil stack, you'll save on labour costs.

23. Maximise space with a mezzanine

Scandinavian Loft extension

(Image credit: Marcos Bevilacqua)

Provided your ceilings are high enough, a mezzanine area could provide extra space for sleeping, with room beneath for a home office or just a quiet nook to relax in. Scandinavian Loft designs, creates and installs bespoke solutions to fit your space, with a standard or custom-size bed. The lofts come in a range of different materials and finishes including mdf, birch ply and metal and in your colour choice or with natural varnished oil. Customise your design with safety rails, bedside tables, ladders, stairs, storage, LED lighting and electrical sockets.

This birch ply mezzanine loft with stair storage costs from £6,000, from Scandinavian Loft.

24. Find a problem-solving builder to cut the cost of an extension

If you want to keep costs down, it's important to find a builder with both experience and an ability to think around problems. This isn't always easy, but with good research locally, you will find a builder (or project manager) who can think on their feet. When you're comparing quotes, don't just ask about extension costs; quiz your prospective builders, too, about their experience, how they've overcome problems, whether they work with a fixed team or a shifting workforce, and whether they have access to surveyors or architects. The answers will all help inform you of their experience and abilities.

25. Visit the site regularly to better control extension costs

Visiting the site to ensure the work is moving along well is a must during extension work. Doing so will allow you to ensure the project is moving along well, that there are no supply problems that your main contractor hasn't told you about, that the workers themselves are happy – and turning up – and that the schedule is being stuck to. There is a caveat if you're looking to cut the cost of an extension: don't use these visits to make changes that will up the price in the long run.

26. Choose a team with experience in extending cheaply

When you have early meetings with architects, surveyors, structural engineers and even builders, ask about their experience in working on low-cost projects and make your budget clear from the start. If they are happy to proceed on that basis, ask if they can produce planning and building regulations drawings for a fixed price.

Find the team you need at: Royal Institute of British Architects, Chartered Institute of Architectural Technologists, Royal Institution of Chartered Surveyors and The Institution of Structural Engineers.

27. Reduce your house extension cost by considering an alternative solution

Could you avoid the cost of extending and consider a garage conversion or loft conversion instead? Both are brilliant options to transform dead space into more useful living space, which is what many of us are looking for at a time when we're spending more time at home. Both are likely to be cheaper than building a new extension from scratch, food for thought.

timber frame garden studio

(Image credit: Chris Snook Photography)

You could even consider adding a garden building, which makes for a sound home office upgrade if you've been working from home for the past few weeks, and due to continue this way for the foreseeable future. You'll be (we can't promise) largely uninterrupted by the chatter of family life and it will help separate work life from time spent at home with others too. 

Architect Richard John Andrews designed the above garden studio for his practice in East London and self-built it with his assistant for just £12,500, using a timber frame modular system with sheets of plywood. It’s clad in lightweight corrugated fibreglass panels, topped with a polycarbonate roof and has bespoke timber sliding doors. 

28. Such as adding a bedroom over an attached garage

Added bedroom with floral fabrics as extension alternative

(Image credit: Tory Mcternan )

Assuming the garage's existing foundations can take the load, building a room on top is much more cost-effective alternative to building a new extension – plus you won't be sacrificing garden space to gain a new room.

Check with an engineer or surveyor about structural challenges, and hire a designer and a good builder to ensure that the new room feels like a natural part of the original house inside, and looks like it's original from outside, too.

Expect to pay from around £15,000 to £25,000 for this type of extension; building regulations and planning permission both apply. 

29. Talk to an estate agent first

Before you even begin to think about extending, talk to a trusted local estate agent about the knock on effects of it on the value of your property overall. Most will be available over the phone and will have their Covid-19 company update on their website to help organise the best way of getting in touch. Forever home or investment for a quick buck – whatever your future intentions, you'll want to ensure that the spend you have planned does not exceed how much value you'll add to your home with the extension. Breaking even in a forever home isn't terrible; coming out in profit with an investment property is ideal; spending more than you increase the value of the house by is never a great idea.

More on extending your home: