Building an extension can be exciting and daunting in equal measure, especially if you’ve never taken one on before. So, it pays to know what’s involved to make the whole process as plain-sailing as possible.
And it doesn’t matter what extension type you choose to build - glazed, single or two storey extension - much of the essentials and processes will be the same.
Our beginner’s guide will set out what to consider before building an extension. From costs per m2 and how to keep on top of your budget, to how big you can extend without planning permission to who will project manage the extension - there’s a wealth of information in our guide to building an extension.
How Much Does Building an Extension Cost?
Try out our Extension Cost Calculator for an accurate estimate for your build project.
How much does an extension cost? The million dollar question for any extender!
Price hikes for materials and labour are occurring across the UK and will affect most bottom lines. Size, shape and glazing will also impact the cost of building an extension.
If you're carrying out a fairly standard single storey extension project, managed on your behalf by a builder, you should be looking to allow £1,800-£2,300/m2 for the finished scheme, with a two storey extension costing slightly less (on account of more cheaper space) at around £1,500-£2,000.
In certain parts of the South and high value areas of the UK - affected by labour shortages - this you could easily be paying £3,000/m2, particularly on smaller schemes. That makes an 8x4m kitchen extension a £50-£70,000 project; a two storey extension on the same footprint around £100,000. But, these figures are there to be broken depending on the chosen specification and the build route.
Keeping on Top of Your Budget
The key to a successful extension scheme is building to what your finances will allow, rather than designing a dream project that your funds will never stretch to cover. If money is tight, invest more in the structure and materials you won’t be changing later – roof tiles, exterior finishes and glazing, for instance. Designer kitchen and bathroom suites can be installed a few years down the line when your bank balance has recovered. In the meantime, specify non-designer but attractive products you can live with until then.
“Making design decisions up front can help you keep in control of your budget so you know how much you’ll be spending on flooring, tiling, kitchen units etc,” says Jo Dyson Dyson, a partner at Mae House Design. Your extension will almost always cost more than you expected it to, so it’s important to have a contingency fund you can fall back on. “Allow a contingency of about 20%. There will always be unforeseen costs no matter how much you plan ahead in advance, especially with old buildings where there are a lot of unknowns,” says Jo.
Who Will Manage the Project
Who Will Project Manage the Extension Build?
There isn’t a one size fits all when it comes to who will manage the build. You can choose a build route that suits you, your budget and the type of extension you want to build.
Hire an architect
The first option is to have an architect or professional designer draw up plans for your scheme who can take you through the process until work gets underway on site. “Generally, if an architect is appointed for a full design service, we’ll assist the client in appointing and then co-ordinating the other specialists and consultants up to a start on site,” says Nimi Attanayake, founder and director at nimtim Architects.
This would encompass the co-ordination of the various consultants required before the project begins, including designers, structural engineers, party wall surveyors, the Building Control Officer plus any other specialists. Your architect will also see you through the planning process.
Once construction begins, project management responsibilities could then be passed onto the main contractor. “This role would encompass appointing, co-ordinating and managing the separate construction packages and contractors, such as foundations, structure and finishes,” says Nimi. “The architect’s role during construction would then be one of contract administrator – facilitating the contract between the client (you) and the builder. We’d inspect the site regularly to ensure the project is being built according to our information.”
Employ a Design and Build company
If you’d prefer to keep everything under one roof, there are design and build companies with in-house professionals who’ll be able to take you through from the start of the design process right up until completion. Or, for larger and more complex schemes, you might consider employing a professional project manager to oversee everything. Going down this route could save you time and potentially stress. Plus, using a professional PM brings the benefit of experience. They will be well-versed in managing the challenges that arise throughout the build because they’ll have done it plenty of times before.
Project manage the scheme yourself
If you’re working to a tight budget, project managing the scheme yourself can be one area to save some pennies. As well as giving you a greater degree of control over the budget and construction, it could be an enormously satisfying experience. “Project managing a construction scheme is always challenging and difficult, but it comes with its own rewards and satisfactions,” says Nimi. “Providing you are comfortable enough to undertake it, the feeling of satisfaction and reward may be an advantage in itself.”
Split the project management
It may be that you can split the responsibilities between yourself and your contractor. For instance, the building company could manage construction until the extension reaches watertight stage, at which point, you take over the role and project manage the rest of the sub-contractors (plumbers, electricians, plasterers etc.) yourself. Every project is different, so the key to managing your extension effectively is to come up with a strategy that maximises your skills, the expertise of the professionals you employ, and your budget.
Build Route and Costs
Which build route you choose will impact costs. Typical build costs based on routes are likely to be:
- DIY: £300-£700/m2
- Self-managed: £600-£1,400/m2
- Main contractor: £1,000-£1,700/m2
- Design and build: £1,200-£2,000/m2
Finding the Right Trades
Whether you’re looking for a reputable builder to bring in his own subbies or sourcing individual trades yourself, personal recommendation is the best way to find a high-quality professional. Family, friends and neighbours are a good first port of call when it comes to putting together your long list of potential companies. Failing that, your architect or designer should be able to recommend professionals they have worked with on successful past projects. Websites such as checkatrade.com and ratedpeople.com can also prove a useful mine of information if you’re required to reach out in search of completely new connections.
Once you’ve compiled a list of potential people, contact them for an informal chat about the project. Ask about their range of skills, experience and workload, using the conversation to try and gauge their level of interest in your project. For main contractors in particular, ask if you can speak to previous clients or see past examples of their work to ascertain the quality of the workmanship. Remember, good word of mouth is one of the best forms of advertising in the industry, so happy clients from past projects will often be more than happy to give their builder a plug. Find out if the person is registered as part of a competent persons’ scheme, too, such as the Federation of Master Builders (FMB) for your contractor or the National Federation of Roofing Contractors (NFRC) for roofers, and so on.
Once you’ve spent plenty of time digging into the details and speaking to various professionals, you’ll be able to begin whittling down the list of people you want to bring onto your project.
(MORE: Find a Builder)
Keeping Costs Down
Can I Build an Extension Cheaply?
One way to keep costs down is choosing a more budget-friendly construction system. Concrete blockwork is readily available and most builders know how to work with the material, too. However, some modern methods of construction, such as structural insulated panels, may cost more to begin with, but may save in labour on site, especially when it comes to insulating your extension.
Keeping the design and shape of your extension build simple — either square or rectangular shaped with a flat roof — will be less expensive than angled designs with more complicated roof structures and lots of glass.
And if you fancy a spot of DIY then there are tasks you can take on to help the budget, too. Plastering walls, laying flooring and tiles are all jobs that can be done on a DIY basis.
(MORE: Extensions for Every Budget)
Can I Build an Extension Myself?
Building an extension you really want and keeping costs within budget means it’s often worth considering doing some of the work yourself. As chartered surveyor Ian Rock says, "Anyone with an aptitude for construction may relish the prospect of learning new skills, taking genuine pride in their achievements. But as a general rule it’s best to leave anything seriously time-critical to the professionals, along with all the heavy duty structural work. If your extension needs to be completed sooner rather than later it makes sense to stick to tackling jobs near the end of the project, such as landscaping."
The type of work you choose to undertake when building an extension will depend on how happy and handy you are with the tools. Pick parts of the build to work on where there’s less risk of causing delays.
Here are some DIY jobs that are most suited to DIY input…
Timber stud partition walls are typically non-structural and should be a realistic project for the average DIYer to get to grips with. However, there’s usually time pressure to get them built so the electricians can start running first fix cabling. Installing door liners in the interior walls can also be worth doing, although these normally need to be in place before plasterboarding and plastering. Cutting and fixing skirting boards and architraves is reasonably straightforward, and can be left until the later stages, along with hanging interior doors and boxing in pipework.
The most widely undertaken DIY flooring project is laying floor coverings. Types of flooring such as tiles, vinyl planks, or thin strips of interlocking laminate or engineered timber are relatively simple to install. However, installing structural timber flooring to the joists upstairs, such as traditional softwood floorboards or chipboard panels, should also be a feasible DIY project. To provide a temporary work platform for the builders, large sheets of oriented strand board (OSB) or plywood can be put in place to use as a deck.
This allows you to postpone fitting the floor towards the end of the project, relieving time pressure and reducing the risk of pristine new floors getting damaged by building work or splashed with plaster. When it comes to the construction of ground floors however, these are not generally DIY-friendly because the insulation and floor screed work tends to be very time-critical and needs to be coordinated with other trades.
Good quality decoration is key to achieving a professional finish, often ingeniously compensating for the less-than-perfect surfaces other trades leave in their wake. Nonetheless, deploying paintbrushes and rollers with a basic degree of competency is well within most people’s abilities, if you make sure you know how to paint a room before you start. Taking on the role of the decorator is also appealing from a DIY perspective because it isn’t usually too time critical, coming towards the tail end of the project.
Wall and floor tiling are popular DIY projects that require dexterity combined with a logical mindset. Using good quality cutting tools is key to a successful outcome (specialist machines can be hired). It also greatly helps if you work to a clear plan and allow yourself plenty of time. As with all finishing trades you are to some extent at the mercy of those who have gone before; tiling onto perfectly true and level surfaces makes the job considerably easier!
What Modern Materials Can I use to Build an Extension?
There are a handful of modern methods that self builders have been using that could well be adopted by extenders to potentially make the job of building an extension that bit quicker and easier.
Beam & block floor
Also known as ‘suspended concrete’, this floor has the advantage of being much less prone to structural problems caused by ground movement. Beam & block floors are more suited to large extensions where the existing house has a suspended timber floor that needs continuity of ventilation.
Liquid screeds are much quicker to install by a specialist than conventional sand and cement screeds, but aren’t cost-effective for a small extension. If you’re adding UFH, then liquid screeds pair well.
The cross-section gives them their name, but also means they can carry far heavier loads than a conventional timber joist of equivalent weight.
Monocouche house rendering can be sprayed or pumped onto exterior walls, which reduces the need for scaffolding and site costs. It’s low maintenance, durable and available in a variety of earthy colours.
Prefabricated Roof trusses
Where extensions need to marry with existing roofs and walls, the need for customisation favours the traditional ‘cut timber’ route (cutting timber to size on site). However, if you’re building a large rectangular extension then prefabricated roof trusses (manufactured off site) can offer labour-saving advantages on site.
Will Building an Extension Add Value?
It’s all about balance. If you're likely to sell on eventually then you don't want to price yourself out of not recouping any of the money you spend on building an extension.
Building a small extension while adding value in terms of space it’ll create might not be that cost effective in how much it’ll add to the value of your house if you come to sell it.
Adding a larger extension should increase the end value of your home but it’ll cost more to build so be mindful not to spend more than you’ll make back on it. Plus, there’s generally a ceiling price that you’ll want to keep within so make sure the numbers add up.
Talking to local estate agents can help paint a picture of the local market and what extended homes similar to your own are selling for.
Building Under Permitted Development
What Size Extension can I Build Without Planning Permission?
The Planning Hub is a new online resource that will help you understand how to get to grips with complex planning rules. Join today for access to easy-to-read guides which will provide you with key information to help you navigate the planning system.
You might find that you'll be able to build an extension under Permitted Development (PD), which means you won't have to go down the formal planning route.
Do bear in mind that if your house is in a Conservation Area or a National Park, the amount of work under Permitted Development is usually reduced.
Under PD rights certain works can be carried out providing you meet certain criteria, such as:
- You can extend a detached property by 8m to the rear if it’s a single-storey extension (6m for a semi or terraced house), or by 3m if it’s double
- A single-storey extension can’t be higher than 4m on the ridge and the eaves, and ridge heights of any extension can’t be higher than the existing property
- Two-storey extensions must not be closer than 7m to the rear boundary
- Side extensions can only be single storey with a maximum height of 4m and a width no more than half of the original building
- Any new extension must be built in the same or similar material to the existing dwelling
- Extensions must not go forward of the building line of the original dwelling
- In designated areas (such as areas of outstanding natural beauty, conservation areas, etc), side extensions require planning permission and all rear extensions must be single storey
- An extension must not result in more than half the garden being covered
From 31st August 2020, the rules changed so that two-storey extensions on detached, semi-detached and terraced houses will be fast-tracked as long as they get prior approval. This means the local authority have to be notified of the details before the project starts and it’s a much more involved process with the Local Authority.
There are some restrictions an extension has to adhere to:
- Once works have been completed, the building must not be more than 18m high (excluding plants)
- The floor-to-ceiling height of any additional storey must not be more than 3m in height or higher than the floor-to-ceiling height of any of the existing storeys
- The overall height of the extension, including the roof, must not be more than 7m high.
If you’re planning on building a large extension then it will, more than likely, need planning permission and you will need to submit an application.
It's wise to engage with your local authority early on and research local planning policies to know what’s likely to get approved before you submit an application.
You can submit an application via the Planning Portal. An application in England for an extension currently costs £206.
Lawful Development Certificate
Do I Need a Lawful Development Certificate?
Even if your extension project can be done under PD rights, it’s worth applying for a Lawful Development Certificate (LDC) from your local authority to confirm that the work was lawful and met PD requirements and didn’t need planning permission. This is well worth doing if you plan to move, too.
It costs £103, half the normal planning fee.
How Close to a Boundary Wall Can I Build an Extension?
If building your extension involves digging or building foundations within 3m of the boundary, party wall or party wall structure, or digging foundations within 6m of a boundary, the work will require you to comply with the Party Wall Act.
Party Wall Act
Does the Party Wall Act Apply When Building an Extension?
You don’t need to have planning approval to serve a Party Wall notice. They are separate.
There are instances when the Party Wall Act applies to building an extension. Chartered surveyor, Ian Rock explains:
“For aspiring extension builders the part of the Act that’s often most relevant is where it applies to the excavation of foundations close to neighbouring buildings or garden boundary walls. In order to trigger this legal minefield, excavation normally needs to be within a critical distance of three metres from the adjoining property where your new trench is deeper than their existing foundations. Because older properties tend to have relatively shallow footings in most cases it’s a ‘given’ that the new ones supporting the extension will be considerably deeper.
"Where there is any doubt, it might be worth consulting any records of foundation depths or seeking an expert opinion, for example from a Building Control surveyor (since the neighbours may not relish the prospect of trial holes being dug next to their home to confirm actual foundation depths). Be warned, however, that in some cases excavating within six metres of an adjoining property can also be covered by the Act. But this only applies where the new foundations are so deep that drawing an imaginary line downwards at a 45° angle from the bottom of the next door’s foundations would hit them, for example on a steeply sloping site or where you’re incorporating a basement.
"There are two other situations where the Party Wall Act often applies. It’s not unusual for new extensions to be designed to maximise floor area by building right up to, or ‘astride’ the garden boundary between two properties. The second situation would be in cases where you physically cut or alter a party wall, such as where you want to build onto your neighbour’s existing wall so it becomes your new extension’s party wall. Happily, minor works like fixing screws or plastering onto party walls are not significant enough to be covered by the Act, but resting a new beam within the wall, as you might for a loft conversion, definitely would be.”
What to Consider Before Drawing up Plans
There are some important aspects to consider before you get to the stage of getting your plans drawn like:
Importantly, notify your insurer of the work. Some may not provide cover during the works, but others offer dedicated extension insurance products.
Employing an Architect
Should I Employ an Architect when Building an Extension?
Most of these bodies require full members to have relevant academic and technical qualifications. Whichever designer you choose, ensure that they carry sufficient professional indemnity insurance.
You've got an idea in mind but now you need someone to turn that sketchy line drawing into a plan.
You can opt for a build and design company, or you can work with an architect or architectural technologist. If you're competent with CAD then you can draw up plans yourself, although more complicated designs will benefit from a more expert eye due to the complexities.
Bear in mind, if you’ve designed the extension yourself you may need the aid of a designer or draftsperson to draw up your plans to submit them for planning approval (if required) and a structural engineer to produce drawings and calculations for Building Regs’ purposes. These will also form part of the tender documents when hiring builders.
(MORE: House Extension Ideas)
To choose the right person, it's always worth asking around among friends and family. And choose someone who's worked on similar projects before and understands what you're trying to achieve with the budget you have.
Do I Need to Comply with Building Regulations When Building an Extension?
Whether you require planning consent or not, you will need to make sure you comply with Building Regulations. To meet the Regs, you can either submit a Full Plan Submission or a Building Notice.
- Full Plan Submission: send plans to your local authority building control or approved inspector prior to the build for approval. The building inspector visits your site at different stages and inspects the work as it progresses
- Building Notice: a statement which lets the local authority know that you will be complying with the regulations in building your extension and gives the building control department 48-hours notice of your intention to start the work. Building inspectors will inspect the work at various stages and will advise you of any problems
A Building Notice is the riskier of the two as you may only find out you have a compliance issue once building work has started, which then needs to be paid to be put right.
All alterations to listed buildings, including internal ones, require consent and it is a criminal offence to alter a listed building without it.
With a listed building the planners will always regard the existing property as more important than what you are proposing to add to it. Any extension will therefore have to respect the flavour, appearance and historic material used in the construction of the original house.
Getting a warranty for your extension is advisable as it will cover you for things such as structural defects or faulty workmanship. Should the worst happen, the builders who did the work will need to return to put right what has gone wrong.
Extending over a Garage
Can I Build an Extension Over a Garage?
"If you’re building over an existing structure building control will normally want a couple of trial holes excavated to expose the existing foundations." Ian Rock, chartered surveyor
Extending over a garage is a popular project. The vast majority of modern garage walls are built from slender single skin brick or blockwork (115mm thick) buttressed internally with brick columns (piers) every couple of metres. This doesn’t necessarily mean the walls can’t provide the required support of a proposed new floor above, but a structural engineer will need to produce design calculations to prove it.
If you’re planning to convert the existing ground floor space for living accommodation the walls will need upgrading to new build thermal efficiency standards to prevent excessive heat loss.
Problems are more likely to arise with older garages where building over is rarely a practical proposition because of their very limited load-bearing capacity. Demolition is the only realistic option.
Heating and Electrics
Check Your Heating and Electrics Before Building an Extension
Before you start work you should reassess your heating requirements and check if your existing system is large enough to cope with the extra rooms you intend adding. If your boiler does not have the capacity it might be more economic to add a second system rather than replacing the boiler.
You may well pay less in the long term by opting for a separate electrical system rather than having to fork out a large lump sum for a new boiler. Many people opt for electric underfloor heating or electric radiators.
If you are adding a kitchen to your house you are likely to have to add a circuit that goes directly from the distribution board. For any other work, unless it is very extensive, it is usually possible to extend the existing ring circuit.
Ring circuits are restricted to 100m² but any number of sockets can be provided on this system.
An extension will give you the opportunity to add to your existing power points. Many people in this position take the opportunity to replace single socket outlets with double ones and install outside lighting.
Financing the Build
How to Finance an Extension Project
There are several options when it comes to borrowing money to finance your extension.
In an ideal world, we would all finance our projects using our savings, but if you need to borrow the money, your best options are:
Credit card: If you need to borrow thousands rather than tens of thousands for your work, an interest-free credit card is a good choice. Shop around — the best deals offer up to 27 months of zero-rated spending
Loan: Personal loans of up to around £25,000 are suited to smaller projects. The loan may be enough to cover the building work and if you are fitting kitchens or bathrooms in your extension you can often buy these on finance from larger stores. Just keep an eye on repayments as they can quickly add up when you borrow from several sources
Remortgage or secured loan: If you are borrowing more than £25,000 you may need to remortgage or take our a secured loan against your home. Many building societies offer a Home Improvement Loan of up to £200,000.
Your builder will generally stage payments for each phase of the project. Try not to make any payments upfront.
If it is necessary to purchase an expensive item perhaps a bathroom suite some months in advance of its installation, you should make sure it is in your name and not that of the builder, just in case they should go out of business.
Your designer will charge fees according to the work involved, so an accurate quotation of the fee would be a very useful thing to have before you give the go-ahead for plans to be prepared.
It is wise to check what any quote includes and who will pay any local authority application fees. Also, ask if any provision has been made should structural calculations be required.
Can I Reclaim VAT When Building an Extension?
VAT is zero-rated on new build in domestic properties including self builds, but this does not include extensions.
If you are renovating or extending a property that you can prove has been empty for 2 years or more, it will be treated as a conversion and therefore be charged at the reduced rate of 5%. The VAT concessions are only available via a VAT-registered contractor.
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