Building an extension is a big financial commitment and emotional experience so you want the build to go as smoothly as it can do.

And for that to happen, it pays to know what an extension project can entail. From the design stage through to planning, from working out costs to the actual build, this beginner’s guide to building an extension covers everything you need to know before you start.

sensitive extension built on a cottage

This listed cottage has been subject to a sensitive extension with glazed walls to let in lots of natural light

How Much Will Building an Extension Cost?

Depending where you are in the UK, for a straightforward extension you should allow around £1,000–£2,000/m2. But, the cost of your extension will all come down to a number of factors, including size, specification and location.

Building a single-storey extension will cost the following per/m2:

  • Basic quality: £1,000 to £1,680
  • Good quality: £1,680 to £1,920
  • Excellent quality: £1,920 to £2,160

Remember: Be mindful of ceiling prices in your area — you’ll want to make a return on the money you spend on an extension so make sure the numbers add up.

Building a two-storey extension will not cost much more per square metre because, aside from the extra interior fixtures and finishes, you are only adding walls and floor joists — a roof and foundations are required whether your extension is single or two storey.

What Should I Consider Before Building an Extension?

Any Shared Walls

If building your extension involves building or digging 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.

Site Access

How easy will it be for deliveries to be made to your home when building an extension? You’ll need to factor in how trucks and lorries will reach the property and unload large items and materials. You’ll also need to determine where trades will park and store their tools.

Demands on Your Services

Don’t forget to give your current services a health check. Don’t assume that your electrics, heating and plumbing will be able to cope with lighting and heating extra space. For example, while replacing the boiler is an option, you could also look at alternatives such as underfloor heating.

This extension features a classic neutral colour scheme throughout with French limestone-inspired Champagne porcelain floor tiles from Valverdi.

Insurance

Importantly, notify your insurer of the work. Some may not provide cover during the works, but others offer dedicated extension insurance products.

Sponsored by Protek

What Insurance Do I Need if I am Extending my Home?

If you are carrying out extension works and are managing the project yourself, you should arrange Extension Insurance to cover the new works and the existing structure. This is because most home insurers will exclude loss or damage whilst the property is undergoing alteration or renovation.

It’s worth discussing your project with a specialist site insurance provider like Protek as extension projects can be complex and often include liability assumed under the Part Wall Act 1996.

Site insurance caters for both the existing element of the property that’s being extended and all the new extension works that go into the process. The existing structure is usually your house – so if the property collapses while creating a new opening for example, the renovation insurance will cover it and completely replaces the requirement for buildings insurance, which is not suitable.

All the works, including any temporary works, materials, plant tools and equipment need to be covered. Public liability and employers liability is automatically included to ensure you are adequately protected.

You may also need to consider a 10 year structural warranty on the new extension works.

Extension insurance needs to be in place from the moment you plan to start works on the property and should continue to the point the project is completed and taken into full use.

two storey house extension

A run-down cottage is now double in size thanks to a striking timber-clad extension. Image: Simon Maxwell

And You Should Also Think About…

Other important aspects to consider before you get to the stage of getting your plans drawn are matters like:

  • soil conditions on the site
  • services
  • surrounding trees
  • any history of flooding
  • rights of way.

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Pairing old with new, this listed barn conversion features a single-storey extension constructed using cross-laminated timber (CLT)

Pairing old with new, this listed barn conversion features a single-storey extension constructed using cross-laminated timber (CLT)

Do I Need Planning Permission When Building an Extension?

Not necessarily. In many cases you will be able to build an extension under Permitted Development (PD) (these rights allow certain works to be carried out to your home providing you meet the criteria).

Under PD, the following rules apply:

  • You can extend a detached property by 8m to the rear if it’s a single-storey extension, 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

You should 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.

If you’re planning a significant extension you’ll likely need planning permission and will need to submit an application. Engaging with your local authority early on and researching local planning policies to know what’s likely to get approved is a good idea.

You can either apply for consent via the Planning Portal or through your local authority. An application in England for an extension currently costs £206.

What is a Lawful Development Certificate? And Do I Need One?

Even without the need for planning permission, 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. It costs £103, half the normal planning fee.

single storey house extension

This copper-clad single-storey extension was added to a detached family home by Andris Berzins & Associates / ArchitectYourHome Camden and features sliding doors by IQ Glass UK

Building Regulations for 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.

Listed Buildings

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.

Warranties

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.

How to Design an Extension

One of the first things you’ll need to think about is who is going to design your new extension. You can choose to design it yourself, opt for a build and design company, or you can work with an architect or architectural technologist.

Side extension to stone property

This larch-clad extension has provided much needed extra space in this converted barn

(MORE: 20 Extension Design Ideas)

Finding an Architect or Designer

Choosing the right person to draw up your plans is as important as choosing the right builder, and the prospect can be just as daunting.

The key is to do your research, look at previous work and ideally speak to past clients, and choose someone who most closely aligns with your design aspirations, and who you feel you can work with best.

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.

Useful Contacts

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.

A stark contrast against the existing home’s London stock brick façade, the new rear extension has been clad in Marley Eternit’s fibre cement cladding — the colour of which is mirrored in the powder-coated aluminium window frames

A stark contrast against the existing home’s London stock brick façade, the new rear extension to this home has been clad in fibre cement cladding

Building an Extension – Planning the Work

Who Will Manage the Work?

You can use a design and build company who will manage the build project for you but if you’ve used an architect or designed the extension yourself then you’ll need to find a main contractor to manage the project.

Alternatively, you could manage the build and hire subcontractors, or take on a project manager to sort it for you.

Sites like Checkatrade.com are a good place to start your search. But, word of mouth is often a great way to find someone locally — speak to friends and family and ask them for recommendations.

Finding a Builder

If there is one thing that is more difficult than finding the right builder, it is getting on with him throughout the project.

Obviously a good recommendation helps, but it is wise to talk to previous clients of the builder. You should also ensure that they have contractors all risks insurance.

If the builder has sufficient information there is no reason why they should not be able to give you a fixed price detailed quotation. Day work rates can be a recipe for disaster for all sorts of reasons. Try to avoid them except for extras requested at your behest although sometimes it is impossible to do so.

If VAT is to be added to the price given, make sure any quotation has a VAT registration number on it and a VAT receipt is provided when payment is to be made. It has been known for some less reputable operators to use the VAT system as a means of adding more onto the bill which they then keep for themselves.

Tradesmen

It is enormously helpful if you manage to find a small group of tradesmen, all of whom have several trades under their wing. This can be crucial in keeping continuity. Time wasted can prove very expensive for a variety of reasons: e.g. scaffold and plant hire, additional labour and rent.

You should also remember that anyone hiring his/her own labour effectively becomes the builder. There is no contract with a main contractor and the buck stops with you. If you take on this role, you will have to manage:

  • trades
  • materials
  • services
  • keeping the site tidy
  • relevant site insurances and public liability insurance.

Safety and Disruption on Site

Try to keep the mess to a minimum. You can do this by:

  • Using masses of protective plastic sheeting wherever appropriate
  • Sealing off the rooms being worked on.
  • Provide welfare facilities (rest area, toilet and access to a kettle) for your builders.

A frameless glass extension to this period property provides a stark contrast between old and new

A frameless glass extension to this period property provides a stark contrast between old and new, and offers uninterrupted views of the garden

Should You Move Out While Extending?

Decide from the outset whether you’ll be able to cope with the disruption.

A large extension project with remodelling work planned might be less stressful if you move out, but for others, you may be able to seal yourself off from the dust and the work, and find it cheaper to stay put.

Always factor the cost of accommodation into your extension budget.

Building an Extension Over a Garage

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.

Adding Heating and Electrics When Building an Extension

Extending Your Central Heating

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.

Extending Your Electrics

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.

How to Pay for Your Extension

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

Two extensions have been added to this period property as part of a major remodel

Two extensions have been added to this period property as part of a major remodel

Payments

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.

VAT

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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Comments
  • Chris Garbett

    Hi. I want to extend the back of my house. I want to do a wooden construction to save cost. I will be building on top of solid decking that’s well built. I will be joking onto my side garage and next doors garage. The front of it will be white cladding or timber like in some of your photos on the website. I will have a felt roof sloping and French doors and a window. This will be built onto my brick house. My next door are ok with it so can I just go straight ahead. I know my limits on size. Any help appreciated. Thanks.

  • Chris Garbett

    Hi. Any advice on the above would be appreciated. Thanks.

  • Lindsey Davis

    Hi Chris,

    The question is whether you can do it under Permitted Development. It is hard to say going just off the information you have given as it is to do with widths, and how much of the space in your garden has already been used up by outbuildings or existing extensions. It also depends how close to the boundary it is. Whether your neighbours are happy or not is sort of irrelevant because the needs of future neighbours needs to be considered too, so building so near to the boundary may be an issue.

    My immediate concern would be about constructing an extension on decking. I would suggest you seek professional advice from someone who can advice on the structural needs.

    I would also recommend something more hardwearing than felt. It has problems from UV exposure and also the chippings often wash off and block gutters. The lifespan is only 15-25 years so it has the shortest life expectancy of roof coverings. Try a synthetic polymer which will last 25+ years of a Polymer modified asphalt which can last up to 60. They are more expensive, but it is a false economy installing something which you need to replace more often and which could cause other problems like blocked gutters.

    I would seek more advice on this if I was you. Contacting the local authority planning department would be advisable too.

    Lindsey

  • Chris Garbett

    Hi. Thanks. My widths and length are ok and my garden size is ok. My next door have done the same as me. Roof materials I can change. I am getting advice on the decking. Thanks for the help. Does this info provide anymore help.

  • Mary Sunderland

    can anyone tell me where this extension is? I want this extension!!!
    Seriously – admin – can you give out the owners or builders details?

  • Lindsey Davis

    Hi Mary,

    The house is in Eskbank in Lothian, Scotland. Their architect was Craig Amy of Edinburgh: http://craigamy.com

    You can see more about this project here: http://www.homebuilding.co.uk/completedprojects/a-striking-cedarclad-extension-0

    Lindsey

  • Mary Sunderland

    thanks!

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  • Patrick Taft

    Undoubtedly useful article for many however I must provide clarification as there are some untruths which are misleading:

    1. RIBA fees scales were abolished in 1982. So to say that fees scales are the reason why many do not choose architects for small projects is entirely false. Disappointing to see that a specialist website appearing to offer help is actually stating false information.

    2. Architectural tehnology is an entirely different discipline to architecture and it is incorrect to say that a technologist has pursued the same path as an architect only to not complete part three. Becoming an architectural technologist has a route of its own and there is no crossover.

  • Patrick Taft

    3. The article suggests that there are alternatives to architects that all provide an equally adequate service for designing an extension. This again is false and shows a real lack of understanding about the roles of the disciplines mentioned. A structural engineer is an expert in structure and absolutely the best person to provide information for all structural elements, e.g. beam calculations, foundation design, etc etc. A building surveyor is an expert in carrying out surveys, assessing construction defects, advising on building maintenance etc. However none of the above are architects! They are not designers and it is staggering how this article puts forward the notion that anyone of those disciplines can simply have a go at designing a building. This is why there are so many bad buildings and so many bad extensions out there. You wouldn’t let an orthopaedic surgeon operate on your heart would you! Architects do not proport to know all about structure, neither do they claim to be maintenance experts – so why is everyone all of a sudden an architect!?

    Sadly due to the abolition of the RIBA fees scales the fees of architects are very low – so you’re not going to save a huge amount in getting a non-architect to do the work. And more to the point – it is a false economy not getting the right person to do the job because ultimately it will not be anywhere near as good in every way!

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