Whether it’s a simple kitchen diner or a two-storey wing, extensions are the most popular home improvement project. They can also be the answer to creating the extra space you need when moving house isn’t something you either can or want to do.

Before the building work begins, our beginner’s guide will cover everything you need to know, including:

Why Should You Extend Your Home?

Moving house costs money — there are agent fees, legal fees, stamp duty, as well as the premium for that desired extra bedroom or bigger kitchen diner. And for the same price, you might well find that you can build a decent extension, adding both space and value to your existing home in the process.

This listed cottage has been subject to a sensitive extension

This listed cottage has been subject to a sensitive extension

What Do You Need to Consider Before Extending?

Your Neighbours

This is particularly important if you live in a semi-detached or terraced property and share a party wall, where any work carried out could potentially damage their property.

If 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.

Before building work can start, you will need a written Party Wall Agreement from all affected neighbours. To obtain this, you must submit a Party Wall Notice (a letter of acknowledgement for the neighbour to complete and return; it must give two months’ written notice before any works start which may affect the party wall or boundary).

Site Access

Secondly, you will need to factor in how deliveries, trucks and lorries will reach the property. Access to site is an issue which can often get overlooked, and if there is no suitable driveway space, narrow lanes, etc, then you will need to assess how large items and materials will be unloaded, and also where trades will park and store their tools.

This 1930s home features a single-storey extension to house a new kitchen diner and offer the family with an improved living arrangement

This 1930s home features a single-storey extension to house a new kitchen diner and offer the family with an improved living arrangement

Demands on Your Boiler

Before you start work you should figure out whether your existing boiler will be able to cope with the demands of the new extension. While replacing the boiler is an option, you could also look at alternatives such as underfloor heating.


And, importantly, notify your insurer of the work. Some may not provide cover during the works, but companies such as Self Build Zone offer dedicated extension insurance products.

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.

Another wise move is to get to know someone who has done a similar extension. They might have a builder or particular tradesman to recommend (or not recommend) but either way they will be full of useful tips on how long to allow for different tasks and many other matters.

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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)

How Much Does an Extension Cost?

As with any project, the cost of your extension will come down to a number of factors, including size, specification and location.

Depending where you are in the UK, for a straightforward extension you should allow around £1,000–£2,000/m2. Experienced renovator Michael Holmes says 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.

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.

Single storey extension to brick property

The ambitious and well-thought out kitchen extension project has improved this home’s suitability for modern life

How to Finance Your Extension

There are several options when it comes to borrowing money to finance your extension. However prudence is the watchword — if you borrow more than you need you might do unnecessary things in the course of the job and have difficulty paying it all back. So always be conservative.

In an ideal world, we would all finance our projects using our savings, but with recent times having offered poor interest rates, the financial landscape has been one that favours borrowers over savers. If you need to borrow the money, your best options are:

  1. 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
  2. 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
  3. Remortgage or secured loan: If you are borrowing more than £25,000 you may need to remortgage your home or take our a secured loan against your home. Many building societies offer what is known as a Home Improvement Loan of up to £200,000
Side extension to stone barn

A contemporary timber extension provides a great contrast to the more traditional granite farmhouse


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.

If necessary you should go to the place where it is stored and make sure the ownership is transferred to yourself.

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 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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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 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

Will I Need Planning Permission for my Extension?

When it comes to designing your extension, you might find that what you have in mind can be carried out without needing to apply for planning consent — Permitted Development (PD) 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 one can do 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 planningportal.co.uk or through your local authority. An application in England for an extension currently costs £206.

Contemporary single storey extension to Victorian home

Despite being on an awkward corner plot, these homeowners have added a strikingly contemporary and contrasting extension to their Victorian home

Listed Buildings

All alterations to listed buildings, including internal ones, require consent and it is a criminal offence to alter a listed building without this consent.

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.

Depending on the size of the extension you propose, you might need planning permission as well as listed building consent to make your alterations.

Local Authority Grants

Unless your house is in an exceedingly poor state or is listed Grade I or Grade II*, or you are disabled, you are likely to have great difficulty in obtaining a grant for your work. However some organisations will offer help to maintain a period property (usually only to bring it to a basic standard of living to avoid demolition), so enquire with your local authority.

Building Regulations

Whether you require planning consent or not, you will need to make sure you comply with Building Regulations. The Building Regulations cover a range of factors, from access and fire to insulation and drainage. 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.


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

Architect or Architectural Technologist?

What’s the difference? Only those who have successfully completed seven years of training and are registered with the Architects Registration Board (ARB) can officially be called an architect.

They can often (but not always) offer a complete service, from the creative design right through to producing drawings for Building Regulations’ purposes, and most become a member of the Royal Institute of British Architects (RIBA).

Architects are a good choice if you’re wanting to build an innovative extension or live in a listed or period property and want to extend.

The skill set of an architectural technologist tends to sit somewhere between the creative side of building design and in building science, engineering and technology. However, many chartered architectural technologists will be very creative and capable of producing innovative designs. Likewise, many architects will also be highly technical in their approach.

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.

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

Updating the Access

Access problems can be a nightmare if you are extending. The issue usually raises its head in towns or on bends in the highway.

The scenario is that more bedrooms mean more bodies and so probably more cars. If your house is in a town and there is a shortage of off-street parking it might be a requirement that you provide parking space on your land.

If this worries you, remember that you do not need planning permission to provide a new access off an unclassified (i.e. lower graded than a B) road. Fortunately an awful lot of suburban roads come into that category.

Building an Extension

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 or the Federation of Master Builders (FMB) are good places 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. Time and time again it is the human element that fails. Put simply: can you get on?

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.


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.

Heating and Electrics

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.

If you are remodelling the entire house or a large section of it try to have all of the pipework and any other first fix work completed before starting on the plastering to avoid having to start hacking plaster off again.

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.

Articles like this 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.


  • 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


  • Mary Sunderland


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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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  • […] or even as a home office. If you are happy with your home but just need the extra space, then an extension to your current home could be a great alternative to packing it all up and moving. This might not […]

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