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Building an Extension? Follow This Complete Guide to Transform Your Home

Building an Extension
This homeowners of this house had a single and two storey extension built to transform the house into a light-filled family home. Glazing designed and installed by Thames Valley Window Company. (Image credit: Thames Valley Window Company)

Building an extension is no mean feat but the rewards when you get it right can change how you live in your home for the better. 

But, there’s no denying there’s a whole host of things to consider, even before you lay a single brick, and it doesn't stop once you're in the throws of the project either.

So if you’re building an extension for the first time and are looking for comprehensive advice to walk you through what you need to know from your budget right through to the build, then our complete guide is well worth a read.

(MORE: Need a quote for your extension?)

How Much Will Building an Extension Cost?

The costs of building an extension are dependent on a number of factors, namely where you are in the UK, the size of the extension, how many storeys you're building and the level of spec. 

For a straightforward extension you should allow around £1,000–£2,000/m2. 

Although building a two-storey extension seems like a lot of extra building works, it won't actually cost much more per square metre than building a single storey extension. 

Aside from the extra interior fixtures and finishes, you are only adding walls and floor joists — a roof and foundations are required how ever many storeys you're building.

(MORE: How Much Does an Extension Cost?)

Editor's Note: partners with the UK's best extension specialists to match your requirements with their services. Simply answer a few questions on what you need from your extension and we’ll put you in touch with a suitable partner.

Building a kitchen extension with rooflights

This rear and side extension to a Victorian house in Enfield has provided the owners with the light-filled open-plan kitchen they craved. The contractor, Parkwood Builders, suggested Roof Maker’s Luxlite unit to complement the Crittall doors, allowing light to flood into the space. (Image credit: Roof Maker)

What's the Cheapest Way to Build an Extension?

The cheapest way to keep costs down when building an extension is to keep the design and shape simple so a single-storey square or rectangular shaped addition with a flat roof will be less complicated than angled designs with more complicated roof structures. 

You can also keep costs down but considering the building materials and construction system you choose. A concrete sub floor and concrete blockwork to construct the walls will be a more budget friendly option than say oak frame or timber frame for example. 

Concrete blockwork is readily available and most builders know how to work with the material, too. 

“There are three main types of concrete blockwork known as ‘dense’, ‘lightweight’ and ‘aerated’,” explains chartered surveyor Ian Rock. “Costs range from as little as £10/m2 for cheap dense blocks to around £30/m2 for high performance aircrew blocks.”

And if you fancy a spot of DIY then there are tasks you can take on to help the budget, too. Plasterboarding the internal walls, laying flooring and tiles are all jobs that can be done on a DIY basis. 

Will I Add Value by Building an Extension?

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. 

Adding 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 a light-filled extension

This kitchen from Higham Furniture, which combines contemporary and classic elements in equal measure, has a striking roof lantern above to flood the space with lots of natural light. (Image credit: Higham Furniture)

How Big an Extension Can I Build Without Planning Permission?

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

(MORE: 23 things you can do without needing planning permission)

kitchen extension

Martin Moore provided the bespoke kitchen suite for this oak frame extension to a Grade II* listed 14th century cloth hall in Kent.  (Image credit: Martin Moore)

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.

Do I Need a Lawful Development Certificate to Extend?

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.

Building an Extension

The homeowners of this detached home wanted a better and more seamless connection between house and garden so built a contemporary side extension garden room. The glazing system was supplied by Thames Valley Window Company. (Image credit: Thames Valley Window Company)

What Should I Consider Before Building an Extension?

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.

1. The Design

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: 20 Extension Design 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. 

French limestone inspired Champagne porcelain floor tiles from Valverdi

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

2. 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.

3. The 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.

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

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

Timber clad extension to run down cottage

A run-down cottage is now double in size thanks to a striking timber-clad extension.  (Image credit: Simon Maxwell)
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What Building Regulations Do I Need to Adhere to 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.

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

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 (Image credit: IQ Glass)

How to Project Manage an Extension

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 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.

(MORE: Choosing a Build Route)

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.

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.

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.

Light and airy kitchen with vaulted ceilings in new extension

The vaulted ceilings in this extension and neutral colour scheme really open up the space to create a light and airy kitchen (Image credit: Naked Kitchens)

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.

building a small extension

Kloeber supplied the aluminium KustomFold bifold doors for this extension to a Victorian house in Cambridge. The doors were manufactured to be taller than standard units to match the high ceilings of the property. (Image credit: Kloeber)

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.

How Should I Finance my 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 added to Victorian house

Two extensions have been added to this period property as part of a major remodel (Image credit: Jeremy Phillips)

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 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.