Skip to main content

Building an Extension: The Ultimate Beginner's Guide

building an extension - single storey side extension
(Image credit: David Barbour)

Building an extension is an exciting project to take on. The ideas you have of making your home work for you by adding the space you need is what building an extension is all about.

But, it's a big commitment - financially and emotionally - so you'll want to get the design, build and budget right.

And this is where our ultimate guide to building an extension can help set you off on the right path. Beyond how many storeys you're going to build and where you want to add the extension, there are lots of things to consider - from site access to services like heating and electrics; from Party Wall agreements to the materials you'll use - all of these elements will affect how long building an extension will take to build and how far your budget will stretch.

So before you build an extension, take a look at this complete guide for an overview of where to start and how to design, plan and build what you need.

(MORE: Need a quote for your extension?)

How Much Will it Cost to Build an Extension?

How much it costs to build your extension will be dependent on a number of key factors, such as the shape and size of the build, the construction system and materials you will use to build and clad the extension and whether you project manage the works, for example. 

Costs around the UK will vary but also be mindful of current material shortages and a demand for trades as these all impact costs.

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: Homebuilding.co.uk 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 an extension kitchen furniture

(Image credit: Higham Furniture)

How to Build an Extension Cheaply

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 when building an extension by 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. 

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. See 'Can I Build an Extension Myself?' below.

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…

Internal joinery 

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. 

Flooring

The most widely undertaken DIY flooring project is laying floor coverings such as tiles, vinyl planks, or thin strips of interlocking laminate or engineered timber. 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 screeding work tends to be very time-critical and needs to be coordinated with other trades. 

Decoration

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.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. 

Tiling

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! 

Building an Extension

(Image credit: Thames Valley Window Company)

Using Modern Materials 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 Screed

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. 

I-joists

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 render

Monocouche render 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 to My Home?

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 an extension with porcelain flooring

Porcelain tiles used inside and out can create a seamless transition between the new extension and garden. Midlake Grey porcelain tiles from Porcelain Superstore have been used in this open-plan kitchen living space. (Image credit: Porcelain Superstore)

How Big Can I Build an Extension Without Planning Permission?

Join The Planning Hub

Join The Planning Hub

(Image credit: Future)

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

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

extension built in oak

(Image credit: Arboreta)

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.

What is a Lawful Development Certificate and do I Need One?

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)

Do I Need to Comply with the Party Wall Act When Building an Extension?

Good to know

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.”

Plan the Design 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. 

barn conversion ideas and extension

(Image credit: Jean-Christophe Godet & Soup Architects)

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)

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.

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 When Building 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 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.

(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.

Can I Build an Extension Over a Garage?

Trial Holes are Essential

"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

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.

Check Your Heating and Electrics Before Building an Extension

Modern, Light Kitchen Extension

(Image credit: Getty Images)

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 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.

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.