Building an Extension: A Beginner’s Guide
Before you start designing, planning and building your extension read our comprehensive beginner’s guide for everything you need to know
This beginner’s guide to building an extension covers everything you need to know before you start. From what to design to getting plans passed, from working out your budget to the trades and build – it pays to know what building an extension involves.
After all, you’ll be investing financially and emotionally in building an extension so you want the build to go as smoothly as it can do! So let us guide you through the things you need to know.
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.
Use our free extension cost calculator
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.
Don’t Forget
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.
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.
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.
(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
- Royal Institution of Chartered Surveyors (RICS)
- Chartered Institute of Architectural Technologists (CIAT)
- Association of Building Engineers (ABE)
- Chartered Institute of Building (IOB)
- Institution of Structural Engineers (ISE)
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.
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