Looking to extend your home?

🕑 6 minute read

Building an extension on your house

Building an extension is one of the best ways to leave your mark on your home. It could even add more value to the property when it comes to selling up.

However, it takes more than just finding the right contractor to arrange an extension. A lot of planning happens before work begins and you’ll need to consider permissions, budgets and timelines before moving forwards.

Where to start when building an extension

Firstly, you need the right builder on board to help arrange your building extension. Many building firms specialise in extensions and can take care of everything from laying the bricks to fitting out the interior.

You should also find a tool hire company in your local area. You may need a digger, cement mixer, a skip and plenty of other equipment to build an extension. Your builder should be able to advise here.

Types of extension

When it comes to building an extension on a house, there are plenty of options – most of which will have planning permission considerations. The most common types of extension include:

  • Single storey extensions - built onto your house, this extension type needs careful planning for openings, roofing and plumbing
  • Double storey extensions - similar to single storey but on a bigger scale, you may need to consider structure more carefully
  • Outbuilding, orangeries and conservatories - a simple version of a single storey extension
  • Cellar or basement conversions - an underground extension that may be restricted due to causing neighbourhood disruption. You’ll also need to comply with Building Regulations for ventilation, drainage and more
  • Wrap around home extensions - forms an L-shape, usually behind your current home

Do I need planning permission for an extension?

In most cases, you’ll need planning permission to build an extension. Planning permission is the green light from your local council to go ahead with the construction.

There are many reasons why planning permission is required, primarily to ensure your extension can be built safely and securely. Without it, your local council are within their rights to halt your extension and order the removal of any building work already constructed.

To get planning permission, you’ll need to contact your local planning authority (LPA) through your local council. You can also use websites such as Planning Portal to start, view and submit your application online.

Once the council has your application, they will look at your proposed project and make a judgement on whether to approve it or not. This includes exploring factors such as:

  • Layout and design - will your extension block light, appear unsightly or affect your neighbour’s privacy?
  • Noise - how much disruption and noise will the construction cause?
  • Conservation - will the build impact a listed building, protected species or a nature reserve?

For smaller projects, you may not need planning permission beforehand. This is because it might fall under the government’s ‘permitted development rights’. Speak to your builder or LPA to check this.

How much does a home extension cost?

As with most things, the cost of building a home extension depends on several factors, including the:

  • Size of the extension
  • Quality of the materials
  • Location of your home

For example, a single storey extension in London can cost around £2,200 to £3,300 per square metre.

For more complex projects, like a basement conversion, costs can vary depending on the level of work and use. You might need to consider things like lowering the floor level, digging a new space and creating an access point.

Outside of construction costs, you’ll also need to consider factors such as:

  • Hiring a consultant - architects may charge around 7% of the construction cost to plan and design your extension. So, for a 30 square metre single storey extension in London, that's around £5,000
  • Applying for planning permission - a householder application costs £206 in England
  • Acquiring a site survey - it can cost from £350 for a single storey extension to get a structural report for your home
  • Taking out insurance - many types of home insurance don’t cover damage caused by building works. So you may need to take out an additional renovation insurance policy
  • Furnishing the space - windows and doors, plumbing and even furniture can set you back thousands of pounds

What to think about when building a home extension

There are all sorts of potential pitfalls when building an extension. Fortunately, it’s easy to plan for most of them.

These are some of the things you should consider before embarking on an extension project.


Architects and structural engineers can be expensive. But if you want a job done right, it’s best to work with the professionals.

They can draw up plans that abide by Building Regulations and hand them to your builder, making everyone’s lives easier. Make sure you choose a professional with the right credentials and a portfolio that includes projects that are similar to yours. For example, a chartered architect ensures work of a high standard.

Building Regulations

Any extension built on your land must comply with Building Regulations. This includes a list of rules, from fire and insulation to drainage and access.

To make sure your plans are safe and secure, you will need to submit a Full Plan Submission or Building Notice to your council. Without it, you could face fines, legal disputes and your extension may be stopped.


You’ll need to make sure you can connect your extension to the vital networks in your home, including the electricity, central heating and plumbing systems (including water and sewers). For this, you’ll require detailed plans of your existing utilities and will need to hire professionals for the job.

Ground quality

If you are building on new ground, you will need to make sure there’s enough support to manage the weight of the structure. You should also check that the ground is not at risk of flooding or subsidence. A land surveyor can certify ground quality before you start planning.

Site access

Throughout the project, builders will need to bring materials from outside to the construction site. For this, you’ll need to make sure there’s a good pathway between the road and your site.

A good builder will advise on solutions, even if the only site access is through your home. You may also want to consider hiring a skip to store any waste and keep the site clean – your builder will be able to help getting a skip arranged.

Right to light

Always consider your neighbours when building an extension, especially when it comes to their ‘right to light’.

Every homeowner has the right to enjoy the natural light that passes over someone else’s land. So, if your extension project will cast a shadow over their property, you may not receive planning permission and will need to adjust your plans.

How long does it take to build a home extension?

There is no set timeline when it comes to building an extension on a house. This is because many factors can delay your build, including:

  • Waiting for planning permission
  • A string of bad weather
  • Complaints from neighbours
  • Size and difficulty of your build

Usually, the whole process can take anywhere between 6 and 15 months. This includes time for designing, planning and sourcing the right contractor.

For the build itself, smaller projects such as loft conversions and single storey extensions can take between 6 weeks to 4 months. For wrap around and double storey extensions, it can take more than 6 months. These timings are guidelines only, and as with all construction work, timelines can shift once the work begins.

Paying for a home extension

If you’re considering extending your home, there are lots of options you can look at when funding an extension.

The best way to stay on top of the costs is to set a budget in advance. This should be realistic, accounting for factors such as design, professional fees and decorating the space afterwards.

Many contractors and builders work on a fixed budget contract. This can give you a clear picture of the overall costs and when payments are due.

A few ways you can keep the costs of your building extension down include:

  • Limit the skip - instead of throwing everything into a skip, look for ways to repurpose materials like flooring and fixtures. If you don’t need them, you might make money selling them on
  • Use reclaimed materials - you can save money by using second-hand materials. Search salvage yards nearby or check the marketplace for private sellers
  • Keep the design simple - the most cost effective build is a square outline with a pitched roof. Try to avoid building on tricky terrain, such as near drainage, to keep costs down
  • Shop around - when looking for an architect, contractor or builder, don’t be afraid to negotiate the price. Get multiple quotes from various professionals, and ask about their experience of working on a budget

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