Building an Extension

A Beginner’s Guide to the Planning Process

Before you start designing, planning and building your extension read our comprehensive beginner’s guide for everything you need to know.

From what type of design you dream of, to getting the actual plans passed, from working out your budget to choosing the trades and build – it pays to know what building an extension involves. After all, you’ll be investing financially and emotionally in improving your home 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 actual cost 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

Building a double 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.

What should I consider before building an extension?

Any Shared Walls

If building 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

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 .

And Don’t Forget to Think About…

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.

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.

Do I need planning permission?

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.

Does your planned home improvement project need planning permission? If you are wanting to knock down a rebuild or build a huge extension, then yes, you will need need permission from your local authority before you begin.

But there are numerous smaller improvements that you may be surprised to learn can be undertaken without planning permission. Projects such as installing rooflights, converting your garage, changing your cladding or adding a conservatory can all be completed with implied consent known as permitted development (PD).

General Questions

The typical questions we get asked for all types of home extensions with key information.

We Can Help & Advise

There are limitations to these rights, especially if you have already made considerable alterations to your home.