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Design Your Extension in 10 Easy Steps


It can be a daunting prospect to realise your dream extension. With so much to consider and budget for, it can be challenging to know where to start. This handy 10-point checklist will help you prioritise the important stuff and get the ball rolling on your project.


  1. Planning

Extensions often come under Permitted Development rules unless you are dealing with a listed building or designated build area. Permitted Development allows for quite of bit of work but it’s handy to familiarise yourself with the particulars to avoid any issues. Here is a quick summary:

  • Detached house rear extensions can be:
    • Up to 8m long in a single storey dwelling
    • Up to 3m long in a double storey dwelling
  • Semi-detached or terrace house can only be extended by 6m in a single-story home.
  • Side and rear extensions cannot be higher than 4m to the ridge and eaves.
  • Side extensions are limited to being single storey and must not be more than half the height of the original dwelling.
  • Double storey extensions can be no closer than 7m to the rear boundary of the property.
  • Extensions must not cover more than 50% of the garden.
  • A continuity of materials must be kept between the original home and the extension.
  1. Function

Why do you want an extension? It is very easy to become obsessed with the look and feel of a new space without thinking about its use. How will your room work amidst your day-to-day life? What will be the implications of your new extension on the adjoining rooms? You want to create a space that you’ll want to actually use. Brand new spaces can become redundant if they aren’t built to meet your needs.

  1. Services

Your standard services should be a big part of your designing process. Making notes of drains, cables and other pipe works need to be mapped out fully to avoid an unwanted discovery during building.

  1. Trees

Trees are big obstacles to overcome before you begin building. The space they take up can be a big problem during construction so it is often the case that they will need felling. Contact your local council to ensure that there are no tree preservation orders in place before you get the axe out.

  1. Soil

The type of soil you will be building on can have a massive impact on the construction process. Get in contact with your Local Building Control to find out which type of soil you are working with. The type of soil you are building on will determine the type of foundations you need. It might be worth conducting a soil survey (yes that is a thing) to get the full picture.

  1. Access

How will you get that big crane onto your property? Access is something that regularly ends up being a last-minute thought on projects but without proper consideration, it can be a real problem. Diggers and other bits of machinery don’t tend to fit in small estate gardens.

  1. Brief

Architects produce their drawings to meet your design and functionality needs. The more information you give them, the closer the design will be to what you want. Collate pictures of extensions you like and research materials that you would be interested in. Be sure to give the architect a realistic budget, this will allow them to achieve a bespoke design that isn’t impossible to achieve.


  1. Getting a design

When your chosen architect produces their complex drawings there are few important things to consider:

  • Will the extensions interior and exterior work seamlessly with your home?
  • Does the design capture the best views of your property?
  • What can you fit in your new space? Will there be enough room for everyone?
  • Is it what you want? Don’t be afraid to say!
  1. Lawfulness

Apply for a Lawful Development Certificate on the interactive Planning Portable website. This will relieve of any concerns relating to Permitted Development. It will also prevent any post-build reviews from the council as you will have already been given the go ahead.


  1. Planning Application

Most planning applications are decided within a period of eight weeks. It is always advisable to give yourself more time to prevent any close calls. Submit the application via the online Planning Portal website or ask your designer to do this for you.