How big can I make my extension?

In the past, your local authority would decide on how far  out you could construct your (rear) extension. They would also be responsible for deciding the size of your side extension where applicable. 


In 2012 Parliament introduced Permitted Development, this new relaxed approach to predominately extensions was introduced to encourage development and helped to take the pressure off planning departments processing applications.

Extension size under Permitted Development
Extensions fall under “Class A” of Permitted Development; This allows a householder to build a single-storey side extension up to half the width of the existing dwelling; a single-storey rear extension up to 4m in length for a detached dwelling and 3m long for a semi or a terrace house.

May 2019 Permitted Development Changes
The changes that took effect on 30 May 2019 now make permanent the decision that larger single-storey rear extensions of up to 8m (6m for semi or terrace) are permissible under Class A — but do require prior notification.

Permitted Development Classes
As well as extensions Permitted Development covers all other aspects of building works;

Class B – Additions to the roof – applicable for Loft Conversion
This allows for rear dormers and hip-to-gable extensions if the additional volume created does not exceed 50m3 (40m3 for semis and terraced homes).
Class C – Other alterations to the roof
Class D – Porches
Class E – Buildings etc. (outbuildings) – applicable for Annexes and Garages

How far out can you build your extension?

Under the new updated Permitted Development rules, you can go out as far as 8 metres from your existing house if you own a detached property. This reduces to 6 metres if you own a semi detached or terraced property.

Designated Areas and Permitted Development
In some areas of the UK, the Government has created 'designated areas', in these areas Permitted Development rights are more restricted.

Current Designated Areas:
· A Conservation Area
· A National Park
· An Area of Outstanding Natural Beauty
· A World Heritage Site or
· The Norfolk or Suffolk Broads

Check with your local planning office

Your local planning authority may have removed some of your Permitted Development rights by issuing an 'Article 4' direction. We always recommend that you check with your local planning office and enquire about Permitted Development restrictions as well as if you fall under a designated area.

Remember if you are building on or near one of your neighbours’ wall/Boundary Line you will need to have a Party Structure Notice served. Also, Permitted Development is different from building regulations and you will need to have a Party Wall agreement for rear extension or party wall agreement for loft in place where applicable – get in touch with us for further advice of Party Wall Matters.

Thinking of adding an extension on your house? Need some advice on house extensions? Contact us to find out more and get assistance with your project.

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