Information Sheet 22


When people look at extending their home one of the first questions is often "will I need Planning Permission?" There is often a reluctance to engage with the Planning process; a fear of potential delays, objections, appeals and escalating costs. Not to mention the stress. While the Planning Process is nothing to fear if approached in a professional and practical manner the process can be avoided if your extension is within the 40m2; allowed under the legislation for exempted development. While 40m2 (430ft2) may not seem much at first it equates to an extension of slightly over a of the average three-bed semi of 1200ft2.

If time is an issue and the accommodation you require can be provided within the restrictions, then exempted development maybe the way to go. However, there are many other issues to be considered other than just staying within the 40m2 limit and it is critical to have an architect involved from the earliest stages to ensure that the extension complies with the legislation. It is also important to have someone willing to sign off on the development as complaint, and that person should be an architect.

Summary of some of the considerations in designing an exempted development:

The proposed extension must be to the rear of the property and must be "invisible" from the front elevation.

The area of the extension, measured from the internal face of the external walls, must not exceed 40m2. If you are planning a two-storey extension to a semi-detached or terraced property only 12m2 is allowable at first floor level while 20m2 is allowable at first floor level for detached houses.

The height of the extension cannot exceed the height of the eaves or parapet of the existing house.

The highest part of the roof of the extension cannot be higher than the highest part of the roof of the existing house.

The height of the walls of the extension should be no higher than the rear wall of the house.

The new extension should not reduce the area of private open space at the back of your house to less than 25m2.

Any windows at ground floor level should be set back a minimum of 1 metre from the boundary they face while any windows at first floor level should be set back at least 11 metres from the boundary they face.

It should be born in mind that under the current planning system you will require planning permission to demolish any existing extension, even if you are proposing to replace it with an exempted development, i.e. a new extension under 40m2 etc.

If you are planning to add to an existing extension the maximum exempted floor area is 40m2 for both the existing and proposed extensions combined.

The construction of a garage is exempted so long as it does not extend in front of the existing building line, is a maximum of 25m2 does not exceed 4m in height for a pitched roof or 3m for a flat roof.

Conversion of a garage etc.:

The conversion of a garage, store, shed etc. which is attached to the side or rear of your house is normally exempted development so long as it complies with the 40m2 limit.

©Holly Park Studio Ltd