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3 Tips on Maximum Permitted Extension Size

Updated: Oct 23, 2019

This Article was created in order to bring a closer view to the answer that is the concern of the many, which is to define a maximum size of an extension which is permitted within their Local Area. 

There are two types of Rear Extensions: 

  • Extensions, which will fall under Permitted Development Rights (meaning they won't need Planning Approval)

  • Extensions, which will require Planning Application

How to find out whether your extension requires Planning Permission?

Tip no. 1 - Understand the 45 degree Sunlight Rule

The 45-degree rule is a method used by the Local Planning Authorities to measure the impact of the proposed extension on the right to sunlight and daylight of the neighbouring properties. 

If you're planning to build an extension, you need to consider how this will influence your neighbours. It’s your local authorities responsibility when considering an application to safeguard the neighbouring properties amenities.

The 45-degree rule is assessed on both plan and elevation. An extension should not exceed a line taken at 45 degrees from the centre of the nearest ground floor window of a habitable room in an adjoining property.

Rooms such as bathrooms, halls, utilities and landings/stairs are not generally considered and overshadowing to garden areas rarely constitute sufficient grounds to justify a planning refusal.

Protecting Existing Daylight

Extensions and new buildings should be designed to minimise shadowing on to neighbouring properties.

Factors affecting over - shadowing:

  • Height of the proposed development

  • Size of the plot

  • Building orientation

  • Distance from boundary

  • Topography

In practice, and from our experience, the above rule tends to disqualify two storey rear extensions from mid terrace properties. End of terrace, semi detached and detached properties are more likely to be able to accommodate a two storey extension

Developments under Permitted Development?

The 45-dregree rule does not apply to Permitted Development legislation.

Tip no. 2 - Maximum Depth

Not all changes or improvements to your home need permission from the planning department. There are many that you can carry out with implied consent, known as Permitted Development.

For semi-detached properties the maximum acceptable depth of the permitted rear extension is 3.5 metres, provided that a minimum of 50% of the back garden remains.

With terraced houses, including end of terrace, the maximum permitted depth, is 3 metres, provided a minimum of 50% of the back garden remains.

For detached properties this dimension grows to 4 metres. Anything above 4m will require a Full Planning Application. 

Tip no. 3 - Maximum Height

In general the maximum height for nearly all types of single storey rear extension is 4 metres, which includes the roof ridge. From our experience most extensions fit within 3.5 metres height including the roof. 

The maximum acceptable height measured from ground level to the eaves of the roof shall not be higher than 3 metres. Based on our experience an average household eaves height is between 2.5 - 2.8m.

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