"THE 45-DEGREE RULE" Sunlight, Daylight And Your Proposed Extension

What is the 45-Degree rule?

The 45-degree rule also known as the 45-degree code and 45-degree guide is a method used by Local planning authorities to measure the impact from a proposal on sunlight and daylight to neighbouring properties.

If you’re planning on building an extension, have you considered its impact on your neighbours? When planning authorities receive an application to develop a property one of the things they will consider is the effect of the proposal on neighboring properties. This includes natural sunlight and daylight.

So before we continue, what do we need to know about the sun:

  • The sun rises in the East and sets in the West
  • The Sun is due South at noon
  • The Sun is also at its maximum height around noon
  • The sun is lower during winter months with shorter days
  • The sun is higher during the summer months making our days longer
  • Direct sunlight is brighter than ambient daylight
  • Daylight is the natural light created between dawn and dusk

It’s your local authorities responsibility when considering an application to safeguard the neighbouring properties amenities.

When designing an extension we need to consider the level of sunlight and daylight currently enjoined by the neighboring properties. Some extensions can be poorly designed and sited, resulting in shadowing that can adversely affect the amenities to your neighbours main inhabited rooms to unacceptable levels.

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 minimize shadowing on to neighboring properties.

Factors effecting shadowing:

  • Height of the proposed development
  • Size of the plot
  • Building orientation
  • Distance from boundary
  • Topography

Your local planning authority will generally follow BRE guidelines and apply the “45-degree guide” in cases where a proposed development may affect neighboring properties.

The purpose of the 45-degree guide is to make sure the proposal does not take away too much daylight. It is based on the notion that it is reasonable to expect a certain level of light and unobstructed view from a habitable room window.

There are two methods used for applying 45-degree rule.

Method 1: Considers the depth and width of the extension.

Method 2: Considers the height of the extension.

If the proposed extension breaks one of the 45-degree splay lines you local authority may view your proposal as unacceptable.

To make these methods more clear please see the diagrams below.

Method 1:

In method 1 we mark the centre point of the neighbours nearest window and draw a 45-degree splay line towards the proposed extension.

45degree bev 1a.png


Example A is to the left and B is to right.

In example A the extension is reduced in depth where it is located outside of the 45-degree splay. This would normally be considered an acceptable development.

In example B the extension depth is too much and the new proposal falls within the 45-degree splay. This would normally be considered unacceptable.


Method 2:

In Method 2 we mark the centre point from the middle of the nearest habitable ground floor window of your neighbouring properties. From this point we draw our 45-degree splay line out towards the proposed extension.



Example A is to the left and B is to right.

In example A the proposed extension height is located outside the 45-degree splay line. This would normally be considered acceptable.

In example B the proposed extension height is too great and falls within the 45-degree splay. This would normally be considered unacceptable.


Developments under Permitted Development?

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