"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.


Do I need planning permission for a conservatory?

conservatory planning

This is one of our most common questions and is normally followed up by will we need building regulations too?

Firstly I want to give you a building regulation definition of a conservatory, this is important as these factors determine to your local council the difference between your conservatory being a conservatory or an extension:

“A single storey part of a building at ground floor level, where the roof and walls are substantially glazed with a translucent or transparent material, intended for growing plants, but often used as occasional living space. A conservatory has not less than three-quarters of the area of its roof and not less than one-half of its external walls made of translucent material."

This development although the roof is over 75% translucent and has over 50% translucent walls would not be considered a conservatory due to not being separated from the main house. In this case this development is not exempt of building regulations.

This development although the roof is over 75% translucent and has over 50% translucent walls would not be considered a conservatory due to not being separated from the main house. In this case this development is not exempt of building regulations.

So we want to address this as simple as possible to make it understandable for all. Do you need planning permission?

There are some exceptions but in general no. Your conservatory can be built under permitted development rights, subject to the limits and conditions below:


  • Conservatories (including any previous extensions) must not exceed more than half the area of land around the "original house"*. This includes other buildings including sheds and outbuildings.
  • The conservatory is not forward of the front or side elevation facing a road (highway).
  • Rear conservatories must not extend beyond the rear wall of the “original house”* by more than three metres if an attached house, or by four metres if a detached house.
  • In addition, outside Article 1(5) designated land** and Sites of Special Scientific Interest the limit is increased to 6m if an attached house and 8m if a detached house until 30 May 2016.These increased limits (between 3m and 6m and between 4m and 8m respectively) are subject to the neighbour consultation scheme.
  • The maximum height of a single-storey rear conservatory is four metres.
  • A conservatories maximum eaves height that is constructed within two metres of the boundary must be no more than three metres high.
  • Maximum eaves and ridge height of conservatory must be no higher than the existing house.
  • Side conservatories are to be single storey with maximum height of four metres and a maximum width of no more than half the width of the “original house”*.
  • On designated land* there is no permitted development for rear extensions or conservatories of more than one storey; no cladding of the exterior; no side extensions or conservatories.
  • The permitted development allowances above apply to houses only.
  • Flats, maisonettes and other buildings are not included in the permitted development regime.
  • Conservatories proposed to a listed building need listed building consent.

* The term "original house" means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date). Although you may not have built an extension to the house, a previous owner may have done so.

** “Article 1(5) designated land” - this is land within a National Park, the Broads, an area of outstanding natural beauty, an area designated as a conservation area, and land within World Heritage Sites.


If you are still in doubt you should always check with your local councils planning department or your architectural service professional. Although planning may not be required you can also obtain a “certificate of Lawfulness”.

A certificate of lawfulness is confirmation from you local councils planning department that your proposed conservatory doesn’t require planning permission.


Does my conservatory require building regulations?

Conservatories are normally exempt from building regulations but there are limits and conditions to check before you start construction:

A Building Regulation application will be required for a conservatory in the following situations:-

a) If the conservatory is to be located in a position that will obstruct access by ladder to an escape window e.g. first floor inner room, second floor alterative escape window, etc. (Regulation 3 (2) material alteration affecting B1 – Means of escape).

b) If the conservatory is likely to obstruct access or facilities for the Fire Service (Regulation 3 (2) material alteration affecting B5 – Access and facilities for the fire service).

c) If the conservatory is likely to affect access or facilities for disable people (Regulation 3 (2) material alteration affecting Part M – Access and facilities for disabled people).

d) If the position of the conservatory is likely to impair the airflow across a balanced flue and subsequently the correct functioning of a boiler (Regulation 3 (2) material alteration affecting a controlled fitting).

If the above items (a) to (d) are met, a conservatory will be exempt from the requirements of the Building Regulations providing:-

1) It is single storey at ground level.

2) Your conservatory has a floor area that does not exceed 30m2.

3) Has at least 75% of its roof and 50% of its exterior walls made of translucent material as stated above in the definition of a conservatory.

4) Has glazing that satisfies Part N of Schedule 1 in the most recent Building Regulations document.

5) It is separated from the dwelling by existing doors or windows. (must not be open to the existing dwelling.

6) For energy conservation the conservatory should be unheated, but if fixed heating installations are proposed, they should have their own separate temperature and on/off controls.

As of the 1st January 2005 any electrical work within the conservatory (the source of which is located within or shared with the dwelling) should be carried out by a person registered under a ‘Competent Person Self-Certification Scheme’


We hope this article helps you with your conservatory planning permission questions. This article has been written as advice and we always advise you to thoroughly research your project before making those steps to construction. If you have any question please do not hesitate to get in contact with us here at