Let there be light

1st February 2010

Let there be light

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Environmental light impacts can form a reason for planning refusal and/or expensive design work. Gregory Francis outlines the general principles to guide the non-technical environmental practitioner.

Dense urban environments are coming under increasing pressure as owners of land look to maximise site potential. This is especially true when developments seek to match the bulk and height of neighbouring properties.

But environmental light impacts in this sort of situation can be far-reaching and so consulting local authority case officers early on can often bring benefits, especially at a stage in the design process when changes can more easily be implemented.

Early discussion can also be valuable in terms of gauging local authority expectations of environmental light impacts both within and adjacent to the proposal and can potentially influence later detailed design work.

Early consultation also offers the opportunity to discuss openly any unavoidable impacts and demonstrate the mitigation measures proposed, taking account of the need to make the most efficient use of the site and using an adequate development density.

In this context, lower target values than those set out in the BRE guidelines may need to be agreed with the local authority to enable development to take place.

And negotiating early on can also make life easier when it comes to realistic environmental light impact target values where development master plans and government targets call for greater site density.

These early discussions can give both the client and the design team greater confidence in taking the proposals forward to planning submission, mindful that the council has inputted at an early stage.

The local authority can also then obtain a good understanding of the potential objections that might be raised and allow it to request additional assessments if needed.

In addition, not all local authorities possess a detailed knowledge of environmental light issues so early consultation can also benefit in providing a clear understanding of the tests and impacts.

Guidance documents

There is a suite of guidance documents used for the assessment of environmental light impacts. These are considered nationally-applicable guidance documents and, in the absence of mandatory legislation, inform planning policy documents such as local development frameworks, unitary development plans, code for sustainable homes, BREEAM and supplementary planning guidance/documents.

Littlefair, P J (1991), ‘Site layout planning for daylight and sunlight. A guide to good practice' published by Building Research Establishment (BRE)

This document, commonly referred to as ‘the BRE guidelines', forms the backbone of environmental light impact assessment methodologies.

The document contains methodologies for the testing of external daylight, sunlight and overshadowing to sensitive receptors, with references to further documents containing tests for internal daylight and solar glare. As the guidelines are not mandatory, there is a degree of flexibility afforded in their application.

BSI (2008), BS 8206-2:2008: ‘Lighting for buildings - part 2: code of practice for daylighting'

This document, referred to in the BRE guidelines, contains the suggested methodology for testing internal daylighting. It has recently been updated to highlight that windows, and the light they allow into spaces, have an important role to play in affecting people's health (both mental and physical) over and above simply achieving minimum illumination for task performance.

Building Research Establishment, Department of the Environment (1987), BRE information paper 3/87 ‘Solar dazzle reflected from sloping glazed facades'

This document, also referred to in the BRE guidelines, contains guidance on avoiding excessive solar glare and also a methodology for predicting the duration of periods when it might be an issue to indentified sensitive receptors.

The Institution of Lighting Engineers (ILE) (2000): ‘Guidance notes for the reduction of light pollution'

This document contains design guidance on avoiding excessive spillage of light from new developments, offering a method to determine significance according to context.

Technical terms

There are a handful of quantitative assessments which are undertaken in order to understand the severity of environmental light impacts. A qualitative assessment of their significance normally accompanies the result.

The 25° test, given in the BRE guidelines, is a quick initial check to see whether external daylight to properties adjoining a proposed development is likely to be affected. It is a simple test offering an initial steer only and therefore has several flaws.

The vertical sky component (VSC) test, meanwhile, is outlined in detail within the BRE guidelines and is considered the most appropriate test for measuring external daylight impacts to sensitive environmental receptors as a result of a proposed development.

This test compares external daylight both pre and post development and gives an indication of the severity of any impact.

But BRE guidelines only provide pass/fail impact assessments; therefore when included in an EIA an author must also express their professional opinion as to the significance of the results, taking into account any relevant associated factors.

Average daylight factor (ADF) is briefly mentioned in the BRE guidelines, but is covered in greater detail within BS 8206-2:2008. The assessment quantifies the level of internal daylight in a habitable room, using the VSC and taking into account the glazing details, room dimensions and internal finish.

The room use is taken into account in determining the significance of an ADF result. Again, a qualitative statement by the author is often required to determine significance.

Another test looking at skyline, the no-skyline test, is outlined in the BRE guidelines and gives an indication of the likely distribution of internal daylight within a room. For example, it will indicate whether light is concentrated in a certain area of the room or spread evenly throughout.

When assessing a proposed development, comparisons are made between the existing and proposed conditions and these are judged against simple pass/fail criteria contained within the BRE guidelines.

Annual probable sunlight hours

The annual probable sunlight hours (APSH) test, also outlined within the BRE guidelines, considers the potential number of sunlight hours a window assessment point will receive compared to the maximum possible.

It also distinguishes between an entire year's total and the number of hours in winter months, giving guidance on the recommended amounts of each.

Once more, when assessing a proposed development comparisons are made between the existing and proposed conditions and these are judged against simple pass/fail criteria contained within the BRE guidelines.

The BRE guidelines also contain an area-based methodology to assess the amount of permanent shadow to an identified sensitive environmental receptor, giving guidance on recommended levels of overshadowing.

If the target values are achieved, a certain level of reduction is permissible when comparing the baseline condition.

External daylight - ambient light from the sky which can be observed at the external face of a window

Internal daylight - ambient light from the sky penetrating the room, based on the external daylight and taking into account the volume of the room, size and type of windows and colour of walls, floor etc

Sunlight - direct light from the sun which can be observed at the external face of a window. As the sun follows a southerly path in the UK, the potential to receive sunlight is orientation specific

Overshadowing - shadow(s) cast by buildings, normally assessed for amenity spaces where people would be expected to spend extended periods of time such as parks, picnic areas and gardens

Light trespass - artificial light which is emitted from a development or specific light source at night. Can also contribute to light pollution over cities and other centres of population

Solar glare - reflected sunlight from building facades. Can cause hazardous dazzle to pedestrians and road users


There are common design methods for overcoming issues related to light, once they have been identified. Windows are an obvious choice and need to be a minimum size and specification in order to achieve the target level of internal daylight compliance within a proposed development.

Balconies are another area to look at. Their presence on both existing and proposed buildings often creates issues by limiting the access of daylight and sunlight. We frequently advise design teams on the optimal positioning and sizing of balconies for new development in order to achieve target levels of daylight and sunlight compliance.

In addition, detailed early consideration of orientation can reduce the potential for excessive overshadowing of amenity spaces and increase the potential for sunlighting to habitable rooms. Reduction of solar glare can also be achieved through orientation.

The location of tall elements of a scheme can greatly affect the degree of compliance both within the proposed development and for adjacent properties. Careful consideration of the location of reflective elements can also reduce solar glare impacts.

BRE guideline flexibility

The BRE guidelines are not mandatory. The guidelines allow for flexibility in their application, and this flexibility needs to be considered with the overall benefits a proposal may bring.

Clearly daylight, sunlight, overshadowing, solar glare and light spillage are important considerations in the preparation of planning applications and accompanying environmental statements.

But they can be emotive topics, often raised during public consultation. Changes to lighting conditions can be amongst the impacts most keenly felt by local residents and other land users.

Early consideration of these issues in the design process allows potential impacts to be managed effectively and reduced as far as possible as part of the design evolution. This early involvement can help avoid unnecessary delays and costs further down the line.


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