BRE revises guidance on access to daylight and sunlight

In June 2022, BRE published a third edition of Site layout planning for daylight and sunlight: a guide to good practice to reflect BS EN 17037: 2018 Daylight in buildings.

This is widely used by local authorities, designers, surveyors and consultants across the UK and Ireland when assessing daylight and sunlight issues for planning applications; although these planning assessments and guidance should be distinguished from the issue of rights to light, a separate area of law that is not part of deciding planning applications.

While parts of the daylight and sunlight guidance remain the same as in the second edition, there have been significant changes and additional advice in other areas, particularly the way provision to new dwellings should be assessed.

Assessments of daylight and sunlight for planning purposes should therefore now be carried out with reference to the latest guidance and the methodologies it outlines.

These methodologies include simple initial angular criteria and rules of thumb such as the 25° obstruction angle, which serves as a basic measure of the external obstruction to daylight for a window.

More detailed assessment methods including vertical sky component (VSC) – a measure of daylight to the outside of a window – and daylight distribution, measuring how much of a room can receive direct daylight. The latter is also referred to as no-sky line, which separates areas in a room that can receive direct daylight from those that cannot.

Probable sunlight hours for the whole year and for winter are measures of direct sunlight, which are still used to assess loss of sunlight to existing living rooms and conservatories. Loss of sunlight to gardens is assessed using the hours of sunlight received on 21 March, with the spring equinox offering representative conditions between summer and winter as an average for the year.

The revised version of the guide includes additional advice to practitioners, for example on how to use a weighted average related to glazing size for a room lit by multiple windows, as well as on appropriate presentation of the results.

It also suggests numerical targets to assess the potential loss of radiation to an existing solar panel if a development is proposed nearby, to ascertain whether any such loss is significant.

This is based on a comparison of the radiation that would be received with the proposed development in place and existing conditions, as well as the slope of the panel from the horizontal.

The average daylight factor (ADF), for instance, is no longer used as a measure of daylight in such dwellings, and has been replaced by a choice of two methods.

The first of these uses a target illuminance – a measure of the amount of light incident on a surface – that should be achieved over a proportion of an assessment area for at least half of daylight hours, referred to as spatial daylight autonomy.

This involves climate-based daylight modelling, which uses the weather data closest to the site's location to determine average natural lighting conditions throughout the year on at least an hourly basis.

The alternative method uses a daylight factor target based on a value also to be achieved over a proportion of an assessment area. This daylight factor is calculated using a standard overcast sky, so – unlike the illuminance method – the results do not vary by site location or orientation. However, site location is still accounted for different target values by latitude.

Either method can be used in an assessment. A discussion and comparison of the two methods is given in CIBSE Research Insight 07.

Meanwhile, the UK national annex in BS EN 17037 and the revised BRE report give additional recommendations. These include minimum illuminance targets that bedrooms, living rooms and kitchens should achieve over at least half of an assessment area for at least half of the annual daylight hours.

Equivalent daylight factor targets are also given, which are referred to as median daylight factor targets. The previous recommendations were in contrast based on mean targets; that is, ADFs.

The recommendations given for dwellings in the UK national annex and BRE guide are lower than those in the main BS EN 17037 standard, and were designed to be closer to the previous ADF targets. However, it is important to note that the targets should be seen as minimum recommendations rather than aspirational ones.

The revised guide sets out best practice for these calculations, including advice on which surface reflectances to use, the assessment grid of calculation points, window framing, and a glazing maintenance factor to account for dirt on windows.

When assessing sunlight provision to new dwellings the BRE guide no longer uses the probable sunlight hours methodology, in response to BS EN 17037. Instead, this is assessed according to the amount of time that windows can receive direct sunlight on 21 March, assuming cloudless conditions.

Similarly, assessment of sunlight provision to proposed gardens and open spaces is based on the areas that can receive at least two hours of sunlight on the spring equinox.

To ensure their assessments follow the latest recommendations, therefore, designers, surveyors, consultants and local authorities using the BRE guide now need to be clear about the implications of all the changes made to it.