3D Design Bureau

Following the publication of the 3^rd Edition of BRE Guidelines in June 2022, it is now a requirement to include trees within a daylight and sunlight assessment. Depending on the metrics being assessed for Impact Assessment or Scheme Performance, evergreen and deciduous trees need to be included in different states within the analytical 3D model.

This blog aims to help you understand how and when to include trees in a daylight and sunlight assessment.

It is important to note that it is generally not possible to accurately represent trees in a digital 3D model as the size and shape will differ greatly from tree to tree. When modeling trees in 3D for daylight and sunlight assessments, assumptions may need to be made and tree geometry may need to be simplified. Models of proposed trees within the development should be included according to the information provided by the landscape architect and at the full intended height and spread at maturity.

Trees for testing Vertical Sky Component (VSC) and Annual / Winter Probable Sunlight Hours (APSH / WPSH)

The BRE Guidelines state that when assessing the effect a new development would have on existing buildings, it is usual to ignore the effect of deciduous trees. This is because daylight is at its scarcest and most valuable in winter when most trees will not be in leaf. Evergreen trees should be included, particularly where a dense belt or group of evergreens is specifically planned as a windbreak or for privacy purposes.

Trees for testing Sun On Ground (SOG)

The BRE Guidelines state that when assessing the impact of buildings on sunlight in gardens:

“…trees and shrubs are not normally included in the calculation unless a dense belt or group of evergreens is specifically planned as a windbreak or for privacy purposes. This is partly because the dappled shade of a tree is more pleasant than the deep shadow of a building (this applies especially to deciduous trees).”

As such, deciduous trees are not included in the calculation of SOG in either the impact or scheme performance assessments. Evergreen trees should be included, particularly where a dense belt or group of evergreens is specifically planned as a windbreak or for privacy purposes.

Trees for testing Sunlight Exposure (SE)

The BRE Guidelines state that as deciduous trees would not be in full leaf on the recommended assessment date (March 21st), sunlight would be expected to penetrate deciduous trees. However, as trees have so many variables, it is impossible to accurately represent how they would affect sunlight at a given time. In BRE 209 the suggested methodology to allow for this is to run the sunlight exposure study in two states. Once with trees as opaque objects and secondly without deciduous trees in the assessment model. This gives a range of potential sunlight hours.

Tree for testing Spatial Daylight Autonomy (SDA)

BRE 209 recommends when assessing daylight in a proposed building, it is appropriate to run the assessment with trees represented in both winter and summer conditions. Light transmittance values should be applied to deciduous tree canopies for winter and summer assessments respectively. A light transmittance value should also be applied to evergreen trees throughout the year.

I.S. EN 17037 does not give any guidance on how trees should be represented.

For testing No Sky Line (NSL)

Because some sky can usually be seen through a tree canopy, deciduous trees are not included in the No Sky Line assessment model. Evergreen trees may be included in this assessment, particularly if there is a dense belt or group planned for windbreak or for privacy purposes.

If you need advice on your project with regard to daylight and sunlight, get in touch with our team who will be more than happy to shed more light on your project! For more information on daylight and sunlight assessments, you can also visit the articles below or book a free presentation with the team by contacting us.

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• What’s New in the BRE Guidelines 3rd Edition 2022
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