3D Design Bureau

Daylight and Sunlight Assessment: Importance of the analytical 3D model to protect your planning application from judicial review

A daylight and sunlight assessment is a critical report of many planning applications. Whether your development is in the early stages of design, or already well advanced towards a full planning submission, it’s essential to give close attention to this type of environmental assessment to assist in a positive outcome of your planning application. A poorly executed study can raise questions about the validity of the assessment and leave the application exposed to refusal, further information or worse still, a judicial review. A key component of an accurate daylight and sunlight assessment is the analytical 3D model.

In this article, we provide five essential considerations for your analytical model to ensure an accurate and robust daylight and sunlight assessment.

1. Model Geometry: Proposed Scheme

It is paramount that the 3D model for the proposed development (buildings) is based on the architectural drawings and details. This is to ensure that accurate results are generated, particularly from a Scheme Performance point of view. Key details that should be included as standard in analytical 3D models are:

  • Accurate massing design including exact footprint at correct levels and proper building/parapet heights.
  • Window design and their accurate placement – fenestration, exterior and interior reveals etc.
  • Balcony details including balustrade design.
  • Accurate internal layouts.
  • Inclusion of internal features such as kitchen cabinets and built-in wardrobes.

Whilst a simple massing model is sufficient for initial Daylight and Sunlight impact assessments, the same cannot be said for a pre-app submission or full application. Assessment of buildings that are inaccurately modelled will result in inaccurate daylight and sunlight results and potentially leave them open to a challenge.

Despite having the same size and height, variations in window design can provide different glazing results. This highlights the importance of having a detailed 3D model.
Figure 1: Despite having the same size and height, variations in window design can provide different results. This highlights the importance of having a detailed 3D model.

2. Proposed Landscaping Design

It is inappropriate to have 3D models of buildings floating without context or simply with a single plane applied underneath the building models. Failure to include proper hard landscaping design within the analytical 3D model (which should include areas of paths, paving, grass, planting zones, roads etc) will impact the accuracy and credibility of the assessment.

The reason for this is that light acts differently depending on the type of surface it hits. Light bounce from grass is less than light bounce from bright-coloured paving. Therefore daylight results within ground floor units, for example, can be affected by the types of surfaces that are located directly outside of them. So it is important to identify all ground surface types as accurately as possible in the analytical model.

Consideration to proposed hard landscaping will ultimately help advise the landscape architects, and the wider design team, to make changes if required. This may, for example,  involve modifying the type of material used outside ground floor windows in a courtyard setting. This may help ensure the development meets the recommended levels of daylight (SDA) within ground floor units in what can be a constrained area of a development.

3. Trees

Whether you like it or not, trees are to be included within your analytical model, and not just existing evergreen trees. Depending on the metrics being assessed for Impact Assessments or Scheme Performance, evergreen and deciduous trees, existing and proposed, need to be included in different states within the analytical 3D model. Therefore early communication with the landscape architect on a project is important.

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 modelling 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.

Read more

What’s New in the BRE Guidelines 3rd Edition 2022
An Expert Guide To Daylight and Sunlight Assessments

BRE 209 provides guidance on how deciduous trees should be treated depending on the study being carried out. Not correctly including trees in assessments will lead to less accurate results and expose the application to a planning refusal or a potential JR if the scheme is granted. This is because the planning authority could be accused of making a decision to grant without all of the correct information on hand.

A perfect example of this would be the following: Not including proposed trees in a SDA assessment despite some trees being shown on the landscape plan located directly outside the windows of well-performing rooms. It would be extremely difficult to argue that such proposed trees have no effect on daylight results. Furthermore, Appendix C of the new BRE Guidelines 2022 (BRE 209) it clearly states:
‘If trees would impact the daylight to the new development, they should be taken into account’.

For full details on the inclusion of trees within daylight and sunlight assessments (for all daylight and sunlight metrics) please visit our dedicated blog on the subject. Our approach to the inclusion of trees has been validated by the BRE.

4. Surrounding Environment: Baseline Model State

A 3D analytical model of a proposed scheme, incorporating neighboring structures. The image highlights a window featuring a 25-degree parapet angle, intersecting with the top of the proposed building, requiring an assessment of the existing space.
Figure 2: An analytical 3D model of a proposed scheme, incorporating neighbouring structures. The image highlights an existing window featuring a 25-degree projection from its centre point, intersecting with the top of the proposed building – therefore requiring an impact assessment.

Surrounding context, and the state of the existing subject site, must be included as accurately as possible within the analytical 3D model. This should include modelling of neighbouring buildings and environments, such as back gardens, boundary wall details, and the inclusion of existing trees (see point 3 above). All existing context such be modelled, where available, from survey information and geo-located. Surrounding buildings, and their receptors (i.e. the windows to be assessed) should not be shown floating in the overall analytical model. The proposed development should be accurately federated to the surrounding context through the use of the site’s topographical survey information.

“Neglecting to include proper surrounding context in assessments will result in more favourable results due to less obstruction of daylight and sunlight. Whilst initial reactions or thought may be GREAT we get better results, this is NOT a true reflection of the real world conditions and validity of the results can be very easily called into question.”, – Nick Polley, Managing Director at 3D Design Bureau.

5. Material Palette and Reflectance Values

The image highlights the impact of material differences on reflectance values. Notice how the lighter brick material yields more favorable results it generates.
Figure 3: The image highlights the impact of material differences on reflectance values. Notice how the lighter brick material yields more favourable results.

Reflectance values, as per the third edition of the BRE Guidelines, updated in June 2022 are numerical values assigned to surfaces that determine the amount of natural light that is reflected and absorbed by the surface. Different surfaces, such as walls, floors, ceilings, and ground cover, will produce varying daylight results depending on the level of reflectance applied. All reflectance values used in an assessment MUST be stated within reports.

This means that reflectance values must be accurately applied to ALL surfaces within the analytical 3D model, with no exceptions. This in itself feeds back into the importance of properly detailed modelling of the proposed development in particular.  

As the saying goes, the results you achieve are only as good as the information and work you put in. This rings very true with regard to the analytical 3D model for daylight and sunlight assessments.

If you would like to know more about the subject or would like our team of specialists here at 3D Design Bureau, to review your project, do not hesitate to get in touch. Our high-quality assessments are fully compliant with current guidelines and can be a crucial factor in securing the success and approval of your development.

We will be more than happy to shed more light on your project!