Daylight, sunlight and overshadowing assessments
Ian Roach, from URS, gives a practitioner's view on applying BRE's guidance on considering a development's impact on access to light in urban areas
It is right that tall buildings should be designed to minimise, as far as possible, their impacts on the daylight and sunlight levels received by neighbouring properties and the levels of shadow cast in existing open spaces.
There are well-established design principles for this, notably, Site layout planning for daylight and sunlight: A guide to good practice published by the Building Research Establishment (BRE) (the "BRE Handbook").
The introduction to the first edition of the BRE Handbook from 1991 states that it was created to provide advice on site layout planning to achieve “good sunlighting and daylighting, within buildings and in the open spaces between them”.
The guide was created for building designers and their clients, consultants and planning officials, and BRE was clear that its advice was not mandatory, but aimed to “help rather than constrain the designer”.
While providing numerical guidelines, the BRE stressed that these should be interpreted flexibly because “natural lighting is only one of many factors in site layout design”.
The second edition, published in 2011, makes a subtle change, stating that the “numerical target values within it may be varied to meet the needs of the development and its location”.
Similarly, guidance for developments in Edinburgh acknowledges that in some circumstances there may be sound urban design reasons why new development should not satisfy lighting standards. For example, where there is an established townscape of high quality, such as a historic street network, which itself would not satisfy the requirements of the guidance.
Anecdotal evidence from consultants suggests that the benchmarks in the BRE Handbook for daylight and sunlight assessments are based on a low-rise suburban environment, not a high-rise urban environment.
Paragraph 1.6 of the handbook states that different target values can be used “in special circumstances” such as in high-rise areas or historic city centres, suggesting that the recommended target values are not necessarily suitable for high-rise environments.
Furthermore, the diagrams that form part of the BRE Handbook are based on a suburban housing development model.
The most significant change in the second edition of the guidance was the introduction of a new section (Appendix I) on environmental impact assessment (EIA).
This move was welcomed by many practitioners because, prior to this, the benchmarks in the handbook had been applied to EIA at the discretion of the EIA professionals involved, with varying results.
In particular, the assessment of the significance of effects was interpreted differently. Appendix I of the BRE Handbook now provides clear guidance on this issue, stating: “Where the loss of skylight or sunlight does not meet the guidelines in this book, the impact is assessed as minor, moderate or major adverse.”
Appendix I also lists the factors that will contribute towards a major adverse impact, including:
- the development will affect a large number of windows or large area of open space;
- the loss of light is substantially outside the guidelines;
- all the windows in a particular property area affected; and
- the affected indoor or outdoor spaces have a particularly strong requirement for skylight or sunlight – for example, a living room in a dwelling or a children’s playground.
Similarly, the appendix lists the factors that indicate a “minor adverse impact” and it also recognises beneficial impacts.
However, anecdotal evidence suggests there is a tendency to use this methodology for the assessments of high-rise buildings in urbanised environments to conclude a relatively high number of “major adverse” effects.
Conclusions to these assessments sometimes then refer back to the introductory text of the BRE Handbook, its “purely advisory” nature and the flexibility of its target values, as a way of excusing the results.
Some practitioners believe that a debate should be instigated to refine the methodology to account for this urban factor. This approach does not to seek to “water down” assessments, to present more favourable conclusions for clients, but to propose a refinement that takes greater account of the nature and form of highly-urbanised areas and the expectations of those living and working in them.
It may be that consultants working on urban projects need to identify different numerical target values that are appropriate to the urban environment and make those clear in their assessments. Such an approach would appear to be consistent with the introductory text from the second edition of the BRE Handbook.
As part of this, it may be beneficial to undertake some meaningful consultation with members of the public living in urban environments to better appreciate how they understand and live with these issues.
Different numerical target values would change the assessment outcomes once the EIA criteria in Appendix I are applied and presented in an environmental statement.
This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.
Ian Roach is an associate – environment and planning at URS Infrastructure & Environment