Daylight and Sunlight Impact Assessment

When do you need to submit a daylight and Sunlight report?

  • Any new construction that might have an impact on the neighbours’ access to daylight and sunlight
  • Side extensions that might have an impact
  • Roof extensions

There are two types of Daylight & Sunlight Assessments. The first is for the development itself. The second is the assessment we carry out to test the impact of your proposal on the neighbouring or surrounding buildings, which we cover here.

Local authorities might request this impact assessment, if they are concerned about the impact of your development or extension on neighbouring buildings in terms of daylight and sunlight. You can also do it voluntarily to give neighbours the reassurance of preserving their light access.

Our analysis is based on the BRE 209 guidelines and recommendations. Models of both the existing and proposed build would be created to test the following:

  • Vertical Sky Component (VSC)
  • Average Daylight Factor (BRE Appendix F criteria applies)
  • Average Probable Sunlight Hours (APSH)
  • Daylight Distribution/ No Sky Line

Alternatively, if you are at the early design stage we advise on how to design a development to reduce or avoid loss of light. We can also help you maximise the development potential by calculating a maximum envelope.

Local authorities might also request an Overshadowing Assessment.

daylight and sunlight, Daylight and Sunlight Impact Assessment

Why Pro Sustainability?

We will provide an initial assessment to determine if a full analysis is required or if it can be avoided by following ‘rule of thumb’ guidelines
We use a sophisticated software (IES-ve). Modelling the existing and proposed scenarios
If required our team would visit the site and collect data and photographic evidence to use in the report
We would discuss options with your design team, and model those options to reduce any possible impact, with minimum effect on your envelope area

Frequently Asked Questions

What is the difference between Daylight and Sunlight?

Daylight is the volume of natural light that enters a building to provide satisfactory illumination of internal accommodation. Sunlight on the other hand refers to direct sunshine and is very much brighter than ambient daylight.

What is the 45 degrees rule of thumb?

A method applied only where the nearest side of the extension is at a right angle to the window. Where a significant amount of light is likely to be blocked if the centre of the window lies within the 45° lines of both plan and elevation of the extension.

What is the 25 degrees rule of thumb?

This approach is used when the new development directly faces the affected window. Suitable daylight is achieved when a 25° vertical angle taken from the centre of the lowest window is kept unobstructed.

What is Daylight Factor (DF)?

It is the ratio of the light level inside a structure to the light level outside the structure. It is used in building design to assess the internal natural lighting levels as perceived on working planes or surfaces.

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