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Sunlight and Daylight Assessment

https://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Sun_through_Trees_%2811424340453%29.jpg/512px-Sun_through_Trees_%2811424340453%29.jpgIn circumstances where there is a potential adverse impact upon the current levels of sunlight/daylight enjoyed by adjoining properties or building(s), including associated gardens or amenity space then applications may also need to be accompanied by a daylight/sunlight assessment. Further guidance is provided in, for example, BRE guidelines on daylight assessments.

A sunlight / daylight assessment can be carried out to assess these impacts, and to ensure that the development in mind makes the best use of possible light available.


Further guidance.

Image: BLMOW [Public domain], via Wikimedia Commons

A Sunlight / Daylight Assesses whether a new development will have adequate natural lighting and or impact of the development on existing neighbours. If using the BRE Guidance this is deduced in the following manner.

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Sunlight & Daylight Problems

Taking a reference point from the centre point on the lowest windows, draw line at 25 degrees upwards, does it bump in to the new building, reverse for the proposed development. If lines are obstructed further assessment may be needed.

Assessing Daylight

sunlight daylight assessment

Daylight is assessed using the Vertical Sky Component (VSC) Concept, as the diagram below shows tall building close together have a lower VSC value than say low buildings spaced far apart. You might call this overshadowing if you were talking about it in lay mans terms.

Ratios of building height vs distance are used to quantify effects. Using special analytical tools these are then recorded as VSC values.

Assessing Sunlight

When we talk about sunlight in the context of sunlight assessment we mean the suns rays. What effect will a new development have on surrounding existing properties? Will the new building cast a shadow over existing neighbours windows for example. Sunlight is assessed using the Annual Probable Sunlight Value.

Sunlight values are assessed according to the site latitude zone, the sun takes a different path in the say in say Aberdeen, as it does in London.

Ratios of building height vs distance are used to quantify effects. Using special analytical tools these are then recorded as Sunlight values, if the reduction sunlight post development is over 20% then perhaps the design of the proposed development will have to altered to allow more sunlight through. Over the last 9 yeras we have seen many different way of achieving this and can offer advice and ideas.

Assessing Sun on the Ground

sun_on_ground_assessmentAssessing sunlight and sun on the ground use two different methodologies, and two types of indicator. Sun on the ground assessments assess the hours of sunlight enjoyed at a notional day (21st March).

The indicators are overlain a fixed scale plan, and using known obstruction heights values for probable sun on the ground hours are calculated.

As with other lighting a 20% reduction is allowable. Although there is also "total sun on the ground" hours to consider, when checking proposed spaces for adequate lighting.

Notes on BR 209 Method

Method used is BR 209. The method involves the use of height distance ratios and a transparent indicator is overlain on to project plans. VSC are presented as points and “obstructed" point numbers are compared before and after development. A 20% reduction is allowable and the VSC value for any effected window must not fall below 27%.  

Calculations are for the ground floor window. Ground floor is used as worst case. Other higher windows will experience higher VSC, but this is irrelevant as all windows must pass.   Layout behind the window is irrelevant when conducting assessment.

Layouts are used when calculating ADFs (average day lighting factor), but in conducting the assessment the only two variables assessed aside from sunlight will be Post Development VSC and the Percentage Reduction VSC. ADF might be used to calculate adequate lighting for the proposed building but, can only be used as a side point for existing buildings.

ADF Lighting Requirements for New Developments

When a new building is suspected as having more natural light internally then the planning authority may request that ADF values are provided for the proposed building. There are certain values that must be met in order for the room to "pass".

The ADF value is worked out by a formula that combines room size, available daylight and other factors such as paint colour, to a give a level of day lighting described as a percentage. If the calculated value is below the allowable value, then window size may have to be increased or lighter coloured paints employed to give an adequate value.

In reality a smaller sized window in a normal sized room will nearly always give adequate light, proving it is not badly overshadowed. More of ADF Daylight Assessments.

Example Projects List

Please find below links, to or "What's new" posts on our Business Places Page:

And link to posts on our Project Blog

Extremely Rough Guide to Daylight Assessment Video