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Sunlight and Daylight Assessment

In 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.
sunlight assessment

A Sunlight / Daylight Assesses whether a new development will have adequate natural lighting. If using the BRE Guidance this is deduced in the following manner.

Is there a problem?

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 developement. If lines are obstructed further assessment may be needed.

sunlight daylight assessment

Assessing Daylight

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 building spaced far apart. You might call this overshadowing if you were talk  ing 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 gardens for example. Sunlight is assessed using the Annual Probable Sunlight Value.

Sunlight values are assessed according to the latitude, 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 VSC values.

Assessing Sunlight on the Ground

Assessing 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 lane over a fixed scale plan, and using known obstruction heights values for probable sun on the ground hours are calculated.

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.