Overshadowing - New Dwelling

Flow Analysis Ltd were commissioned to assess whether or not a proposed new house would cause significant overshadowing to a neighbouring property.

A methodology has been developed by Flow Analysis Ltd (based on computer modelling) to precisely quantify the extent of overshadowing based on accepted guidelines. Furthermore, the process has been largely automated, so that the cost of performing such analyses is relatively low.

The question of the extent of overshadowing by one building on another is answered by considering the loss of daylight and sunlight. When examining a case of claimed potential loss of daylight and sunlight resulting from a new development, it is important that any analyses are as objective as possible. To achieve this objectivity, the guidelines laid down in the widely accepted BRE guidebook ‘Site layout planning for daylight and sunlight: a guide for good practice’ by P J Littlefair are adhered to.

To quantify the potential loss in sunlight, the first stage is to calculate the number of annual ‘probable sunlight hours’ which the interior receives. The ‘probable sunlight hours’ means the total number of hours in the year that the sun is expected to shine on unobstructed ground, allowing for average levels of cloudiness for the location in question.

In the analysis, the number of annual probable sunlight hours is calculated at each of the same reference points used to calculate the VSC. The loss to sunlight is then quantified by considering the change in probable sunlight hours due to the planned extension with reference to the BRE publication guidelines.

If both the loss to daylight and sunlight due to the planned development is within the guidelines laid down in the BRE document, then the development is deemed acceptable. In this case, it was found that the new building would cause a loss of daylight which is outside acceptable limits, but due to the orientation, the loss of sunlight is within acceptable limits.

However, the Planning Officer would need to take into consideration that fact that the majority of affected windows are obscured glass and therefore almost certainly serve bathrooms, which need not be analysed according to the BRE guidelines.

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