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Overshadowing - house extension |
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Flow Analysis Ltd were commissioned to assist in an appeal against a planning decision to reject a house extension on the grounds of perceived overshadowing to a neighbour's 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. Any potential reduction in daylight received by a vertical wall which contains one or more windows, is quantified by calculating the ‘vertical sky component’ (VSC) at the centre of windows. The loss to daylight is then quantified by considering the change in VSC due to the planned extension with reference to the BRE publication guidelines. |
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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. this was found to be the case with the proposed extension analysed here. The animations above (before extension) and to the right (after extension) are also produced as a visual aid to demonstrate the change in overshadowing. |
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