The Building Research Establishment’s daylight standards are often quoted but it is easy to forget how they actually work. Here is a handy guide, lifted from an inspector’s decision (DCS Number 400-012-993).
The 25° rule is a simple initial assessment. If a development proposal does not obstruct a 25° line in a vertical section from a point at the centre of the lowest window of property that might be affected, there is no need to go further and the effect is deemed to be acceptable. If that is not the case, a more detailed assessment is required which takes account of open space or obstructions either side of the vertical section. This is the Vertical Sky Component.
Vertical Sky Component is a measure of the amount of sky visible from a centre point of a window. A window that achieves 27% or more is considered to provide good levels of light, but if with the development in place the figure is both less than 27% and would be reduced by 20% or more, the loss would be noticeable.
That’s the maths done. The next step, of course, is to relate the information to national and development plan policy.
PS Don’t confuse daylight and sunlight. Many do but you know better.
The following DCP chapter is relevant: 4.1353