Sunlight Availability is a measure of the average number of hours of sunlight one would expect to receive at a given position, as a fraction of the unobstructed total number of hours at the same location. The BRE have compiled data sets consisting of a statistical sample of solar positions convolved with local meteorological data. Using these to calculate Sunlight Availability, one would simply calculate the number of solar positions visible from a point, compared to the total number, expressed as a percentage. The diagram below, taken from the BRE report, shows the solar positions, relative to a reference point, used to calculate Sunlight Availability for London (51.5° N).

BRE Criterion

The BRE report states that for windows within a new development, if a point at the centre of a window on the plane of the inside surface of the wall "... can receive more than one quarter of annual probable sunlight hours, including at least 5% of annual probable hours during the winter months between 21 st September and 21 st March, then the room should still receive enough sunlight." For windows in surrounding properties which experience a change in sulight availability, it goes on to say that, "Any reduction in sunlight access below this level should be kept to a minimum. If the available sunlight hours are both less than the amount given and less than 0.8 times their former value, either over the whole year or just during the winter months, then the occupants will notice the loss of sunlight."