British Standard BS8206-2:2008
British Standard BS8206-2:2008 states that the average daylight factor is used as the measure of general illumination from skylight. It is considered good practice to ensure that rooms in dwellings and in most other buildings have a predominantly daylit appearance. In order to achieve this, the average daylight factor should be at least 2%.
If the average daylight factor in a space is at least 5% then electric lighting is not normally needed during the daytime, provided the uniformity is satisfactory. If the average daylight factor in a space is between 2% and 5% supplementary electric lighting is usually required.
Minimum values of average daylight factor in dwellings
Even if a predominantly daylit appearance is not achievable in a dwelling, it is recommended that the average daylight factor should be at least the relevant value as shown below.
Room type Min ADF%
Bedrooms 1.0 %
Living rooms 1.5 %
Kitchens 2.0 %
Where one room serves more than one purpose, the minimum average daylight factor should be that for the room type with the highest value. For example, in a space which combines a living room and a kitchen the minimum average daylight factor should be 2%.