The purpose of this standard is primarily to ensure that an adequate standard of day lighting is attained in habitable rooms in dwellings to allow domestic activities to be carried out conveniently and safely. A kitchen or toilet is not deemed to be a habitable room in terms of the building regulations.
Conversions - in the case of conversions, as specified in regulation 4, the building as converted must be improved to as close to the requirement of that standard as is reasonably practicable, and in no case worse than before the conversion (regulation 12, schedule 6).
Every apartment should have a translucent glazed opening, or openings, of an aggregate glazed area equal to at least 1/15th of the floor area of the apartment and located in an external wall or roof or in a wall between the apartment and a conservatory.
A conservatory may be constructed over a translucent glazed opening to a room in a dwelling provided that the area of the glazed opening of the internal room so formed is at least 1/15th of the floor area of the internal room. There are other recommendations relating to ventilation in clause 3.14.8 and the size of windows in Section 6, Energy.
An extension however constructed over a glazed opening to a room, because of its greater solidity, can seriously restrict daylight from entering the dwelling and the existing room and extension should be treated a single room. The area of the translucent glazed opening to the extension should be at least 1/15th of the combined floor area of the existing room and the extension. A new translucent glazed opening should be provided to the existing room but, where this is not practicable, the wall separating the 2 rooms should be opened up to provide a single space. To ensure sufficient 'borrowed light’ is provided, the opening area between the existing room and the extension should be not less than 1/10th of the total combined area of the existing room and the extension. Clause 3.14.8, covering ventilation, also recommends that the existing room and extension are treated as a single space.