Whilst recognising that the amount of daylight available within a building is constrained by the amount of useful daylight outside, the aim of the AECB Daylighting Guidance is to provide adequate daylight between 9am and 5pm for 70% of the year and acceptable daylight 80% of the year between 10am and 2pm. To satisfy these criteria the Guidance requires you to assess the quality of daylight within each habitable room. The AECB Daylighting Assessment is used to verify that a design meets this Guidance. To determine the quality of daylight you must evaluate how evenly daylight is distributed within a room (the Daylight Uniformity), and the proportion of daylight within the room relative to outdoor conditions (the Daylight Factor). Satisfactory Daylight Uniformity is achieved when more than 80% of a habitable room receives natural daylight. Acceptable daylighting is achieved when the specified Daylight Factor is achieved (Appendix 1 in the Daylighting Standard Guidance, describes the Daylight Factor criteria). To reduce the risk of overheating, high daylight factors (oversized windows) should be avoided. Daylight Uniformity and the Daylight Factor must be calculated in accordance with BS 8206-2 using the AECB Daylight Calculator.