Belfast·SPG003 Residential Design·Page 35·Figure 3.16
45 Degree Rule for Daylight Assessment
The 45 degree rule is used to assess whether a proposed dwelling maintains adequate daylight levels for neighbouring properties. A 45 degree line is projected from the centre point of the main habitable room window closest to the proposed dwelling, and adequate daylight is maintained if this line does not cross the proposed adjoining dwelling in both plan and elevation.
To apply the 45 degree rule, a 45 degree line is projected from the centre point of the main habitable room (i.e. bedrooms, living rooms, dining rooms or kitchens) window that is closest to the proposed dwelling. Adequate levels of daylight can be maintained if the line does not cross the proposed adjoining dwelling in both plan and elevation.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/belfast/SPG003 Residential Design.pdf