Generally speaking, the 45-degree guideline is used to ensure that a new rear extension on a semi-detached house does not overshadow your attached neighbours’ windows. This guideline is not usually applied to a ground floor extension, whether existing or yet to be built, as you are allowed to build to a 3m depth, full width under your permitted development rights.
It is applied to a plan drawing by extending a 45-degree line from the nearest habitable room windows belonging to your neighbour, outwards towards your property. A bedroom or study is a habitable room; a bathroom is not a habitable room. Your upper storey should not cross this 45-degree line. Typically, planners look for a 2m set-off at the boundary with the other semi.
The start point of the 45-degree line varies according to each Council. Some have a start point at the centre line of the window and others use the nearest edge. However, this is just a guide used by Planning Officers and compliance with local habits does not preclude a rejection of an application if the design is poor, overbearing or overshadowing.