Common Wall and Roof Rooms.
There can sometimes be confusion among assessors about what constitutes a common wall and when a space should be recorded as a roof room or as a separate storey.
To clarify:
A common wall is the vertical extension of the external wall of the storey below. It is not the same as a party wall, stud wall or the room height.
Diagrams A, B, C and D above all represent roof rooms because the common wall does not extend up to the top floor.
The height of the common wall must be less than 1.8m for at least 50% of the common wall (excluding gable ends and party walls) to quailfy as a roof room.
The diagrams above show a roof room, a roof room with a small common wall (less than 1.8m) and a separate storey as the common wall is more than 1.8m.