In terms of RdSAP and roof rooms, the common wall is a vertical continuation of the external wall of the storey below. This frequently confused with the ceiling height within the roof room.
The common wall height excludes the gable end of a property and the party wall, but looking from the gable can help in identifying the common wall height.
On the property below, a large dormer room-in-roof had been considered an additional storey but the assessor due to a room height over 1.8m, however, the common wall height has been highlighted to show that it is less than 1.8m
The image below is from an audit appeal where the assessor was adamant that the property had an additional storey and not a room-in-roof:
Remember, gable ends are excluded as parts of the common wall, so the common wall height for the upper portion of this dwelling is effectively zero.