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.



EXAMPLE 1

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



EXAMPLE 2

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.