Only the external walls above ground level are required to be assessed under ‘external walls’.
The external walls below ground (i.e. within a basement area) perform a specialist function, these are not comparable with other external walls in a building. These are included in the ‘basement retaining walls’.
20/08/2021 Scope clarified in accordance with the New Rules of Measurement (NRM) by the Royal Institute of Chartered Surveyors (RICS).