The paper examines proposals for nD BIM with respect to what may be considered a dimension and how dimensions relate to information in a symbolic representation. It establishes that ‘dimension’ is often used metaphorically to indicate information-processing capacities – an unfortunate usage in an area where the term is used literally. The paper proposes that a dimension in symbolic building representations should be a primary property of a symbol, not derivative, and moreover essential for the identity of the symbolized object, i.e. not subject to abstraction. On the basis of these principles, it is reasoned that BIM can only be 4D.