<jats:p> The arrangement of buildings along roads creates one of the most fundamental patterns of three-dimensional streetscape skeletons, primarily defined as a set of building heights and setbacks in a district. Under zoning regulations, building heights and setbacks are indirectly controlled by the building coverage ratio (BCR) and the floor area ratio (FAR). Variations in the BCR result in variations in streetscape skeletons. Moderate complexity of streetscape skeletons is a necessary condition for aesthetic streetscape. Understanding the relationships between variations in the BCR, building heights and setbacks is thus important in order to harmonise streetscape skeletons, smaller variations in building heights, and setbacks, however, this relationship has yet to be theoretically investigated due to the complex relationship between buildings. The objective of this paper is therefore to formulate the relationship between variations in building heights and setbacks as a function of the standard deviation of BCR, and, based on this formulation, to discuss how to indirectly harmonise variations in streetscape skeletons under zoning regulations. This formulation enables us to analytically investigate the relationship between these two functions and the standard deviation of a BCR. An indirect scheme for harmonising variations in streetscape skeletons under zoning regulations is proposed on the basis of this formulation. The external diseconomies of inharmonious streetscape skeletons are quantitatively defined in order to incentivise plot owners to harmonise streetscape skeletons. The optimal building height and setback criteria are computed, which minimises the social cost of inharmonious building heights and building setbacks in a district. This scheme for incentivising plot owners to reduce their social costs is expected to contribute to indirectly harmonising streetscape skeletons. </jats:p>