<p>Aggregate dwellings are a common construction configuration in urban environments in the Americas, where space constraints favour terraced construction. However, seismic interaction between adjacent dwellings can significantly alter their structural response, especially when there are differences in height or in the stiffness of horizontal diaphragms. Despite advances in the seismic analysis of masonry buildings, most previous studies address these factors independently, without considering their combined effect in structurally heterogeneous complexes. This paper analyses the seismic behaviour of clusters of terraced dwellings built with reinforced masonry using partially grouted concrete blocks (PG-RCM) through non-linear dynamic analysis. Twenty-six structural configurations are studied, composed of three units that vary simultaneously in height (one, two, and three levels) and in the stiffness of the roof diaphragm (rigid or flexible). The results show that height heterogeneity induces pounding effects that increase displacement demand at upper levels, while the combined interaction between height difference and diaphragm type with different stiffness amplifies lateral response and damage distribution, even when the most rigid units are located in apparently favourable positions. The findings show that the seismic response of aggregated dwellings cannot be assessed in isolation, as it depends on the dynamic interaction with adjacent dwellings. This study provides quantitative evidence on the combined effect of variability in height and stiffness, offering useful criteria for seismic design in contexts where this type of construction is common.</p>