Building aggregates represent the most widespread housing typology found in the many small historical centres of Italy. They consist of a non-homogeneous group of buildings, connected to each other by more or less structurally effective connections, which may interact under seismic actions. In this paper, the seismic vulnerability of ancient masonry building aggregates is investigated through macroseismic and mechanical approaches with reference to a case study located in the historical centre of Atella, a small town in the Basilicata region of Italy. Firstly, the main historical and structural features of the selected aggregate are illustrated. Secondly, following the LM1 Risk-UE method, the surveyed structural information is used to carry out a quick preliminary vulnerability analysis of the structural units having intermediate and heading positions, thereby providing their vulnerability index. Thirdly, the whole aggregate is modelled and non - linear static analyses are performed. The seismic response of the structural units is also assessed by modelling them both as isolated constructions and considering their position in the aggregate, so to consider the influence of other constructions. Aiming at assessing the most appropriate structural modelling for the units inserted in the aggregate, a comparison of the results is carried out in terms of capacity parameters and fragility curves. Finally, fragility curves derived by the mechanical approach are compared with those obtained by the macroseismic one to validate the vulnerability index-based method.