Many historic centers in Italy are composed of masonry aggregates—clusters of adjoining buildings developed over time. Within these aggregates, it is often possible to identify structural units (SUs), which represent the smallest portions with potential for independent structural behavior. When retrofitting interventions are required, it is essential to correctly define the extent of the intervention unit and understand how to model it accurately, without needing to simulate the entire aggregate. In this study, a case located in Todi (Italy), specifically the northeast corner of a school building, was investigated as a representative SU within a masonry aggregate. Based on historical and structural analysis, the SU was modeled in three different configurations, with varying extents of adjacent portions included. The goal was to evaluate how much of the surrounding aggregate must be modeled to reliably reproduce the SU's seismic behavior. The study further investigated the same representative SUs after being strengthened comparing the behavior of the unreinforced and reinforced configurations. Results were evaluated through capacity curves and local out-of-plane failure mechanisms and validated against damage patterns observed on site. The analysis confirmed that including a limited extent of the adjacent buildings is sufficient to significantly improve the accuracy of the structural model, particularly in terms of stiffness and damage distribution. The reinforced model showed a notable increase in capacity, confirming the effectiveness of the proposed strengthening strategies.