The paper presents an analytical-mechanical based procedure to estimate the seismic overall fragility of existing reinforced concrete building portfolios in town compartments, as reduced areas of a municipality. The proposed methodology is based on two main concepts: (a) to consider all typological parameters characterizing the entire set of buildings located in a certain urban area and their variability through an analytical procedure; (b) to employ a mechanical approach by means of ideal numerical models to estimate the safety level of the focused sample of buildings. Hence, the methodology allows to compute seismic overall fragility curves, obtained by using laws of total variance and expectation and weighing factors proportional to the probability of having a certain configuration of typological parameters with determined values. To test the proposed procedure, some town compartments of the municipality of Bisceglie, Puglia, Southern Italy, were investigated by firstly identifying the most recurrent typological features exploiting multisource data, after by elaborating an extensive campaign of modelling and analysis on different ideal buildings (herein named realizations) and finally by computing fragility curves for each realization and for the set of ideal buildings. The results show overall fragilities curves for the investigated town compartments, which are obtained in a different way from the existing procedures, by avoiding an a-priori selection of one or more index buildings to represent the specific building portfolio and the definition of a specific building taxonomy.