This work presents preliminary results from an ongoing numerical and experimental investigation aimed at supporting the seismic design and performance evaluation of a retrofitting system for reinforced concrete (RC) buildings. Shaking table tests were conducted on two full-scale, three-dimensional, single-bay, three-storey RC buildings, which included internal infills and partitions. Specifically, this study focuses on assessing the effects of a seismic retrofitting intervention installed on one of the two structures based on the output-only operational modal identification of the tested specimens. In particular, a sequence of white noise excitations was applied alternatively in between successive scaled Irpinia earthquake ground motions of progressively increasing amplitude. Frequency Domain Decomposition and Covariance-based Stochastic Subspace Identification were adopted using the PyOMA2 software. The preliminary dynamic identification results were compared with a modal analysis of a Finite Element Model of the RC real-scale specimen implemented in the Scientific Toolkit for OpenSees (STKO) software. The experimental results show quite a good agreement in terms of natural frequencies and the expected damping ratio. Future studies may also explore retrofitting assessment with input-output dynamic characterization or even machine learning-based methods.