In this research work, the seismic assessment of a 5-story building damaged during the Mw 5.1, May 11th 2011, Lorca earthquake (Murcia, Spain) is presented aiming to introduce an original and practical damage index, which decouple the effects and contribution to damage of structural and non-structural elements in the damage evaluation of buildings in a post-earthquake scenario. An advanced numerical model of the building has been developed by taking advantage of a monitoring campaign carried out by using a non-invasive remote sensing interferometric technique. The proposed damage index has been compared with five benchmark damage indices. The results show that the proposed damage index is reliable when well-calibrated numerical models of the buildings are available. The calibration of the numerical models is possible from remote or in situ monitoring of buildings or by taking into account empirical formulations to estimate the fundamental periods of undamaged buildings. The results of this research work point to the importance of developing realistically calibrated and quantified damage models for non-structural elements with the objective of developing complete and reliable numerical models of buildings.