Recent earthquakes have demonstrated that buildings with irregular configuration are more vulnerable to earthquake damage. Moreover, the configuration irregularities introduce major challenges in the seismic design of building structures. One such form of irregularity is the presence of re-entrant corners and torsional irregularity that causes stress concentration due to sudden changes in stiffness and torsion amplification in buildings. Constructive research into re-entrant corner and torsion-irregular buildings is therefore needed to evaluate the seismic response demands for reducing the potential damage. The aim of the study reported in this paper is to grasp the seismic performance of L-shaped irregular buildings with moment-resisting frames through an evaluation of the irregularity effects on measured seismic response demands. The results for inter-storey drift, storey shear force, overturning moment, torsion–moment responses at the base and along the building height, top-floor displacement and torsional irregularity coefficient prove that buildings with irregularity are more vulnerable than those with a regular configuration resulting from stress concentration and coupled lateral–torsional behaviour.