This paper presents a numerical study on the fire behaviour of prefabricated modular reinforced concrete buildings, developed within the framework of the (on-going) R2UTechnologies project. The main objectives of this work are: (i) to verify the fire resistance of the structural elements with respect to the (Portuguese) legal framework; and (ii) to use insights from numerical analyses to design experiments targeting critical aspects. Thermal and mechanical sequentially coupled analyses were performed, using the commercial software ABAQUS. It was found out that the critical scenarios are (i) the post-fire residual behaviour of the connections between walls and (ii) the possibility of spalling during fire exposure. Fire resistance tests, under preparation, will validate the numerical models presented in this paper, and study the above-mentioned phenomena.