Cyber-physical systems are slowly emerging to dominate our world. Cyber-physical systems (CPS) are systems that tightly integrates users, devices and software. Whereas many of these systems are obviously safety-critical systems, some of them become so under special circumstances. This is the case with our focus CPS, i.e. building management systems (BMS), which are not always safety critical per se, but under special circumstances they can become such. This certainly depends on the purpose of the building. We can easily imagine BMS of hospital buildings as safety-critical, but also BMS of buildings that store sensitive materials and equipment that could be of biological nature or encompassing sensitive technology that would need special temperature, humidity and light settings. For this reason, in this paper we would like to emphasize on the importance of reliability of CPS in general, with a special focus on BMS, as our area of interest. Furthermore, we also propose a classification of buildings with respect to the necessity of having their reliability evaluated.