The paper presents the results of studies of individual variations in permanent EPN / IGS stations positions over time, as well as the sources of such variations. In order to eliminate an influence of those perturbation sources, which does not affect station position directly but over surrounding territory, only very small subnets, ranging in size from several meters to several tens of meters, were used for research. It was found, that systematic variations have seasonal nature of behavior (annual variations with extreme values at winter and summer) in most cases and the stations, whose antennas are placed on buildings, have the largest seasonal variations. It was shown, that both the largest individual seasonal variations and the short-time inter-seasonal ones of such stations positions are strongly correlated with local temperature changes. It was shown as well, that varying such stations positions corresponds to expanding-compressing effects of a base where antenna mounted (such as building roof) affected by temperature changes. Some instances of such EPN/IGS station position behavior both from this investigation and other sources were cited. It shows that the main reason of seasonal variations of position of a station, which antenna placed on a building, is the expanding-compressing effect of the building due to changes in ambient temperature.