The aim of the research is to investigate the ways in which various structural systems and time influence the cost of construction of RC structures of residential buildings, and to identify and quantify key parameters that are of highest significance in this respect. The data taken from the database of completed projects are analysed using the leave-one-out-cross-validation and regression analysis. Limit numerical values confirmed by the redesign of two structures from the database are identified as key parameters. These values define in quantitative terms the rationality of the design solution and the rationality of construction work scheduling. They can also be used in practice in order to optimise the cost and time of construction work on residential buildings.