Large amounts of energy and carbon are embodied in the frames of buildings, making efficient structural design a key aspect of reducing the carbon footprint of buildings. Similarly to a previous study which analysed real structures had observed that the unused mass of steel framed building could amount to nearly 46% of the total mass due to over-specification of the sections, we find a value of 36%. We observe that this value correlates with the design method, with software-aided design bringing significant improvements and with the design stage, where most of the optimisation seems to occur between the preliminary and tender stage. We find that neither the regularity of the structure nor the cost, independent of the measure used, correlate with the mean utilisation ratio (UR). Conversely, we observe an apparent reluctance to design beams above a 0.8 capacity UR. This reluctance explains most of the unused mass in buildings. The rest of unused mass consists in cores, trimmers and ties (6%), some of which bear loads not captured in this analysis but are otherwise necessary for stability reasons, and in edge secondary beams (3%) which design is constrained, and should not necessarily be considered as ‘unused’ mass.