Material efficiency is not currently a common driver of building design. Indeed, in previous studies, we estimated that 12% of the mass of steel used in structural frames would be saved by more accurate specification of steel members. However, this inefficiency is not the main reason structural frames are light or heavy. We show here for the case of steel structures that it is the layout of the grid and the choice of the decking which have the largest impact on the embodied carbon of frames. Using a database of real designs, associated to a generative design model, we quantify the impact of grid and decking selections. Using our model, we find that real designs are relatively efficient economically, but less so environmentally: the typical building frame could have 40–60% less embodied carbon, and be approximately 10–20% cheaper with the right selection. We show how more complex frames have higher embodied carbon than simpler grids. From our findings, we establish a list of design considerations that architects and structural engineers should account for when creating an initial design to lower the embodied carbon: the complexity of the layout, the optimisation of the design and the choice of the decking technology.