One common strategy to include more downstream lifecycle dimensions in early design is to enrich modelling and simulation techniques embedded in decision support systems. However, downstream dimensions are difficult to trade against more traditional engineering objectives. This research studied through individual interviews how six disciplines use models to negotiate design trade-offs. References to models were categorised according to whether models supported or hampered the duration of trade-off identification and how they impacted the duration of trade-off resolution. The results point to the difficulty of applying downstream lifecycle issues earlier in the design process because of the characteristics of the models that are used. A list of characteristics promoting and limiting the use of four models as boundary objects (CAD models, simulation results, total cost of ownership and decision matrices) is provided. The cross-analysis of these characteristics provides insights into how models need to be organised in decision support systems.