This paper investigates the potential involvement of “epistemic uncertainty” in mediating between complex design requirements and strategic switches in software design strategies. The analysis revealed that the designers produced an initial “first-pass” solution to the given design brief in a breadth-first manner, with this solution addressing several easy-to-handle requirements. The designers then focused on adding relatively complex-to-handle requirements to this initial solution in what appeared to be a depth-first manner, as reflected, for example, by detailed mental simulations that spanned many transcript segments. Furthermore, such depth-first development of complex requirements was linked to increases in epistemic uncertainty, a finding that supports the predicted role of uncertainty in mediating between complex requirements and depth-first design. Overall these findings support a view of software design as involving a mixed breadth-first and depth-first solution development approach, with strategic switching to depth-first design being triggered by requirement complexity and being mediated by associated feelings of uncertainty.