Behaviour change design has much to gain with the integration of insights from the behavioural sciences in the design process. However, this integration needs to be done without hampering the creative process. In two rich design cases aimed at health and safety behaviour change, we describe our efforts to develop a method for theory driven design based on the Double Diamond. Our method attempts to integrate insights from the Persuasive by Design-model (PbD) for behaviour change into the entire design process. Our case studies demonstrate that our method indeed augments the integration of theory and evidence in our designs, but only if the Double Diamond process model is complemented with an evaluation phase, and insights from the PbD-model are derived using rich, well developed tools.