It is contended that legal proceedings, as they have evolved from Antiquity onwards, embrace important and effective principles for collaborative competition in pursuit of a common goal, in the considered context, justice. Seven principles contributing to this goal can be recognised: “hear both parties”, reasoned judgment, right to appeal, use of both logical and rhetorical arguments and reasoning, standardized proceedings and documents, public nature of proceedings, as well as dedicated and structured space. It is contended that the court of law can be used as a metaphor of what is happening in design. There are wishes, concepts and solutions competing against each other. For reaching the best outcome, each wish, concept or solution needs to be promoted and defended in the best possible way, and a reasoned judgment among them has to be done. Then, the question arises whether the seven principles found in legal proceedings have relevance for this collaborative, yet competitive pursuit of a common goal in design, namely the best solution in view of customer requirements. For initial exploration of the relevance and validity of the seven principles in design, a case study was undertaken. It turns out that all the seven principles are being implemented. The outcomes of the project are clearly better than in projects managed in the traditional way; although it is not possible to trace back the benefits only to the collaborative principles and related practices, their emergence, and continued use, provide solid circumstantial evidence on their efficacy.