The establishment of a platform architecture is a critical task but there is no methodological support for this in the first phases of development. There are several approaches for evaluating designs and architectures when an initial design is present, however, not for the phases foregoing the embodiment of ideas into concepts. This paper fills this void by introducing a new design methodology for modelling, assessing and narrowing down the architectural design space. It allows exploration of more alternatives in the earliest phases of development, which ultimately may produce better designs. The result is a design space of a manageable and desirable size for subsequent embodiment and detailed design with traditional engineering tools. The advantage is that feasibility of the candidate platforms have been established to a high degree of certainty. The approach is illustrated with a case of redesign showing how a manufacturer of parts for a jet engine can use the methodology to model and assess platform concepts in the earliest phase of development.