1. A method of designing a system, the system including an application having software components and an architecture having hardware components on which the application is run, the system has IT platforms including cards and electronic modules, the system has to satisfy at least one functional requirement and one non-functional requirement, the method comprising the following steps: a functional analysis step, for obtaining a breakdown of the functional need relating to said application, a step for defining the architecture, a step for designing hardware components according to said architecture, a step for designing software components based on the breakdown of the functional need, a step for integrating the software components in the hardware components, a step for validating the functional requirement of said system, a step for validating the non-functional requirement of said system, wherein it also comprises an upstream step for validating the non-functional requirement of said system, preceding the steps for designing hardware components and software components, said upstream step for validating the non-functional requirement comprising the following steps: modelling of the functional need based on said hierarchical breakdown, modelling of the architecture according to predefined architecture rules, said modelling of the architecture being performed with processors linked by buses, allocation of the model of the functional need to the model of the architecture, making it possible to obtain a functional model allocated to the architecture model, computation of at least one non-functional parameter from the functional model allocated to the architecture model, comparison of the computed non-functional parameter with the non-functional requirement, if the non-functional requirement is met, design of the hardware components and software components, if the non-functional requirement is not met, definition of a new architecture or search for a new allocation of the model of the functional need to the model of the architecture.