This paper describes the methodology that has been developed and applied to derive the principal geom-etry of the main DEMO tokamak systems, in particular the radial and vertical cross section based on the systems code output parameters, while exact parameters are described elsewhere [1]. This procedure reviews the analysis of the radial and vertical build provided by the system code to verify critical integration interfaces, e.g. missing or too large gaps and/or insufficient thickness of components, and updates these dimensions based on results of more detailed analyses (e.g. neutronics, plasma scenario modelling,etc.) that were carried out outside of the system code in the past years. As well as providing a 3D configuration model of the DEMO tokamak for integrated engineering analysis, the results can also be used to refine the systems code model. This method, subject to continuous refinement, controls the derivation of the main machine parameters and ensures their coherence vis-à-vis a number of agreed controlled physics and engineering assumptions.