Reuse in systems engineering is a frequent, but poorly understood phenomenon. Nevertheless, it has a significant impact on estimating the appropriate amount of systems engineering effort with models like the Constructive Systems Engineering Cost Model. Practical experience showed that the initial version of COSYSMO, a model based on a "build from the scratch" philosophy, needed to be refined in order to incorporate reuse considerations that fit today's industry environment. The notion of reuse recognizes the effect of legacy system definition in engineering a system and introduces multiple reuse categories for classifying each of the four COSYSMO size drivers - requirements, interfaces, algorithms, and operational scenarios. It fundamentally modifies the counting rules for the COSYSMO size drivers and updates the definition of system size in COSYSMO. In this paper, we present (1) the definition of the COSYSMO reuse extension and the approach employed to define this extension; (2) the updated COSYSMO size driver definitions that are consistent with the reuse model; (3) the method applied to defining the reuse weights used in the modified parametric relationship; (4) a practical implementation example that instantiates the reuse model by an industry organization and the empirical data that provided practical validation of the extended COSYSMO model; and (5) recommendations for organizational implementation and deployment of this extension.