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Reuse in systems engineering is a frequent but poorly understood phenomenon. Nevertheless, it has a significant impact on system development and on estimating the appropriate amount of systems engineering effort with models like the Constructive Systems Engineering Cost Model (COSYSMO). Practical experience showed that the initial version of COSYSMO, 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 the four COSYSMO size drivers-requirements, interfaces, algorithms, and operational scenarios. It fundamentally modifies the driver counting rules and updates its definition of system size. It provides an enabling framework for estimating a system under incremental and spiral development. 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 to be 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.