The problem of the retrofit design of a multiproduct batch plant is considered from a new perspective that involves explicit consideration of inherent reliability and maintainability characteristics of existing and new equipment. Until now, in multiproduct batch plant retrofitting formulations, the production capacity has been specified by the limiting batch size and limiting cycle time. We propose a more robust retrofit solution that is obtained by defining the effective production capacity by three parameters: the limiting batch size, the limiting cycle time, and the overall plant availability. The novel simultaneous optimization framework developed in this work combines a process model and an availability model to obtain the optimal size, optimal operating mode, and optimal allocation of inherent availability for new equipment during the retrofit stage. The overall problem is formulated as a mixed integer nonlinear programming (MINLP) model, and its applicability is demonstrated through the solution of a number of examples. This framework provides the designer with the opportunity to select the initial inherent availability of new equipment during retrofitting by balancing the costs of design investments against the costs of downtime.