Decisions made in the early design phase enormously contribute to the performance of a product during its life cycle. Since users' preferences may change over time, a product design should be revised under the preference change. Providing accurate data for designers ensures an optimal decision for product design; this research presents a new method to assess effects of the quantified changes on product cost and development time. In addition, two models to optimize design under unexpected disturbances are proposed. Normally, optimal parameters require several search iterations in design process before finalizing a product. The design time in terms of number of iterations can be reduced by adding resources in each iteration using modern control engineering methods. However, adding resources will increase the design cost. The proposed method in this research minimizes the total product design cost and environmental impacts under changes of users' preferences. The method is validated using an example of the smartphone design. The research novelty is a method of applying quantified changes of external disturbances (such as changes in users' preferences) in the design process, addressing a real problem in industry, and proposing optimal models of products for reduced cost and environmental impacts.