The "Extended Target Weighing Approach" presented here describes a holistic, cross-component, function-based lightweight design method - in terms of conceptual lightweight design - for the identification and evaluation of lightweight design potentials in the concept phase of product development. It systematically extends the existing, purely mass-based "Target Weighing" approach by the key factors of CO2 emissions and costs. The selection of concept alternatives to be pursued in the early phase of product development often depends on the experience of the product developer. Therefore, the potential of some concepts is not recognized correctly or the risk according to the intended solution caused by missing knowledge or uncertainties is misjudged. This paper presents a method for the evaluation of uncertainties of concept ideas based on the PGE - Product Generation Engineering. Four factors – Impact, Carryover Variation Share, Reference Product - Technology and Reference Product - Application Scenario - to assess the total uncertainty of a concept idea are described. The total technological uncertainty of a concept idea is gained by summing up the influencing factors. The method is used in the context of this paper for the Extended Target Weighing Approach to evaluate concept ideas in terms of their uncertainties. The capabilities of the proposed method to evaluate uncertainties in the context of the Extended Target Weighing Approach are demonstrated in a real world application scenario as a part of the ALLIANCE project. Based on an existing product generation of a vehicles front part, lightweight design potential and the linked efforts costs and CO2-emissions of two concepts are analyzed in regard to the technological uncertainties and show how decision making during the development process can be supported during an early phase of concept design.