This paper proposes the use of liaison to develop a knowledge based system for variant design application to assist and guide designers at earlier stage of product development. A knowledge based framework has been proposed to support the joining process selection during variant design, where liaison act as an interface. Various liaison knowledge is represented in a complete and systematic manner. These liaison knowledge includes various geometric and non-geometric information. A user interface has been developed. A query engine is built and used to enable reasoning the joining process based on the requirement of a variant product. An industrial case study of two variants of automobile front bumper is provided in order to illustrate and validate the proposed knowledge based framework.