The design approaches of new exterior additions to historic buildings have been among the ongoing debates in the field of architectural conservation. The aim of this study is to develop an assessment method, which can be used to determine the compatibility in architectural expression of the new addition in relation to the characteristics of a historic building. This method is based on the architectural analysis, which includes environmental relations, building-lot relations, mass relations and the facade composition of the historic building both before and after the new addition in relation to the value of a historical building. In this study, a selected group of historic buildings in İzmir, Turkey with new exterior additions was analysed with regard to their exterior architectural characteristics in order to evaluate the compatibility of the new addition, by employing the use of contemporary conservation principles. The importance of this study is to guide architects to form the basis of a decision for developing an integrated approach in designing new additions in the course of the actual design process. On the other hand, the proposed method can be evaluated as a contribution to the emerging field of heritage impact assessments as well as scientific assistance to local governments to criticize specific projects of cultural heritage assets submitted for appraisal. Analysis results showed that the value of the historic building should first clearly be identified and a new addition should bear the identity of its own period. However, instead of altering the scale or form of the historic building, a new addition should complement and contribute to the sense of proportion, disposition and historical pattern.