Historical building extension design, a subset of sustainable development for historical environments, is a complex process as it concerns both ahistorical and a new building. In Turkey, decisions, and practices for the protection of the historical environment often result in products lacking anaesthetic value, being deceptive in their imitation of the existing historical building, and/or physically and contextually detrimental to the historicalbuilding. The decision-making authorities responsible for new constructions in the historical environment in Turkey are the Regional Heritage Conservation Boards (RHCB). For various reasons the project approval process from RHCB, resulting in a product with questionable design quality becomes adifficult and weary process for all stakeholders, especially the architect. Focusing on contemporary extensions to historical buildings, this study presents an assessment model that aims to provide aesthetic, theoretical, and constructional compatibility between a historical building and its new extension. Two main objectives have been determined to achieve the desired assessment model. The first is to define the main criteria for the integrationof the historical building and the new extension, and the second is to develop a model so that the optimal material and structural combination is selected for the historical building and extension interface. The criteria and sub-criteria for the integration of a historical building and its new extensionwere developed through research and literature review. Each criterion was explained in terms of its advantages and disadvantages and the optimumrange in conservation and extension design. After defining criteria, the assessment model was constructed using the Analytic Hierarchy Process (AHP)tool. The Super Decision (SD) program, the digital interface of AHP, was used to apply the model to different extension design cases. Thus, the operation of the model was tested and results were obtained. Defining criteria for all aspects of extension design for a historical building, this study focuseson the structural interface between the two buildings. The initial design should match the preliminary limitations of the model. One main limitationis that the historical building and the new extension parts of the combination should separately exhibit the characteristics of a single building. Asecond limitation is that the initial design should not exceed the plot and height limitations stipulated by the relevant legislation on new architecturein historical environments. Two main results were obtained from this study: the definition of the criteria for integrating historical buildings with theirnew extension and the assessment model. To define criteria provides a basis for the understanding, discussion and possible standardization of thehistorical building extension design process. Given the criteria for each extension design case, the model can assess different combinations materialand structure and establish a relative assessment point separately for each based on the selected criteria. As a result of the model’s assessment, theoptimal solution is chosen among the solution variations. The model’s output can be used to criticize the design, change design decisions, go back andforth through the design procedure for re-assessment, and create new design solutions. The lack of architect’s access to integrated, inter-disciplinarydesign and the lack of a systematized set of the RHCB principles are among the main reasons for inefficiency in the process. In order to facilitate thehistorical building extension design process and elevate the design quality above a certain level, effective criteria should be determined in the designand implementation of extensions of historical buildings. This model can be used by firms in projects requiring expertise from various disciplines andtherefore suitable for use by architecture, engineering, and consulting companies. Thanks to the user-friendly and flexible nature of this model, it canbe easily adapted and improved for future studies. However, the selection of criteria and the creation of different design alternatives are crucial to theeffective use of the model. The presence of a team of experts from different disciplines would enhance the discussion and assessment of the model. Thisstudy contributes to the literature in an attempt to determine the criteria for historical building extension design with its advantageous/disadvantageous effects on the design. Further investigation and collaboration can make it possible to provide numerical values for construction-related criteria,to contribute improving the regulations and standards in the field. Additionally, the model is an effective tool to improve and facilitate the HCB processby opening a discussion ground that can also be used in formal HCB procedures for professionals and authorities.