In the big cities of Europe large-scale construction of apartment buildings took place at the end of the 19th and at the beginning of the 20th century. In the process, buildings were created based on unique plans, but very similar to each other, containing similar technological solutions and built from similar building materials. Over the past 100 years, some of the buildings have been continuously maintained, while the condition of other buildings has deteriorated significantly. The renovation of these buildings has now become necessary and in many cases, unavoidable. In the current economic and energy situation, it is important that maintenance or conversion is carried out in a sustainable manner to the necessary extent. The method and extent of the interventions can be provided in a uniform manner with help of a computer system. We have developed a condition assessment and decision support model and algorithm that can be used for this purpose. We call it Complex Building’s Decision Support System based on Fuzzy Signatures (CBDF system). We use fuzzy signature-based model to handle uncertainties, inaccuracies and possibly missing data that occur during the condition assessment. The presented decision model prepares the status assessment based on 4 main components (project info, knowledge base, preparatory work process, fuzzy system). After defining the objectives (e.g., general condition assessment, evaluation from the perspective of accident prevention, examination of the possibility of roof installation), the system requests the necessary data and generates the fuzzy signature required for the condition assessment of the given building. Based on the input data for the specific project and the knowledge base, the decision model searches for failures and anomalies in the building based on the preparatory work process, manages the existing uncertainties and inaccuracies, and determines the load bearing surplus of the examined load bearing structures. Using the existing information and conclusions, based on various fuzzy set-based descriptors and aggregation operators, the condition assessment is prepared, and then, if necessary, the intervention proposal as well. The final goal of the decision model is to put a tool in the hands of experts examining the condition of buildings, which can be used to prepare uniform and objective assessments (also suitable for ranking) and to reduce error in condition assessment.