This contribution reports on the development of a computational environment for performance-based building design and retrofit optimization. The underlying technologies for this environment were developed within the framework of the research project SEMERGY. One of the key objectives of this research project was to explore and utilize the potential of Semantic Web Technologies toward provision of effective support for building performance assessment and optimization. The motivation behind this line of inquiry was grounded in pertinent results of earlier research and development efforts concerning the application of performance assessment tools by the design community. Such results identified, as a key impediment, the extensive amount of time, effort, and expertise required to accumulate and provide input information for performance evaluations tools and routines. This circumstance was found to represent a disincentive particularly with regard to the application of such tools for improvements in early stages of the building design and delivery process. Even the most rudimentary performance inquiries require large quantities of data regarding building materials and systems, climatic conditions, building codes, etc. Much of this data is available on the internet, yet its use is hampered by lack of appropriate structure and format. The SEMERGY project explored thus the potential for acquiring and utilizing the abundant, yet ill-structured building-related data available on Web, through implementation of Semantic Web Technologies. This web-based data is deployed within a flexible, multi-objective design optimization environment, incorporating various performance assessment applications.