Life Cycle Sustainability Assessment (LCSA) is an integrated method that combines environmental, economic, and social assessments. Its methodological development remains under discussion, mainly regarding the building design. This paper aims to provide a systematic, interoperable, and open-source approach towards implementing LCSA in Building Information Modelling (BIM) in five steps. A harmonized data structure that enriches BIM objects is proposed. Automation in the principal evaluation step is provided by integrating new parameters into the current Industry Foundation Classes (IFC4). A Dynamo script verifies its utility in a case study in Spain using real-time calculations and visualizations. Two alternative structural systems are assessed, and identification is made of the lowest CO2 emitter, the lowest cost, and the most beneficial system for local employment. The approach can be employed to evaluate other indicators and building systems in other countries. Challenges and limitations in the standardization and harmonization of the three dimensions are identified.