The adoption of computation in the building industry has led to the development of a range of tools intended for design. Although commonly used for detailed analysis (FEA) and documentation (CAD/BIM), few tools address the needs of the early design stages for flexibility and quick, insightful feedback and documenting the justification of design choices. This absence of suitable tools leads to missed opportunities for computation in early-stage analysis and documentation of design progression and rationale. StructuralComponents has been developed as a tool for the early conceptual stages of structural design, until recently mainly focused on high-rise structures (Rolvink [13]). This paper describes the next steps of the development of a more generally applicable building model which supports composition of design justification (Coenders [5]): the collection of analysis models, reasoning, mental models, scenarios, etc. which form the backing of the choices which form design. This paper discusses the proposed building model and its underlying concepts, based on research into design theory, modelling approaches, and structural design practice as well as its application in practical examples. As a proof-of-concept, a prototype has been developed which demonstrates this modelling approach is flexible and versatile enough to document various types of designs, processes and reasoning. Opportunities for future developments include extending functionality for geometry and visualisations, as well as investigation into standardization.