|CAD models of buildings represent architects’ views of buildings. Data definitions that represent a given building conform to the internal data structure of the CAD software that is used to define the building and typically include data which facilitate the representation of the building in CAD, but do not necessarily by themselves define anything about the building. CAD representations of buildings are based on detailed definitions of building geometry; those definitions thus contain significant amounts of information needed by CAD tools but not used by other types of tools – those tools need only rudimentary definitions of building geometry to operate.Building energy performance simulation tools, as well as many other types of simulation and analysis tools (like acoustics and fire propagation simulation tools) have their own internal data models of building geometry. Such internal data models represent views of building geometry typically used by the disciplines served by these simulation and analysis tools, and are usually much simpler than the geometry data models of CAD tools. Consequently, CAD building geometry representations must be “simplified” and “reduced” before they can be directly used by other (non-CAD) tools.Most simulation and analysis tools define building geometry as systems of surfaces (i.e. surfaces that delineate walls, slabs, roofs, columns, beams, windows and doors) which are all part of the definition of spaces identified in the model of the building. Such surfaces are called “space boundaries” and are the critical part of the building geometry definitions for non-CAD tools. This paper clarifies and systematizes what a “simplified” building geometry for building energy performance and similar simulation and analysis tools must contain, and should help prevent misunderstandings and misrepresentations often encountered in the AECOO industry today. It describes the five “levels” of space boundaries, why and how they are defined, and how they are used by simulation and analysis tools. The paper discusses the 55 test cases pertinent to semi-automated modeling of building geometry for energy performance simulation that were developed to test space boundaries defined and exported in IFC format by model based CAD tools. It also discusses the process and tools that check instances of IFC definitions of building geometry exported by CAD tools.