ABSTRACT. Digitisation in the construction industry is advancing and covering an increasing number of areas. One of these is the involvement of building authorities in the BIM process through the introduction of digital building permission processes. The building authority of the City of Vienna (Austria) has conducted a research project on this. One of the main objectives was to develop automated code compliance checks for complex building regulations based on information in BIM models. This paper discusses the challenges and potential benefits of automated checking of one such regulation: The verification of the light exposure of common rooms. In Austria, there are complex requirements for determining the light entry area of windows, as only the area with unobstructed light incidence may be considered. We begin by analysing the relevant building code requirements and then describe the methods used to determine whether the light incidence is obstructed and, if so, how the area of unobstructed light incidence is determined. The key to this step was the use of a parallel projection into a plane in 3D space. A software implementation demonstrated the ability to determine the precise geometry and area of each light entry area. This makes it possible to precisely check the light exposure of a room, which would hardly be possible manually. In addition, the concept converts implicit information in building models into explicit elements that are much easier for building authorities to access in the long term.