In this paper the design and execution of a simple AutoLISP routine for generating a map of plan isovists (in the sense of Benedikt) are discussed. Such a plan field of isovists is a registration of visibility from multiple station points within and around a building. More precisely, the plan field records the cumulative effect, over a spatial matrix, of occluded vision of a distant horizon. Thus, the plan field is termed an occlusion map. An occlusion map registers the effect which an observer's position in space has on their perception of architecture's visual permeability. Occlusion maps are shown here to be an important tool for comparing existing buildings in a historical sense and also as an effective design tool, particularly when an addition to an existing building is being contemplated, as an addition invariably affects the visual permeability of its host.