1 Introduction Benedikt (1979) proposed the isovist as a means of quantifying perception of environ- ments as distinct from objects. Simply expressed, the isovist is the set of all points in an environment visible from a fixed station point. Defined in this way the isovist can be understood as equivalent to the viewshed (Turner et al, 2001). In his major papers on the subject, Benedikt largely restricted his commentary to plan isovists, or two-dimensional (2D) sections through three-dimensional (3D) sets of points, similar to Porter's spatial boundary diagram (Porter, 1979). Benedikt and later researchers tend to use the word isovist to refer to these 2D diagrams rather than to their 3D counterparts. By focusing on 2D isovists, Benedikt was able to define measurable properties such as isovist area or isovist perimeter as these properties varied across an environment (Benedikt, 1979; Davis and Benedikt, 1979). He used the term isovist field to refer to a graph of these properties. Despite Benedikt's interest in measuring and mapping properties of isovists distributed over an environment, he did not provide an illustration of iso- vists deployed in an environment [such as figure 2(b) later in this paper]. Instead, Benedikt's illustrations are limited primarily to two types: illustrations of individ- ual isovists and illustrations of isovist fields. Furthermore, the idea of preparing an illustration of isovists deployed in an environment appears to have been largely overlooked by subsequent researchers, in favor of illustrations of other properties of isovists and associated isovist fields (Batty, 2001; Turner and Penn, 1999; Turner et al, 2001). In this paper I explore the possibility of mapping isovists deployed within an architectural environment and identifying potentials which this mapping has for the study of that environment. To develop this possibility I devised a simple AutoLISP routine for preparing plan isovists. The case of an addition to an existing building is presented as a test case. Registering visual permeability in architecture: isovists and occlusion maps in AutoLISP À Mike Christenson College of Engineering and Architecture, North Dakota State University, 650 NP Ave, Fargo, ND 58102, USA; e-mail: mike.christenson@ndsu.edu Received 29 June 2009; in revised form 21 December 2009; published online 4 October 2010 Environment and Planning B: Planning and Design 2010, volume 37, pages 1128 ^ 1136 Abstract. 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. doi:10.1068/b36076 À See http://usa.autodesk.com