Existing urban open space typologies within dense urban fabrics cannot meet society's open space requirements in developing countries' metropolitan cities, such as Istanbul. Because of high building densities, it is a challenging task to create new open spaces within urban cores. Developing new tools that work with the existing built environment is crucial to reveal 'opportunity spaces' that can act as breathing points within dense urban fabrics. In this research, a new model is developed to evaluate the 3D spatial enclosure of open spaces using basic geometrical properties and geographic information system (GIS) tools. As a case study, Istanbul's changing spatial organization is analyzed using this model.