Churches have always been the centre of Dutch society to gather under one roof (De Kruijf & De Roest, 2010). Due to the secularization of the Netherlands, church vacancy is increasing. Currently, churches still fulfill a central function within a city, village or neighborhood (Kerkmagazine, 2022). The Grote- of Mariakerk is one of them, and the vacancy of buildings in such prominent areas of the city will detract from social and physical quality of these areas (De Kruijf & De Roest, 2010). The church should not withdraw from the public domain (Kennedy, J. C. K., 2010). A transformation towards a public interior could avoid this. Kloos (1993) describes public interiors as buildings in which the city’s public space is continued. The social and functional diversity of urban buildings is reflected in their façades (Jürgenhake, 2014), especially around their interior and exterior; the ‘transition zone’ (Gehl, 2011). Therefore this paper focuses on the role of the transition zone when transforming the Grote- of Mariakerk towards a public interior, whilst maintaining its heritage values. By combining literature research , case studies, and field research, the role of the transition zone in public interiors is clarified. By analysing the existing transition zones of the Grote- of Mariakerk and linking them with the criteria of a public interior, opportunities and challenges for the transformation towards a public interior result in design approaches regarding the transition zone.