Thanks to several foundational contributions (De Angelis 2010; Hardt and Negri 2009; Harvey 2012; Ostrom 1990), the topic of urban commons has recently gained much interest, even if there is room to further investigate the relationship between urban commons and planning (Dellenbaugh et al., 2015; Muller 2015). On 5 May 2012, the artists’ collective M^C^O (henceforth Macao) drew public attention by squatting in the iconic Galfa Tower in Milan, a private property abandoned since 1996. Symbolically, it served to shine a light on the need for a radical change in urban policies regarding the reuse of abandoned sites in town (Valli 2015). In opposition to the current planning tools and resolutions adopted by the City Council of Milan, Macao’s activists developed and proposed the Constituent City manifesto (Macao 2015). Starting from these premises, the paper interrogates the issue of how urban commoning can challenge conventional planning procedures and seeks to identify the mutual influences between these practices,local governance and planning tools. It draws on Macao’s commoning actions and particularly on the case study of the former slaughterhouse Exchange Building (henceforth SEB) in Milan, interpreted as a potential urban common. We conclude by offering a reflection on the roles that urban commoning practices and urban commons may have in defining innovative governance and planning processes.