Traditional planning approaches tend to consider informal open spaces either for built redevelopment or for conventional open public space. In both scenario, the unregulated character of the informal open space is lost. Although, these unconventional characteristics are increasingly recognized for their ecological and social value. This paper identifies strategies that foster the recognition of informal open spaces as official green spaces while conserving their un-planned character. Three informal open spaces in Brussels (Belgium) are analysed. These spaces were waiting for urban re-development for 50 years. Today, they are official dedicated as green zones whereas they are not subject to any official maintenance. Those spaces are commonly called "semi-natural spaces" although there is no official definition for those spaces in Brussels legislation. Data have been gathered through interviews with stakeholders, analysis of regional mapping and exploration of urban forms around those spaces. The case studies show that informal open spaces allow an amalgam of temporary and spontaneous uses and occupations. Multiple stakeholders are permanently using and transforming these spaces. In order to support the unregulated characteristics of informal spaces, planners need to develop an alternative planning approach that accepts this co-production and permanent temporality. In this respect, fringes of informal spaces appear to be strategic. Fringes are room for a better transition between the informal open space and its urban context while the core of the space could remain as informal as possible. Yet, today, fringes of informal open spaces are poorly designed without considering the nearby open space. Research findings show that urban quality of the fringes could be improved by property transactions (acquisition, selling, exchange, re-parceling) and urban form regulation. Future assessments should evaluate how this two strategies -fostering the co-production and the permanent temporality (1) and designing the edges as transitional space (2)- could be implemented on other informal open spaces.