In Brazil, the Legal Reserve is an area in a rural property established by Brazilian law to conserve native vegetation due to its importance in maintaining biodiversity and ecosystem functions for ecological balance. In urban areas, the Law for the Protection of Native Vegetation states that the Legal Reserve is to be converted into green areas. However, it does not make it clear whether this is automatic and mandatory or an option for the public authority, nor what must be the extension of this area or its level of protection. In this chapter, we present an ecological interpretation of Brazilian legislation, arguing that the incorporation of rural areas into the urban perimeter and its subsequent expansion requires the preservation of the Legal Reserve’s ecological functions. We demonstrate that the permanent conservation of these vital areas is supported by a robust legal framework, including the Law for the Protection of Native Vegetation (LPNV), the Brazilian Constitution, and the international Kunming-Montreal Global Biodiversity Framework. This interpretation ensures the continuity of essential ecosystem services and biodiversity conservation as urban areas expand. The characteristics and ecosystem functions of these native vegetation fragments provide an ecological equilibrium for present and future generations when incorporated into the urban areas, mainly due to their capacity for mitigating the negative impacts of urbanization on the well-being of the population, conserving urban biodiversity and for adapting to and mitigating climate change.