Every private rural property in Brazil must maintain a percentage of its area with conserved native vegetation, establishing a legal reserve area (LRA). This percentage is defined by the Brazilian Federal Law on Protection of Native Vegetation and is 20–80% of the rural property, depending on the political boundary and vegetation formation. However, the Environmental Code of Mato Grosso stipulates that vegetation type determines the LRA percentage in the state of Mato Grosso, considering both vegetation formation and floristic composition. In addition, the state adopts a coarse-scale vegetation map as a reference, despite the existence of a more accurate official map. In this study, we calculated the impacts of these provisions by combining legal interpretation, spatial analysis and ecological reasoning, in accordance with the scientific concepts of ecology, and intersecting official maps from the State Environmental Agency and the Brazilian Institute of Geography and Statistics, at the scales of 1:1 000 000 and 1:250 000, respectively. A total of 9 045 065 ha could have their LRA requirement reduced from 80% to 35% under Mato Grosso’s legislation, potentially authorizing deforestation in the Amazon, Cerrado and Pantanal, in apparent contradiction to federal law. These findings highlight that issues related to the concepts, classifications and mapping scales for defining native vegetation have significant implications for biodiversity conservation.