At the crossroads of spatial planning and agriculture, this article explores how planning practices impact on-farm diversification strategies in peri-urban Mediterranean France. It focuses on farm building permit applications, statistically analyzing a database of 860 buildings applied for and interviewing local authorities and farmers’ representatives. Results show the large proportion of applications related to reterritorialization of food systems, development of recreational activities, and production of green energy. These on-farm diversification trends highlight the transformation of peri-urban areas from a natural or agricultural landscape into a multifunctional landscape, posing several planning challenges. Planners don’t know how to assess whether a new building is really essential for farming, or to find the right trade-offs to support farming dynamics while limiting soil sealing. Their assessment criteria generally favor large, full-time, and economically viable farms. Comparing two provinces’ planning practices, we observe significant differences in interpretation of national law, with incoherent or unharmonized criteria applied especially when assessing professional farming activities, new dwellings, and solar energy projects. We also highlight two potential obstacles to farmers’ generational renewal and agroecological or food transitions: i) lack of transparency regarding the rules and the decision-making processes behind building permit assessment, and ii) the frequent rejection of applications for multi-purpose premises and lightweight, mobile, or dismantlable structures. Such results point to the role of planners in transforming peri-urban landscapes and the need for more consistent regulation of new farm buildings, supporting and not hindering farm adaptation strategies and the multifunctionality of peri-urban areas.