Farm diversification is an important component of rural development and policy in Europe. We examine the influence of neighbouring farms on farm diversification decisions. Our analysis investigates spill-over effects between farms and different activities in a spatial econometric framework. Using census data from about 66,000 farms in the Netherlands, we find significant correlations of diversification activities between spatially proximate farms. These are positive for some activities, for example for nature conservation. On a local level, positive spatial dependencies between farms may result from (tacit) cooperation and information sharing within neighbourhoods. However, for other activities, such as on-farm sales, we find negative correlations on a higher spatial level, i.e. within the region, which could result from competition. Spatial aspects of cooperation and competition have important consequences for the success of policies supporting the uptake of farm diversification. Our findings thus reveal that policy measures promoting farm diversification require implementation beyond the scale of individual farms and single activities.