Emerging models for context-aware role-based access control pose challenging requirements over policy administration. In this paper we address the issues raised by the decentralized administration of a spatially-aware access control model in a mobile setting. We present GEO-RBAC Admin, the administration model for the GEO-RBAC model. The model is based on the notion of hierarchy of spatial domains; a spatial domain is an entity grouping objects based on organizational and spatial proximity criteria. In the paper we formally define the model and introduce and prove relevant properties.