The invention relates to a system for erecting a building structure, the system comprising a foundation for receiving the building structure, and a plurality of inner and outer walls, roof and floor panels, the panels being adapted to engage each other for defining at least one story of the building structure, wherein the foundation comprises a first section and a second section, the first section being adapted to bear the load of the building structure and the second section being adapted to provide access to the interiors of the building structure.