A storage system features a fleet of storage/retrieval vehicles and a gridded three-dimensional structure. The structure features a gridded two-dimensional track layout on which the one or more storage/retrieval vehicles are conveyable in two directions, and a plurality of storage columns residing above or below the gridded track layout in spaced distribution throughout the two-dimensional area of the track layout. Upright shafts reside above or below the gridded track layout and provide vehicle access to the storage columns. At least one working station resides outside the two-dimensional area of the track layout, and via one or more extension tracks, is served by the same vehicles that navigate the gridded structure.