A gravity based foundation for an offshore installation includes a caisson of concrete and a hollow shaft. The caisson has a bottom slab, a roof and a side wall extending between the bottom slab and the roof. The roof having a passage for the shaft into the caisson. The shaft support has embedded tensioning bars vertically projecting from the upper side of the shaft support. The shaft is mounted on the shaft support by the tensioning bars. A method of manufacturing includes providing a concrete bottom slab, arranging a full-length formwork onto the bottom slab, arranging a slip formwork onto the bottom slab, providing the tensioning bars and mounting the tensioning bars in a fixed position to the full-length formwork, and concrete pouring of the sidewall and shaft support while raising the slip formwork.