Method for manufacturing a gravity based foundation for an offshore installation, wherein the foundation comprises a caisson of concrete and a hollow shaft, the caisson having a bottom slab, a roof and a side wall extending between the bottom slab and the roof to define a hollow structure delimited by the bottom slab, the roof and the side wall, the roof having a passage for the shaft, the shaft extending with its lower part through the passage into the caisson; wherein the bottom slab is provided with a shaft support of concrete, the shaft support having tensioning bars embedded in the concrete of the shaft support and vertically projecting from the upper side of the shaft support; wherein the shaft has a flange at its lower part, the flange having bolt passages (18, 18a, 18b) and being supported and mounted on the shaft support by means of the tensioning bars extending through the bolt passages (18, 18a, 18b); wherein the shaft support is hollow and has an inner side and an outer side; the method comprising the following steps: - providing a concrete bottom slab, - providing a full-length formwork configured for defining the inner side of the shaft support and having a vertical length corresponding to the vertical length of the shaft support, and arranging the full-length formwork onto the bottom slab, - providing a slip formwork (41, 42) configured for defining the side wall and the outer side of the shaft support and having a vertical height smaller than the vertical length of the full-length formwork, and arranging the slip formwork (41, 42) 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 side wall and shaft support by pouring concrete into the spaces delimited between the slip formwork (41, 42) and the full-length formwork while raising the slip formwork (41, 42). wherein the bottom slab is provided with a shaft support of concrete, the shaft support having tensioning bars embedded in the concrete of the shaft support and vertically projecting from the upper side of the shaft support; wherein the shaft has a flange at its lower part, the flange having bolt passages (18, 18a, 18b) and being supported and mounted on the shaft support by means of the tensioning bars extending through the bolt passages (18, 18a, 18b); wherein the shaft support is hollow and has an inner side and an outer side; the method comprising the following steps: - providing a concrete bottom slab, - providing a full-length formwork configured for defining the inner side of the shaft support and having a vertical length corresponding to the vertical length of the shaft support, and arranging the full-length formwork onto the bottom slab, - providing a slip formwork (41, 42) configured for defining the side wall and the outer side of the shaft support and having a vertical height smaller than the vertical length of the full-length formwork, and arranging the slip formwork (41, 42) 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 side wall and shaft support by pouring concrete into the spaces delimited between the slip formwork (41, 42) and the full-length formwork while raising the slip formwork (41, 42). - providing a concrete bottom slab, - providing a full-length formwork configured for defining the inner side of the shaft support and having a vertical length corresponding to the vertical length of the shaft support, and arranging the full-length formwork onto the bottom slab, - providing a slip formwork (41, 42) configured for defining the side wall and the outer side of the shaft support and having a vertical height smaller than the vertical length of the full-length formwork, and arranging the slip formwork (41, 42) 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 side wall and shaft support by pouring concrete into the spaces delimited between the slip formwork (41, 42) and the full-length formwork while raising the slip formwork (41, 42).