A method for building an offshore oil, gas or wind farm platform, wherein at a first location a topside for said oil, gas or windmill farm platform is fastened on a ship or barge by means of first grillage, whereby the topside is at first height relative to the waterline; said ship or barge sails to a second location where said topside is unfastened, raised, and fastened on a ship or barge by means of a second grillage, whereby the topside is at second height relative to the waterline, wherein the second height is higher than the first height; said ship or barge sails to a third location where said topside is placed on top of a jacket of said oil, gas or wind farm platform; said second location is provided with a lifting device; wherein said lifting device comprises at least four lifting units; wherein the ship or barge with the topside moves between said lifting units; wherein said topside is placed on support points of the lifting units and the first grillage is unfastened from the ship; wherein said topside is moved in vertical direction to a higher position by means of actuators; the topside is fastened to the ship or barge by means of a second grillage whereby the topside is at said second height relative to the waterline.