Abstract: A method and a building system for manufacturing a floating structure, for instance a watercraft, in which method the hull part for the floating structure is formed, it is provided with deck structures or similar support structures, and space units are prefabricated and installed between said deck structures or similar support structures. In the method such space units are manufactured, which comprise engineering units, which are provided at least with WC-equipment, as well as room units so that an engineering unit interconnected with a room unit forms a cabin unit. The engineering units of two adjacent cabin units are prefabricated and joined together to form an engineering unit pair and the thus accomplished engineering unit pair is transferred to and installed in its place on the floating structure.