Boaters in the Turku area may have rubbed their eyes as a barge passed carrying the steel-framed building, which weighs 200 tonnes, en route past Ruissalo to the large island of Hirvensalo.

The structure was lifted using a hydraulic chassis at the STX shipyard and shifted onto pontoons before being placed on the barge.

The building's steel frame was considered sturdy enough to safely transport with the interior decor complete, including tiles.

The company Nostokonepalvelu requested a portable block of flats, as it will be placed on a rented property. If need be, it can be moved again when the lease on the plot runs out.

The structure is built of steel cells, a Finnish invention. The company that produces them, Neapo, describes them as low-cost and low-weight.

"A concrete building of the same strength would weigh five times as much," says CEO Olli Vuola. "This kind of transfer could not be carried out with a concrete structure."

He added that system should be of interest to Asian builders, as it is earthquake-resistant.

Video: Apartment building at sea