K21, Art museum and heritage monument in Unterbilk, Germany

K21 occupies the Ständehaus, built 1876–1880 by Julius C. Raschdorff as a four-wing complex around an inner courtyard, modelled on Italian Renaissance palazzi such as the Palazzo Farnese, with a mansard roof echoing 17th-century French architecture. The 1943 fire left only the outer walls; Hans Schwippert's 1947–49 rebuild replaced the roof and extended the plenary hall. The 1996–2001 conversion by Kiessler+Partner restored the courtyard layout, added a glass-and-steel dome and a hall over 6 m high beneath the building, whose porthole windows look onto and under the Kaiserteich.

The Ständehaus was built between 1876 and 1880 to designs by Julius C. Raschdorff as the parliament and administrative seat of the Rhine Provincial Diet, replacing the Düsseldorf Stadtschloss that burned in 1872. After the 1943 bombing left only outer walls, Hans Schwippert rebuilt it in 1947–49, and from 15 March 1949 it housed North Rhine-Westphalia's first elected Landtag until 1988. The 1996–2001 conversion by Kiessler+Partner turned it into the K21 museum, inaugurated on 18 April 2002.

The name K21 marks it as the second site of the Kunstsammlung Nordrhein-Westfalen, whose main building at Grabbeplatz is K20; contemporary art from about 1980 onward is shown here. In the ground-floor bar facing the Kaiserteich, Cuban-American artist Jorge Pardo designed murals and a light installation. Sculptures by contemporary artists are placed in the surrounding park.

The entrance is on the north facade, where the bronze fountain Vater Rhein und seine Töchter stands. The glass dome on the top floor doubles as a viewing space over Düsseldorf, and the underground hall reaches north to the Kaiserteich, with porthole windows looking out at and beneath the water's surface.

A large glass dome was integrated into the original architecture, creating a striking modern contrast to the building's classical elements. This addition enables new perspectives on the art exhibitions and subtly changes the light within the rooms.