The Art Nouveau movement of architecture and design first appeared in Brussels, Belgium, in the early 1890s, and quickly spread to France and to the rest of Europe.

It began as a reaction against the formal vocabulary of European academic art, eclecticism and historicism of the 19th century, and was based upon an innovative use of new materials, such as iron and glass, to open larger interior spaces and provide maximum light; curving lines such as the whiplash line; and other designs inspired by plants and other natural forms.

Some of Brussels' municipalities, such as Schaerbeek, Etterbeek, Ixelles, and Saint-Gilles, were developed during the heyday of Art Nouveau and have many buildings in that style.

Through Beyaert, Hankar became an admirer of Eugène Viollet-le-Duc, the French architect who advocated the use of innovative new materials such as iron and glass, while drawing from historical architecture for inspiration.

To decorate it, he brought together the talents of several of his artist friends, including the sculptor René Janssens and the painter Albert Ciamberlani, who adorned the façade with sgraffiti, or layers of plaster tinted in pastel colours onto a moistened surface, a technique popular in Renaissance Italy in the 15th and 16th centuries.

He died in January 1901 at the age of 41, but his style influenced the work of younger Brussels' artists, including Paul Hamesse, Léon Sneyers, Antoine Pompe and the modernist Victor Bourgeois.

He subsequently worked for the neoclassical architect Alphonse Balat, who was in the midst of constructing the enormous glass and iron Royal Greenhouses of Laeken in northern Brussels for King Leopold II.

The interior decoration featured curling lines, modelled after vines and flowers, which were repeated in the ironwork railings of the stairway, in the tiles of the floor, in the glass of the doors and skylights, and painted on the walls.

In designating these sites, UNESCO explained: "The stylistic revolution represented by these works is characterised by their open plan, the diffusion of light, and the brilliant joining of the curved lines of decoration with the structure of the building.

He applied the same original combination of a steel frame, open plan, skylights and functional features, without the decoration and luxury materials, for several larger buildings, including the headquarters of the Belgian Workers' Party (POB/BWP) or Maison du Peuple/Volkshuis (built in 1896–1899, and demolished in 1965).

After about 1910, Art Nouveau features gradually disappeared from Horta's work, as his style evolved into a fusion of neoclassicism and early modernism.

He not only created vases in organic and floral forms, but also jewellery, bronzes, lamps, glassware, and other decorative objects, produced mostly for the leading Belgian glass factory of Val Saint Lambert.

[19] Victor Horta, who had worked on the construction of iron and glass Royal Greenhouses of Laeken, was one of the first to create Art Nouveau ironwork.

[19] Many Brussels architects, including Victor Horta and Henry Van de Velde, also designed the furniture and other interior decoration of their houses.

His work often crossed the frontiers between sculpture and decorative art, inspired by the lines of forms ranging from dragonflies to bats to Grecian masks.

[21] Another important figure was Frans Hoosemans, who made small works of sculpture, candlesticks and other objects using ivory, silver, and other precious materials.

[9] The Stoclet Palace, built between 1905 and 1911 by the Austrian architect Josef Hoffmann, one of the founders of the Viennese Secession, has also been listed as a World Heritage Site since 2009.