The Art Nouveau style of architecture defined the transition between the 19th century and the 20th. It was a global sensation that spawned numerous designers from around the world. Hector Guimard, Charles Rennie, Mackintosh, and Victor Horta were three prominent architects whose work helped foster this movement.

The impetus behind Art Nouveau architectural design was to break with tradition and explore newer and freer forms.

Art Nouveau's broad influence across architecture, design, and the visual arts allowed architects to create total works of art, where every element of a structure from its windows to its door handles to its decorative flourishes could be harmonized to create an Art Nouveau style.

Architects finished the interiors of their buildings as ornate as their exteriors and decorated them with exquisite materials, custom furnishings, and hand-painted wall treatments.

By opening up interior spaces, removing walls, and using skylights, architects made the maximum use of light in their interior. Victor Horta's residence-studio and other houses built by him had extensive skylights, supported on curving iron frames. In the Hotel Tassel he removed the traditional walls around the stairway, so that the stairs became a central element of the interior design.

Art Nouveau architecture was a reaction against the eclectic styles that dominated European architecture in the second half of the 19th century. Architects used decoration, either ornamental or sculptural. While faces of people, called mascarons, were ornament, the use of people in different forms of sculpture was also common.

Characteristics of Art Nouveau Architecture
Art Nouveau style is inspired by the natural world, characterized by sinuous, sculptural, organic shapes, arches, curving lines, and sensual ornamentation. Common motifs include stylized versions of leaves, flowers, vines, insects, animals, and other natural elements.
Decorative elements found on the inside and outside of buildings include intricate mosaic work, stained and curved glass, and decorative wrought iron.

Buildings and their interiors were asymmetrical. Architects extensively employed arches and curved forms, including curving, plant-like embellishments. They also used mosaics on both exteriors and interiors. Stained and curved glass also became an integral part of building designs.

Belgium
Paul Hankar and Victor Horta built the first Art Nouveau town houses in Brussels—the Hankar House by Hankar in 1893 and the Hôtel Tassel by Horta between 1892 and 1893. The theories of the French architect Eugène Viollet-le-Duc inspired Hankar. With a goal to create a synthesis of fine arts and decorative arts, he brought Adolphe Crespin and Albert Ciamberlani to decorate the interior and exterior of the Hankar House with sgraffito, or murals. Hankar decorated stores, restaurants and galleries in completely original motifs.

However, it was Horta who became one of the most influential architects of early Art Nouveau, and his Hôtel Tassel was one of the new style's landmarks. Horta worked as an assistant to Alphonse Balat, architect to King Leopold II, constructing the monumental iron and glass Royal Greenhouses of Laeken. Between 1892 and 1893, he designed the residence of a prominent Belgian chemist, Émile Tassel, on a very narrow and deep site. The central focus of the house was the open stairway, decorated with a curling wrought-iron railing, and placed beneath a high skylight. He supported the floors by slender iron columns like the trunks of trees and decorated the mosaic floors and walls with delicate arabesques in floral and vegetal forms. Horta built three more town houses, all with open interiors, and all with skylights for maximum interior light: the Hôtel Solvay, the Hôtel van Eetvelde, and the Maison & Atelier Horta.

Henry van de Velde, born in Antwerp, was another founding figure of Art Nouveau. Van de Velde's designs included the interior of his residence, the Bloemenwerf, built in 1895. The Red House, the residence of writer and theorist William Morris, the founder of the Arts and Crafts Movement, inspired the exterior. Trained as a painter, Van de Velde turned to illustration, then to furniture design, and finally to architecture. For the Bloemenwerf, he created the textiles, wallpaper, and silverware all of which matched the style of the residence. Van de Velde went to Paris, where he designed furniture and decoration for Sigfried Bing, whose Paris gallery gave the style its name. He was also an early Art Nouveau theorist, demanding the use of dynamic, often opposing lines.

A wave of decorative art accompanied the debut of Art Nouveau architecture in Brussels. was accompanied by a wave of decorative art in the new style. Important artists included Gustave Strauven, who used wrought iron to achieve baroque effects on Brussels facades; the furniture designer Gustave Serrurier-Bovy, known for his highly original chairs and articulated metal furniture.

The Brussels International Exposition held in 1897 brought international attention to the Art Nouveau style; Horta, Hankar, Van de Velde, and Serrurier-Bovy, among others, took part in the design of the fair, and Henri Privat-Livemont created the poster for the exhibition.

France
French artists quickly noticed the new style. After visiting Horta's Hôtel Tassel, Hector Guimard built the Castel Béranger, among the first Paris buildings in the new style, between 1895 and 1898. Parisians had been complaining of the monotony of the architecture of the boulevards built under Napoleon III by Georges-Eugène Haussmann. The Castel Beranger was a curious blend of Neo-Gothic and Art Nouveau, with curving whiplash lines and natural forms.

Parisians welcomed Guimard's original and picturesque style. The Castel Béranger was chosen as one of the best new façades in Paris, launching Guimard's career. Guimard was given the commission to design the entrances for the new Paris Métro system, which brought the style to the attention of the millions of visitors to the city's 1900 Exposition Universelle.

In Paris, Art Nouveau was a reaction to Georges-Eugène Haussmann’s strict architectural restrictions imposed on building facades, including a ban on bow windows until 1903. Art Nouveau architects went to the opposite extreme, most notably in the houses of Jules Lavirotte, which were essentially large works of sculpture, completely covered with decoration. An important neighborhood of Art Nouveau houses appeared in the French city of Nancy, around the Villa Majorelle, designed by Henri Sauvage as a showcase for Majorelle's furniture designs. between 1901 and 1902.

Following the 1900 Exposition Universelle, the capital of Art Nouveau moved to Paris. Jules Lavirotte built the most extravagant residences in the new style. He covered the facades entirely with ceramic sculptural decoration. The most flamboyant example is the Lavirotte Building, at 29, avenue Rapp, built in 1901. Office buildings and department stores featured high courtyards covered with stained glass cupolas and ceramic decoration. The style became especially popular in restaurants and cafés, including Maxim's at 3, Rue Royale, and Le Train Bleu at the Gare de Lyon, built in 1900.

The City of Nancy in Lorraine became the other French capital of the new style. In 1901, a group of artists founded the Alliance Provinciale des Industries d'Art, also known as the L’École de Nancy, dedicated to upsetting the hierarchy that put painting and sculpture above the decorative arts. The architect Henri Sauvage brought the new architectural style to Nancy with his Villa Majorelle in 1902.

Germany
In Berlin Jugendstil was chosen for the construction of several railway stations. The most notable is Bülowstraße by Bruno Möhring (1900–1902), other examples are Mexikoplatz (1902–1904), Botanischer Garten (1908–1909), Frohnau (1908–1910), Wittenbergplatz (1911–1913) and Pankow (1912–1914) stations. Another notable structure of Berlin is Hackesche Höfe (1906) which used polychrome glazed brick for the courtyard facade.

Austria
Vienna became the center of a distinct variant of Art Nouveau, known as the Vienna Secession. The movement took its name from the Munich Secession established in 1892. A group of artists founded the Vienna Secession in April 1897. They objected to the conservatism of the Vienna Künstlerhaus, the official union of artists. The Secession founded a magazine, Ver Sacrum, to promote their works in all media. The architect Joseph Olbrich designed the domed Secession building in the new style.

Otto Wagner, who joined the Succession early on, was the most prominent architect of the Vienna Secession . His major projects included several stations of the urban rail network, or Stadtbahn, the Linke Wienzeile Buildings, consisting of Majolica House, the House of Medallions and the house at Köstlergasse.

United States
Louis Sullivan of Chicago was an important figure in American Art Nouveau architecture. Sullivan was a leading pioneer of American modern architecture. He founded the Chicago School, and designed some of the first skyscrapers. Sullivan firmly believed in the concept of form follows function. While the function of his buildings dictated their form, his decoration was an example of American Art Nouveau. At the 1893 World's Columbian Exposition in Chicago, most famous for the neoclassical architecture of its renowned White City, he designed a spectacular Art Nouveau entrance for the very functional Transportation Building.