The Nazi era was a very difficult time for Hundertwasser and his mother Elsa, who were Jewish. They avoided persecution by posing as Christians, a credible ruse as Hundertwasser’s father had been a Catholic. Hundertwasser was baptized as a Catholic in 1935. To remain inconspicuous, Hundertwasser also joined the Hitler Youth.

In 1957 Hundertwasser acquired a farm on the edge of Normandy. Hundertwasser married Herta Leitner in 1958 but they divorced two years later. He married again in 1962 to the Japanese artist Yuko Ikewada but she divorced him in 1966. By this time, he had gained a popular reputation for his art.

From the early 1950s he increasingly focused on architecture, advocating more just human and environmental friendly buildings. This began with manifestos, essays and demonstrations. For example, he read out his “Mouldiness Manifesto against Rationalism in Architecture” in 1958 on the occasion of an art and architectural event held at the Seckau Monastery. He rejected the straight line and the functional architecture. In Munich in 1967 he gave a lecture called “Speech in Nude for the Right to a Third Skin”. His lecture “Loose from Loos, A Law Permitting Individual Buildings Alterations or Architecture-Boycott Manifesto”, was given at the Concordia Press Club in Vienna in 1968.

Beginning in the 1950s Hundertwasser traveled globally promoting ecological causes. In 1959 Hundertwasser got involved with helping the Dalai Lama escape from Tibet by campaigning for the Tibetan religious leader in Carl Laszlo’s magazine Panderma. In later years, when he was already a known artist, Friedensreich Hundertwasser became an environmental activist and most recently operated as a more prominent opponent of the European Union, advocating the preservation of regional peculiarities.

In 1964 Hundertwasser bought “Hahnsäge”, a former saw mill, in the sparsely populated Lower Austria’s Waldviertel. There, far from the hustle and bustle and surrounded by nature, he set up a new home.

In 1972 Hundertwasser incorporated a stock company, the “Grüner Janura AG”, in Switzerland; in 2008 it was renamed as “Namida AG”. Hundertwasser managed his intellectual property rights through this company.

For Hundertwasser, human misery was a result of the rational, sterile, monotonous architecture, built following the tradition of the Austrian architect Adolf Loos, author of the modernist manifesto Ornament and crime (1908). He called for a boycott of this type of architecture, and demanded instead creative freedom of building, and the right to create individual structures. In 1972 he published the manifesto Your window right — your tree duty. Planting trees in an urban environment was to become obligatory: “If man walks in nature’s midst, then he is nature’s guest and must learn to behave as a well-brought-up guest.” Hundertwasser propagated a type of architecture in harmony with nature in his ecological commitment. He campaigned for the preservation of the natural habitat and demanded a life in accordance with the laws of nature. He wrote numerous manifestos, lectured and designed posters in favor of nature protection, including against nuclear power, to save the oceans and the whales and to protect the rain forest. He was also an advocate of composting toilets and the principle of constructed wetland. He perceived feces not as nauseous but as part of the cycle of nature. His beliefs are testified by his manifesto The Holy Shit and his DIY guide for building a composting toilet.

In the 1970s, Hundertwasser had his first architectural models built. The models for the Eurovision TV-show “Wünsch Dir was” (Make a Wish) in 1972 exemplified his ideas on forested roofs, tree tenants and the window right. In these and similar models he developed new architectural shapes, such as the spiral house, the eye-slit house, the terrace house and the high-rise meadow house. In 1974, Peter Manhardt made models for him of the pit-house, the grass roof house and the green service station – along with his idea of the invisible, inaudible Green Motorway.

In 1979 Hundertwasser bought the vast historical garden Giardino Eden including the Palazzo Villa delle Rose, from Alexandra of Yugoslavia via his Swiss company.

In 1980, Hundertwasser visited Washington D.C. to support activist Ralph Nader’s efforts to oppose nuclear proliferation. Mayor Marion Barry declared November 18 to be Hundertwasser Day as a result. Hundertwasser planted trees in Judiciary Square and advocated on behalf of a co-op apartment owner who was taken to court for installing a bay window.

In 1982, Hundertwasser’s only child, his daughter Heidi Trimmel, was born.

In 1999 Hundertwasser started his last project named Die Grüne Zitadelle von Magdeburg (in German). Although he never completed this work, the building was built a few years later in Magdeburg, a town in eastern Germany, and opened on October 3, 2005.

Hundertwasser was buried in New Zealand after his death at sea on the Queen Elizabeth 2 in 2000 at the age of 71.

The Austrian post office used more Hundertwasser motives for the European edition 1987 (Modern architecture, Hundertwasser House), on the occasion of his death in 2000 (painting Blue Blues, under the WIPA 2000) and 2004 National Donauauen (poster: The outdoors is our freedom at civil protests in Hainburg).