They’re like two ends of a see-saw, London and Paris. One rises, the other falls. I think it might be our turn again.” Brendan Macfarlane has been waiting for this moment. The co-founder with Dominique Jakob of the hottest architectural firm in France — Jakob Macfarlane — has just left his calling card: the most architecturally radical building built in the centre of Paris since I. M. Pei stuck a glass pyramid in the Louvre in the 1980s.

There’s no missing the Cité de la Mode et du Design. Bulging out over the Seine beside the Gare d’Austerlitz, its computer-generated angles and facets make the austere, Norman Foster-ish glass and steel blocks behind look instantly dated. On top of that, it’s green. Bright green. The green of peas or freshly unfurled leaves. So green, in fact, that it shimmers, almost metallic, in the winter sun. “This green stuff . . . ,” Nicolas Sarkozy sniffed on its completion, “that must be architecture.” Dissed by the President of the Republic — “You can’t buy credibility like that,” Macfarlane laughs.

Paris — a city seemingly composed of varying shades of taupe — is not a place given to wanton flashes of colour. Macfarlane disagrees. “The Eiffel Tower’s painted green. Look closely and the city’s a riot of colour.” He took his cue, he says, from the water of the Seine, tinted a milky absinthe, and the trees that line it through the city. “The city thought about putting a park here, so we sort of gave them that as well.”

For the Cité — designed to house the most prestigious fashion school in France and a massive new design gallery — is not just a building. It’s a promenade. Cut through it at ground level a wide arcade continues the Seine-side walkways for which Paris is famous. Traversing it, a second path digs beneath from the Gare d’Austerlitz on one side to a new water-taxi station on the other. The green stuff, though, houses a network of paths and stairs that meander up and down the building and over the Seine, like a rollercoaster. “We wanted this to be a porous building, entirely open to the public, one you explore,” Macfarlane explains, “that allows you that intimate relationship with the river.”

The flowing lines of the façade, he says, were inspired by the movement of the Seine below. It’s like a gigantic camera obscura, a device for looking out of, taking in the carefully framed views. But the pièce de résistance is on the roof: a massive public square in oak decking, like the prow of a ship, suspending you high in the air, with the Paris roofscape all around, and topped with plant-covered artificial hills, housing a restaurant, offices and terraces. It takes your breath away.

The Cité is Macfarlane’s most important commission to date. But it’s more than that. It heralds not only the arrival of a new generation of radical French architects, but also, as Macfarlane suggests, a return to form for a city which, architecturally at least, has long been in the doldrums. Paris’s fortune seems inextricably linked to its counterpart across the Channel. Back in the 1980s — when Britain’s architectural scene was drearily stuck in its Modernist v the Prince of Wales zero-sum game — all eyes were on Paris, where President Mitterand indulged his “droit du prince” by building grand projet after grand projet.

When London’s fortunes returned in the 1990s, Paris became stuck in a torpor, while London built Tate Modern, the Eye, the new Great Court at the British Museum. “When Paris lost to London in its 2012 Olympic bid,” Macfarlane says, “and rioters took to the streets in the banlieus — that was the low point.”

In part, he says, “it’s a simple generational shift. For years, architecture in France has been stuck in a time warp. It’s odd. France is so provocative in so many fields, so open to other cultures, yet in architecture it’s seemed trapped. Its schools of architecture have been pretty conservative, inward looking.” They’ve been dominated by the “soixant-huitards”, the May 1968 generation.

France does, though, have a magnificent system of state support for architecture. Its state building programme — schools, mediathèques, health centres, galleries, etc — has been consistent and extensive for decades. Each public project, too, by law must be subject to an architectural competition, judged by a vast network of local juries, and for which each entrant is paid.

“The competition system basically keeps architects alive,” Macfarlane says. “Since 1977 we are the only country in the world which has a law specifically safeguarding architecture,” says Francis Rambert, the director of the Institut Français d’Architecture and curator of the French pavilion at this year’s Venice Biennale. “It states that architecture is of public interest.” A simple, but great symbolic act. With the state underwriting so much building, Rambert thinks that France’s architects might be spared the excesses of Britain’s market-led construction industry.

But this comfortable culture can lead to a deep complacency. “There’s an establishment, for sure,” Macfarlane says. “The buildings are mostly built as designed, with good quality, but anything unusual looking, that doesn’t conform to what that generation likes — clean, neat, modernist, predictable — is rooted out.”

“To be fair,” Rambert counters, “we have had a lot of shadows to emerge from: Le Corbusier, Pompidou, Mitterand. And today, Jean Nouvel.” For a country that practically invented the icon project with Louis XIV, there has of late been a curious suspicion of new architecture — especially recent advances in computer-aided design. You will find little work by Daniel Libeskind, Frank Gehry, Zaha Hadid or Rem Koolhaas in France. While this has spared France cheaper, cheesier, Dubai-apeing icons, it has also left it risk-averse. The new generation, though, has benefited from the Erasmus scheme, which sends students on placements around the world.

“Erasmus is exceptionally important,” Macfarlane says, “because it opens students’ eyes to what’s happening round the world. You get to work in Japan or on the West Coast of America. Then you come back with new blood. These are the people who will take France forward.”

But it’s France’s old tradition — the droit du prince — that is saving the country again. Last year President Sarkozy invited the world’s foremost architects — Foster, Richard Rogers and Hadid among them — to the opening of the biggest architectural museum in the world, the Cité de l’Architecture at the Palais de Chaillot, and called on them to “give back the possibility of boldness to architecture”. And Paris, once again, will be the focus. Its Mayor, Bertrand Delanoë, though politically opposed to Sarkozy, shares — some say even inspired — his architectural patronage. Since 2003 he’s been attempting to right Paris’s urban wrongs — particularly the city’s exclusivity.

Property prices have risen steeply in the past decade, and those who can’t afford to live within the traditional “Cité” marked by the péripherique ring road have simply left. The riots of November last year were, in part, caused by this sense of alienation in the suburbs of Paris. “The riots were, of course, a turning point,” Rambert says. “We have two worlds living side by side divided only by a motorway.” Social housing for key workers has been at the top of Delanoë’s agenda. “But there was a fear that Paris was, to use that cliché, turning into a museum,” Rambert adds.

With successful schemes such as the Paris plage and the vélib bike-hire scheme, Delanoë has been been adding a new edge to the city that he feels is in danger of ossifying. Lately too, suggesting that Paris has been unable to compete economically with London and Barcelona, he has tried loosening the strict conservation rules that keep the city looking so intact.

In September, the Paris council voted to drop the 1977 ban on new buildings taller than 25m (82ft) in the centre, 37m (121ft)farther out. “Paris is on the move,” the deputy mayor, Anne Hidalgo, says. Height will no longer “be a taboo subject”. On the cards are several new skyscrapers: Le Projet Triangle, a 50-storey pyramid at Porte de Versailles in the south, by Herzog & De Meuron; at La Défense, the complex begun in the 1960s as Paris’s financial quarter, there are Sarkozy-backed plans to build three more, including the Tour Phare by the radical West Coast American firm Morphosis, led by the Pritzker prizewinner Thom Mayne, and Jean Nouvel’s 300m Tour Signal. Recent opinion polls, though, suggest that more than two thirds of Parisians oppose them.

“This is not about building skyscrapers anywhere,” says Rambert, one of the judges that picked Morphosis. “But about carefully placing them.” Delanoë has said that he “will accept no project that is not a true work of art”. “For me the horizontal versus vertical debate is the wrong one,” Macfarlane says. “Parisians have this lurking horror of the Tour Montparnasse. We have to get over it. The debate should be about new types of buildings, more complex ones, more environmentally responsive, more durable long term, adaptable. Being clever about it, not getting stuck in this binary debate either up or down.”

Paris has filled with wealthy Nimbys, interested in protecting their way of life and property values but at the expense of the overall health of the city. The biggest project of all is Sarkozy’s “Grand Paris” scheme to knit together the Cité and the banlieues; ten architects have been picked to solve the biggest problem of all. Shortlisted designs will be announced in February. “It’s a little disappointing the architects are so conventional,” Macfarlane says, “but there’s no denying the project’s importance. It’s not about styles or height. It’s about the future of Paris, the future of France. Paris has been so forward thinking about urbanism for so long.” It’s time to reclaim the crown.

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