Barely had the dust settled on Notre Dame Cathedral, after the horrifying fire in April, than the world’s architects started cranking out visions of how the famous spire could be rebuilt.

Norman Foster was one of the first out of the gates, proposing to build a “light and airy” vaulted glass roof, topped with a crystal spire and an observation deck. It should, he said, be “a work of art about light … contemporary and very spiritual, and capture the confident spirit of the time”. Some compared the concept to his acclaimed transformation of the Reichstag in Berlin; others thought it would be more appropriate for a conference centre in Essex.

The Swedish practice UMA proposed installing a giant swimming pool on the roof, with the statues of the 12 apostles repositioned like lifeguards around the edge. The French firm Studio NAB proposed turning the roof into an educational greenhouse, with a new spire conceived as a giant beehive. Mathieu Lehanneur, a French designer, suggested erecting an enormous golden flame where the spire once stood. “I love this idea of a frozen moment in history that can remain for centuries,” he said. “For me, it’s a way to capture the catastrophe and turn it into beauty, turning ephemeral into permanency.”

These provocative visions had been prompted by the hastily spoken words of the French prime minister, Édouard Philippe, who, two days after the fire, declared that there would be an international architectural competition to rebuild Notre Dame. He called for “a spire suited to the techniques and challenges of our time”, following President Emmanuel Macron’s rash pledge the day before, that it would be rebuilt “more beautiful than before” – and completed in just five years’ time.

The surge of promises, pledges and bold plans made in the days immediately following the blaze was accompanied by an orgy of wallet-waving. The world’s billionaires queued up to help, each seemingly eager to be seen to donate more than the next. François-Henri Pinault, owner of Gucci, pledged €100m (£84.5m), only to be outdone by Bernard Arnault, boss of Louis Vuitton, with €200m. The heirs to L’Oréal piped up too, as did Apple, Total and many others. Within 48 hours, nearly €1bn had been pledged. So what has happened since?

As the neo-gothic steeple buckled and folded into the roaring furnace, many feared all was lost. In fact, the building had a lucky escape. The great wooden roof – nicknamed “the forest” for the thousands of trees used in its beams – was gone, along with the 19th-century spire, but the magnificent 13th-century rose window, the great organ, the innumerable sacred relics and most of the stone structure remained intact.

Facebook Twitter Pinterest A Swedish architectural practice, UMA, proposed not rebuilding the spire and instead having a swimming pool on the roof. Photograph: u-m-a.se

The chief complication in excavating the site to determine the extent of the damage has been the fact that the building was under restoration at the time of the fire – and therefore stuffed with a cat’s cradle of scaffolding. Miraculously, it didn’t collapse in the 800C heat, but many of the 50,000 poles melted, fusing together in a mangled matrix of steel that has been fiendishly difficult to take apart without damaging the stonework below. There was also a long hiatus over the summer because of a lead-poisoning scare. More than 400 tonnes of lead on the roof was vaporised by the fire, dispersing into the atmosphere and leading to a lengthy cleanup operation before work could progress.

As the scale of the operation becomes apparent, Macron’s five-year goal is looking increasingly unlikely. At a press conference in October, the French culture minister, Franck Riester, said the five-year timetable had been an “ambition”, adding “our obsession is with quality” rather than speed. Patrick Chauvet, the top administrative cleric of Notre Dame, reinforced the slow and steady approach, saying people shouldn’t expect visible progress any time soon. “We’re still in the first phase, the phase of securing [the monument’s structure], which is lasting longer than initially planned,” he said. “Then there will be the second phase, dedicated to assessing the situation; we will work out how much the restoration will cost. The third phase, which will start in 2021, will be the restoration phase itself.”

No decision has been publicly announced as to how the spire will be rebuilt, but, behind closed doors, the debate remains heated. In November, General Jean-Louis Georgelin, the army chief appointed to oversee reconstruction by Macron, told the chief conservation architect, Philippe Villeneuve, to “shut his mouth”, prompting astonished gasps at a meeting of the national assembly’s cultural affairs committee. Villeneuve has publicly stated that he is not in favour of a “flèche signature” (a trophy spire), a position that seems increasingly common. PThe prominent French architect Jean Nouvel agrees this is not the place for a contemporary addition: “We have to be more gothic than ever,” he has said. An opinion poll found that 55% of the French public want the spire to be rebuilt exactly as it was.

But others maintain this is a once-in-a-lifetime opportunity, beyond rethinking just the spire. “Paris has a problem with modern architecture,” says Jean-François Cabestan, an architectural historian. “Here is a chance for a proper international debate, not only about the spire, but how to replan the surrounding Île de la Cité – to turn it from a no man’s land for tourists into a real part of Paris.”