London has a great number of magnificent buildings and structures that came and went, that were demolished or destroyed. Buildings such as the original Palace of Westminster, Old St Paul’s Cathedral, the Euston Arch or the Crystal Palace are only available to us as paintings or photographs, tantalising in their absence.

But there are other ways in which the development of the capital could have been greatly different – proposals that stood to make changes to the urban landscape which, had they happened, would have changed the course of the city.

King’s Cross airport

In 1931, architect Charles Glover proposed to increase airborne traffic by building an elevated airport above the railway sidings of King’s Cross. It was a remarkable plan: a pinwheel arrangement of concrete runways, supported directly on top of new buildings, allowing planes to take off in different directions across the city. Like other plans for runways built over the Thames, King’s Cross airport didn’t quite see the light of day. But the perennial problem of air capacity and obsolescent air infrastructure could be very different today if they had.

Watkin’s Tower

Although London was home to the Crystal and Alexandra palaces, it been overshadowed in the modern architecture stakes since 1889 by the Eiffel Tower. Double the height of the next tallest structure in the world, the Eiffel Tower was a true engineering marvel that London couldn’t match.

Sir Edward Watkin, a politician and railway entrepreneur, wanted to put this right. He was laying out a park on land he had bought in the north-west London hamlet of Wembley, and decided the showpiece would be a brand-new iron tower that, crucially, would be taller than the one across the Channel. After one of the maddest design competitions ever held generated 68 ludicrous flights of Victorian whimsy, the chosen design was in the end pretty similar to Eiffel’s, but 46m taller. Construction began in the early 1890s.

Unfortunately, the tower was only built up to the first level before it was abandoned and then demolished a few years later. The site of the tower – which would have been 52m taller than the Shard – is now occupied by Wembley football stadium.

What became known as Watkin’s Folly, had it been built, could have set off an early race to the sky. As it was, the Eiffel Tower’s height was only exceeded 40 years later by the Chrysler Building in New York.

Sunken Soho

In the postwar years, as the UK’s rapid growth in car use threatened to grind cities to a halt, proposals for “comprehensive redevelopment” came thick and fast. The 1963 Buchanan Report proposed, among many other ideas, rebuilding Tottenham Court Road as a flying pedestrian precinct; the London County Council’s Sir William Holford tried to demolish three-quarters of the buildings on Piccadilly Circus.

But perhaps boldest of all was a 1954 plan by Geoffrey Jellico, Ove Arup and Edward Mills to remake the whole of Soho as a concrete landscape of sunken roads, plazas and office towers. It would have involved knocking down much of Soho, and building a raised concrete platform, with 24-storey pinwheel towers, gardens and glass-bottomed canals over the streets beneath.

Covent Garden redevelopment

Covent Garden Is Moving was the name of the 1968 GLC report that outlined the redevelopment of the area around the old fruit and vegetable market. The area had become utterly dysfunctional: the market was too small and the streets were unable to cope with the traffic. Something had to be done. The GLC proposed a full repertoire of modernist redevelopment: sunken roads, raised pedestrian areas, residential and office towers.

Locals rebelled. They set up a campaign group that attracted disgruntled planners, disgusted local politicians and angry young architects. Eventually they got their way, and the project was cancelled, saving the Victorian buildings of the area and turning the tide against comprehensive redevelopment in general. What they didn’t save, however, was the market, which decamped anyway, or the local spirit of the place, which was washed away by commerce.

Hook New Town

In the 1960s, a team of ambitious young designers put together a plan for a new town in Hampshire, to relieve London’s population. It was one of most radical bedroom community settlements proposed in the era: a small, super-dense city of modern houses, where pedestrians and vehicles were completely segregated, and paths ran through parkland to an integrated central complex that contained all the civic and economic functions of the town.

It was passed over, but the book describing these proposals, The Planning of a New Town, would go on to be influential for planners across the world. Perhaps it’s a good thing Hook New Town was never built, though: its closest relative, Cumbernauld in Scotland, has since become a laughing stock famed for the dismalness of its very own concrete town centre.

Coin Street

Fresh from designing two of the most shocking buildings in a generation, the Pompidou Centre and the Lloyds Building, the third part of Richard Rogers’ hat trick was to be a mixed-use scheme for Coin Street, on the South Bank of the Thames. Early versions from 1979 depicted a curving glass arcade, surrounded by high-rise housing and offices in his trademark “guts out” style. Had this gone ahead, it would have cemented Rogers’s early, extreme approach to architecture, and might have made a difference in some of the battles with conservative neo-traditionalists that dominated the 1980s.

Instead, a local campaign led to Coin Street being built as a low-rise housing cooperative, in what was widely considered a huge success for the provision of social housing to Londoners on lower incomes. Nevertheless, when Rogers returned to build in London more than a decade later, he had developed a mature, smoother, more commercially tasteful style, and you can’t help wonder if a whole generation of city buildings were far less bold as a result.

The Fun Palace

In the late 1950s, the pioneering theatre director Joan Littlewood hired architect Cedric Price to create a new kind of theatre – one in which the users would have direct input into the activities within, where no performance would be the same twice. Price, in thrall to cybernetics and the new “network cultures” of the age, designed what was essentially a factory shed, a giant frame with all the internal spaces able to be moved around and reconfigured at will.

The Fun Palaces, as these creations were called, would have sat in various locations in London, including Camden and the Lea Valley, where the Olympic site is now. But the effort to get planning permission foundered against opposition from bewildered churches, community groups and city councils, and funding vanished when the scheme was almost through planning. Since then, its themes of indeterminacy and of architecture that responds to users have become massively influential, not only with architects but also within the art world. In a way, however, the fact that it wasn’t built meant that it can never fail to live up to its revolutionary concept.

The Ringways

The Westway section of elevated roadway and the tower blocks that overlook it are, depending on your point of view, a thrilling ensemble of modernism or a horrific example of the dominance of road engineering over postwar planning. But it could have been much much better, or worse. The Westway is actually just one small part of the breathtakingly extensive traffic infrastructure being planned for London at the time.

By the late 1960s, the GLC had decided that four separate high-speed roads would encircle the capital, a system to be known collectively as the London Ringways. They would plough through open countryside at their furthest extents, and at the innermost were to smash through much of inner London.

The Ringways awoke a great level of protest. People campaigned heavily against the destruction of their neighbourhoods, and the plans were abandoned in 1973. What had been built of Ringways 3 and 4 were joined into the M25 orbital motorway – you can still see the awkward links in the north-west and south-east sections – and a handful of grimy inner sections, such as the Blackwall Tunnel approach, remain as a reminder of what could have happened.

Maplin

By the beginning of the 1970s, there were seriously ambitious plans to upgrade south-east England’s airport capacity and take some of the pressure off Heathrow, which even then was straining. One proposal was to build Maplin airport, at Foulness, on an artificial island eight miles long. The development would include a deep-water container port and a whole new town to serve it.

Trial land reclamations began but the project, like so many others, was scuppered by the oil crisis of 1973. It marks the last hurrah of what you might call the “planned” era, after which Britain began to give up on large-scale infrastructure. Today’s fevered talk of airport expansions, Boris Island and an estuary airport underline its failure.