There are other, lesser, schemes, such as a weird warped tower planned for Waterloo by architects RHWL. All of these are to do with the same thing: square footage, office space. Such commercial towers, to be found in any of the world's great financial centres, are like three-dimensional wealth graphs. The more money there is sloshing around, the more people in big firms want to locate there, the more rent you can charge, and the taller the towers tend to go. When space runs out, new space is found: just as Docklands relieved pressure on the City in the 1980s and 1990s, so Paddington, out west and handy for Heathrow, is now spawning its own crop of mini-towers and groundscrapers. As it happens, there is far too much empty office space in London right now. Even new buildings are standing empty. But the tower-builders smell a return of the market: they want to build for the next peak.

There used to be a limit to all this in central London. It might not look like it, but we used to have one of the most protected skylines in the world. A complex planning system of invisible viewing-corridors, radiating outwards from the sacrosanct dome of St. Paul's Cathedral, left only a relatively few places where skyscrapers could be built near the centre. Most people agreed that a cluster of towers in the City, and another cluster at Canary Wharf, was fine. But then the Shard was approved.

The Shard is not in the central cluster, and it gets in the way of St. Paul's from certain angles. But it got its permission anyway. It is outside the City, south of the Thames in Southwark. As Peter Rees, knowledgeable chief planner for the City of London since the mid 1980s, puts it: "Following the Shard decision, I don't know what the rules are any more. Although I'm in favour of tall buildings, I'm also in favour of being able to see St. Paul's." The danger Rees sees is the prospect of London's few picturesque concentrations of tower being eroded by "dragons' teeth" - other towers popping up here and there, all over, spoiling the party. He doesn't want that. He thinks desirable clusters of new towers could emerge in Croydon, and lower ones around Paddington and maybe King's Cross, but he hates the idea that they could just be dropped in willy-nilly. Although it's well outside his patch, Rees dislikes the idea that a colossal redevelopment plan for the Elephant and Castle in south London - for all that it is masterplanned by Lord Foster - will involve a couple of landmark towers. Foster, for his part, talks about another coherent cluster starting to emerge there.

The key thing is that towers - because of all the people they contain, surging in and out of them according to the tides of the working day - have to be built close to public transport interchanges so as not to overload the roads. Hence the ones planned at Paddington, London Bridge, Waterloo, and now Elephant and Castle. But while the emphasis so far has been on office buildings, the story is quite a bit different when it comes to homes. Towers that you live in. It's not just a question of reaching ever higher. It's a matter of re-thinking the whole social and physical structure of such places. And this is where Skyhouse comes in.