All of which means that it's safe once again to re-invent the prefab. Or, as they say these days, "modular housing". This time, there's a lot more architecture involved than there used to be, and the rising generation of young architects is leading the process. But as ever, the drive towards prefabrication is based on economics rather than aesthetics or science. Which is why it is starting up again in London. Because in London, land is dangerously expensive. So if you want to build affordable housing (or, in the private sector, make bigger profits), and you can't get cheap land, then you have to make cheap buildings. And since cheap conventional labour-intensive buildings look cheap and don't last, you have to think of a different kind of cheapness. Economies of scale. Mass production.

It would be easy to conclude that nothing much has changed: that the lower orders, who were shoehorned into tower blocks in the 1950s and 1960s, are now once again being given second-class treatment to the rest of us. Leaving aside the little matter of tower blocks being OK again, not to mention private-sector interest in the subject, this view ignores the fact that the latest generation of prefab social housing is of the medium-rise apartment-block type, looks rather good and is not - so far - aimed at the traditional council-housing market anyway. Instead, it is calculated to appeal to young workers, even professionals, who can't yet afford to buy a house, but who can afford to pay a realistic, less subsidised, rent. It is all a rather Victorian concept - tenements for the artisan class, as they would have described the concept - so it is apt that the pioneering work being carried out here is by one of the oldest and biggest of charitable housing associations, the Peabody Trust.