Skyhouse is not one tower but three. Technically, it shares some genes with Lasdun's little "cluster block" in Bethnal Green in that its bundle of slender towers is supported by a common, exposed, backbone (Marks and Barfield say they are unaware of the precedent). The difference is that Lasdun's pioneering example - his conscious rejection of the slab-block aesthetic - rises to all of 15 storeys: Skyhouse will reach 72.

From the left: (1) Cesar Pelli's Petronas towers, Kuala Lumpur, 450m: (2) proposed London Bridge tower (Renzo Piano), 420m; (3) Skyhouse by Marks Barfield, 305m; (4) Canaray Wharf tower (Pelli), 237m; (5) Tower 42 (former NatWest Tower, by Richard Seifert) 182m; (6) Norman Foster's proposed Swiss Re tower, 180m; (7) London Eye observation wheel (Marks Barfield) 135m.

There is an important difference between an office tower and an apartment tower: the apartment tower can be a lot more slender, simply because half as many people occupy it, so it needs fewer lifts, stairs and lavatories - the things that occupy the fat "cores" of office blocks, and makes too many of them look dumpy. Marks and Barfield go further still by splitting their tower into three sections of different heights, each teardrop-shaped on plan, radiating from that central spine. This means that their towers don't need so much heavy structure to make them stable. Again, the result is lightness - both actual and visual.

Their ideas go further than just providing flats. Returning to Le Corbusier's notion of the "Unite d'habitation", they propose shops, health clubs, nurseries, restaurants and gardens in Skyhouse. Nor will all the apartments be for sale: they want around a quarter of them to be social, low-rent, flats. Another chunk, they want to be "live-work" units, whence people can run businesses. But the idea is to mix these all up together. "We call it 'cohesive living', says Marks. "We don't want to have ghettos. People are more willing today to live in flats than at any time in British history. The key to the idea is to have a real mixed society."

The difference between now and the last time architects designed "streets in the sky" is that in those days, it was all for one class - working-class council tenants. Nobody ever tried a genuinely mixed-society tower in Britain. And this is likely to be the biggest challenge for the architects, because financiers tend to lend money only to projects of a kind they are used to. It's difficult enough even getting funding for apartments and offices in the same building - something that is commonplace in the United States, for example.