Architecture: Out with the old, in with the new

London's skyline has already undergone dramatic change. But two developments are accelerating the transformation from historic capital to modern metropolis. Jay Merrick reports

There's hard. And there's very hard. Two major architectural projects in London cast Dickens' unforgivingly "boney light" on what it takes to make acceptable modern sense of old places. In one site, master-planning was the key; in the other, architectural detail has been the big, and very thorny, issue.

First up, More London. This is the biggest mixed-use conglomeration of new architecture on the South Bank, and Foster and Partners are the architectural authors. As smooth as jelly, the sculpted grey-green riverside buildings gaze straight across the Thames over the six-inch-diameter gun barrels of HMS Belfast, into the very heart of the City, and murmur: "You've got competition."

Competition good enough, at the project's half-completion mark, to have sucked in tenants including Guy Hands' Terra Firma Capital Partners, Ernst & Young, Hewitt Associates and the post-millennial manifestation of the Commonwealth Development Corporation, now known by the neologism- cum-choking-fit Actis Aureos Globeleq. From the ninth floor of Ernst & Young's piece of the action, London's skyline can be meticulously audited from Battersea Power Station in the west to Canary Wharf in the east.

Architecture as power-base. Architecture as prime location - in this case, 280,000 square metres of rent-slabs four minutes' stroll from platform nine at London Bridge station, and about five from the Monmouth Coffee House's trendy boudoir in Borough Market. Architecture as daily bread, daily landscape and daily art. So far, the 13-acre More London estate contains London's second-busiest Marks & Spencer Simply Food outlet, a More Champagne Bar, a Caffe Nero and a Cafe Amore; there are 63 mature tulip and red oak trees, fibre-optic gizmos, and public art by Stephan Balkenhol, David Batchelor and the Turner Prize nominee Fiona Banner.

And thus, architecture as data from the Facts and Figures section of its prospectus; a success story, a new place that is already a tree of urban plenty for the 5,000 people who work there, and for the 20,000 who will, in due course. Any judgement of the architecture itself is duly compromised. If Ernst & Young and the rest love their new pads, then the architecture has met the brief set by More London Development.

Is it enough, though? Despite the deep, clear floor plans, the panoramic views, the refined external detailing, the textbook fracturing of the overall massing of the office blocks, there is something bland about the overall effect. Thomas Heatherwick, designer of the B of the Bang starburst sculpture in Manchester, once suggested to me that "the eye likes to reach out and touch things". He's right, because sensory engagement is a dynamic experience. But it's essentially a one-way ticket with the More London buildings that front the Thames. They're like the shining, closely shaved and studiously expressionless faces that are occasionally to be seen emerging from hairdressers in St James's or Jermyn Street. The architecture, like those faces, radiates withheld certainties.

That is hardly surprising: the cranial Greater London Authority headquarters, also designed by Foster, was the first building in the development. It, too, is smooth-skinned, and has clearly had a No 2 cut. These buildings are inscrutable. Our gazes sheer off them. We pass by, like so many pixels on Southwark's urban screen.

There are, though, two telling moments in the development that belie the suave face it presents to the Thames. The Number 6 More London Riverside building provides a genuine moment of architectural pause. The main facade, nominally triangular in plan, carries a series of accordionish folds that are hazily obscured behind a vast screen held well clear of them. And the eye is well and truly held. So, for a moment, is the mind: how can a large building have been made to look almost delicate? Answer: cleverly arranged voids, angles and contrasting textures.

Number 6 is a key building on the estate. It grants access to More London's single most important architectural and space-planning gesture. The architects made sure that the sharp south-western edge of the building can be seen as one comes out of the Tooley Street exits of London Bridge station, used by 1,000 people a minute in the rush hours. From there, it's a short walk to that alluring edge and the delightful exo-screen of Number 6 looming overhead. And then, the payoff: running straight at an angle of 45 degrees to the Thames, a wide pedestrian avenue that divides the completed half of More London from its uncompleted half. The cut is what makes the site work.

We could give it its proper label: the primary masterplanning axis. But let's think of it in plain terms. This is pure common sense and was surely the first felt-tipped line in the project architect's notebook - an urban division that also delivers a crucial connection. If you stand on the pavement in Tooley Street where it hits the angle of Number 6, you are in a part of London that has usurped the Charing Cross area as the capital's population-density hotspot. And from there, the gaze meets, a la Heatherwick, reaches out and touches a collage of Tower Bridge and, beyond it, the City.

The More London development cannot be classed as wholly riveting, but it has created - one cannot help using the development term - an important new urban desire-line. And, in the end, the ability to walk directly from Tooley Street to the southern flank of Tower Bridge adds a civic patina to these gleaming new corporate molars sunk into the South Bank's clay gums.

It's a very different story on the western edge of the City, where Smithfield meets Farringdon Street. What a messy challenge. Could the architects Kohn Pedersen Fox and the developers Thornfield Properties deliver architecture in a conservation area that would provide a strong but pedestrian-porous Farringdon Street frontage - and develop both sides of West Smithfield as it runs up to the market? And do it having demolished a hotly defended cluster of historically interesting buildings?

KPF's president, Lee Polisano, has proposed a cornerstone building that seeks to inject faux-Victorian structural drama into the West Smithfield side of the building - giant, V-shaped steel supports rippling with alloyed muscle. Less dramatic, but far more important, is that pedestrians can escape Farringdon Street through a ground-floor plaza that takes them through to either Smithfield market or Charterhouse Street. Up close, the architecture facing Farringdon Street promises visual texture. From farther away, it would seem much blander. Why? Polisano couldn't go for broke: the rest of that stretch of Farringdon Street, and much of Farringdon Road, is so iron-in-the-soul ugly that showstopper architecture would have been absurd - and refused by City planners, who will hand down their decision on the scheme in the next month.

Polisano has upped the modern-architecture ante in the area, but with circumspection. The rest of the site - Snow Hill and the cut that joins it to West Smithfield - has also been treated tidily. Crucially, the original design for a new cafe and studio building at the sharply prowed junction of the cut with West Smithfield was junked. It resembled the conning tower of a U-boat and would have obliterated the sight-line to the market. A better-looking, and lower-slung, version has replaced it.

And what of the late-Victorian market buildings by Sir Horace Jones that currently stand, obdurate and derelict, on the site? They are not beautiful, though there is certainly a case for their renovation and re-use as, say, a cafe society zone - something along the lines of Jones' Leadenhall Market. But the debate seems academic. Any redeveloper of Jones' market buildings faces an pounds 18m bill to strengthen the roof of the rail tunnel beneath the site. But the real sticking point is that City planners want more offices and rate income. It's hard to imagine that the triple-whammy of cost, rent-slab demand and English Heritage's long-standing indifference to Jones' buildings can be overcome.

Which leaves the City's urbane planning boss, Peter Wynne Rees, to pore over the fine detail of KPF's architecture. I'll bet More London's designers didn't have to sweat as much as Lee Polisano is sweating right now. Rees, too, will be very slightly humid. The new offices may become a commercial success, but will they bring any distinct benefit to Smithfields - and to the leaden gulch formed by Farringdon Street and Farringdon Road?