City suit: interpreting a very banal brief on a site of extraordinary historic riches, the architects have made subtle suggestions about achieving urbanity in today's marketing-led world.

As the centre of the biggest conurbation and the most powerful economic focus in Europe, the City of London has been rebuilt generation after generation since it was founded by the Romans. After the Second World War, when a good deal of the City was destroyed by Nazi bombing, the texture of the place was more radically reorganized than it had been since the Great Fire of 1666. Now, post-war commercial architecture (little of it of any distinction) is being replaced by a new generation of office buildings, shiny and designed to fill to the full the envelope permitted by the planning authorities who, until recently, would not allow tall buildings. But planning policy has changed somewhat since the success of the American-style tower development at Canary Wharf, which is outside the City, and has attracted much business away from the traditional centre.

Gresham Street was first radically reorganized in 1845, when the lanes that made up its original track were widened into a modern thoroughfare, largely lined with Victorian office buildings. These set a scale that persisted even after post-war reconstruction. The street is being redeveloped again, piecemeal as has ever been the custom in the City, always fiercely jealous of private property rights.

The scale of many of the surrounding streets has altered over the years, especially recently, since height rules were partly relaxed -- for instance there is a particularly gross monstrosity, Alban Gate by Terry Farrell, a few hundred metres to the north on London Wall, long a menagerie of the worst vulgarities of city commercial architecture. But for all its transformations, Gresham Street has largely retained its Victorian scale. At the corner of Gresham and Noble Streets is a new speculative office block by Farrell's old partner Nicholas Grimshaw, whose firm attempts to continue the scale today.

As in most City buildings, the main action is at the front, with sides and back muted. The building looks south over a sunken garden, part of the churchyard of St John Zachary, which was lost in the Great Fire. Owned by the Goldsmiths' Company across the road, the garden was redesigned by Peter Shepheard as a memorial to the fire-watchers of 1941. Now it is being designed again. Behind the garden's fine plane trees, the south front of the new building hovers over an entrance hall that stretches across the south front clad in frameless glass. This transparency is achievable because the building front is cantilevered and suspended from the main structure by diagonal ties.

The central third of the front is a stack of lift lobbies that gradually become narrower in depth as they go up, for the outer wall is gently battered. At each level, there is an external planting bed, and vegetation is already beginning to shade the glass, though not enough to obscure moving views over St Paul's and the city from the lobbies, which themselves have translucent glass floors, making the whole a kind of vertical luminous space linking down to the entrance hall.

Left and right of the lobby stack, office areas press forward to the permitted building limit. Here, the walls curve gently backwards until they get to the seventh floor, where they crank quite severely back to obey planning profile rules. Scale is reduced by the division of the front into three panels, and it is further modulated by the cladding strategy. Under the window strips (glazed in heat-reducing green glass), spandrels are faced in rain-screen strips of greyish-green flame-textured slate. At first these look like louvres, but they are fixed, by specially designed cast stainless-steel brackets, to hangers of the same material which convey loads back to the main structure. All this generates a finely-tailored garment with delicately sewn seams: functional elements of construction generate decoration in the way the Gothic Revivalists anticipated 150 years ago.

This elaborate facade fronts a very straightforward interior in which open plans are free of columns and are intruded on only by a service core. Side and back elevations of the building are clad in slate and glass in a similar way to the south front, but they are vertical until the crank of the upper floors. There are unfortunate chamfers on the back corners of the plan to allow for rights of light of neighbouring buildings. On the chamfers, slate and glass is jerkily replaced by glass and metal.

But overall, 25 Gresham Street is a very thoughtful response to what is normally an imagination-chilling brief, and to a site next to a Wren church, the foundations of the Roman governor's guard's camp and several halls of ancient City guilds. You can't expect much in the way of radical re-evaluation of energy conservation in the City: entirely indifferent to such fancy notions as sustainability. But you can expect decency and respect for context; the architects have provided both.

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Nicholas Grimshaw & Partners

Project team

Nicholas Grimshaw, Neven Sidor, David Portman, Ewan Jones, Declan McCafferty, Ben Heath, Constantine Kaskanis, Angus Denvir, Malgorzata Haley, John Ridgett, Carl Shenton, Grant Starling, Murline Bagalue

Structural engineer

Whitby Bird & Partners

Services engineer

Waterman Gore

Photographs

Edmund Sumner