Through a glass, imaginatively: Richard Rogers' headquarters for Channel 4 mirrors its innovative style of programming, says Jonathan Glancey

Channel 4's new headquarters opens tomorrow. Staff have been working in their Richard Rogers-designed building in Westminster for some weeks, but what they have been occupying is a rough edit of the final version.

Tomorrow, Michael Grade, chief executive of Channel 4, will informally announce the completion of a special building that says much about his high-wire organisation. The channel has been responsible for much innovative television and cinema. It has taken risks (notably in arts coverage) that have been by turns mad, bad, dangerous, downright silly and brilliant. It seems only appropriate that Channel 4 should have chosen, through competition, a firm of architects world-famous for their ability to pull off creative risks, to boldly go where few architects - and even fewer buildings - have gone before.

The last major building the Richard Rogers Partnership completed in London was the bravura and highly controversial Lloyd's headquarters (1981-86). This was greeted with much the same mix of scorn and elation that Gustave Eiffel provoked when he erected his famous tower in Paris 105 years ago. In the intervening years, Rogers' buildings, like the films of Jane Campion, David Lynch and Jim Jarmusch, have become acceptable to a much wider public: popular, commercial, yet still reliably eventful.

Rogers' Channel 4 is not a revolutionary building, as was the Pompidou Centre or Lloyd's when built, but a development of the themes that have inspired most of his practice's work ever since. Here are the lifts climbing up and down the outside walls we have come to expect. Here is the Russian Constructivist-style tower signalling the entrance from surrounding streets, and here a roofscape animated with Alien-like ducts and dishes, aerials and all the ineffable wizardry needed to drive a communications centre. Most of all, here is a machine for working in, that is also a playful stage set. It might be finished for the most part in subfusc white and grey, but this is, without doubt, a building to enjoy working in, not one to endure.

The celebratory fan-shaped entrance from Horseferry Road rises gently to reach a wilful and beautifully engineered concave screen of sheer suspended glass. Channel 4 is a 24-hour operation; day and night passers-by can see straight through the screen, into the lobby and garden beyond and up through a layering of concrete and glass pedestrian bridges that span the building's two rectangular office wings. The effect, especially at night, is televisual.

This is, transparently, no faceless corporate headquarters. From the street, it seems as if you can see right into the heart of the operation. In fact, what you see is the alluring curve of the streamlined staff canteen, the garden beyond and the principal public spaces of the building. You see nothing, however, of its inner workings.

A shallow scallop of steps leads visitors from the street into the lobby under a steel-ribbed canopy, across a glass bridge and over a glazed lantern that half reveals, half conceals a second lobby below. This serves a blue-lit underground cinema. Beneath the cinema, a second basement that few visitors will see conceals the secret ministry of machines which pump programmes into the electronic ether and magically on to our screens.

Six hundred people will work in this building when fully operational. Most of them will occupy open-plan offices that are neither bland, boring nor dominated by technology. Daylight filters through perimeter offices into the core of the work spaces. Above ground, this is a building where daylight is ever-present.

The ground-floor cafe yields to the garden. Several floors above, so does the cool, unpretentious boardroom. The directors can sit out on a yacht-like sun deck admiring a potted cityscape that encompasses architecture from the sublime - Big Ben - to the substandard - the three concrete slabs of the Department of the Environment.

Like Lloyd's, Channel 4's headquarters looks on first encounter like a spaceship come to land in central London. This does not make it an unacceptable foreign presence, however. It animates the streets around it, encouraging visual contrast, allusions and surprises where once there was blandness. This part of Westminster is down-at-heel, a thing of old-style cafes (bacon rolls and instant coffee), street markets selling nylon sheets, an LCC fire station and a Peabody estate.

It is a patch of old working London, not yet treated to Westminster City Council's artless programme of prettifying the streets in its stewardship with red-brick surfaces and pseudo-Victorian traffic lights. The machine-like quality of Rogers' building makes it an appropriate neighbour for its workaday surroundings.

The building is finished throughout in grey steel cladding, much of it perforated, punctuated by red-ochre steel struts. The colour, says John Young, the project architect, was taken from a paint sample provided by the City of San Francisco: it is the same colour as the Golden Gate Bridge.

The local authority's artlessness is, however, apparent in the unimaginative housing blocks rising behind Channel 4. These were a condition of planning permission being granted for the new headquarters. That local people can find new homes here is good news, yet it seems a wasted opportunity not to have commissioned interesting designs here, particularly as the new blocks are tall and bulky.

An excuse might be that these homes are built to a very tight budget, but so, too, is Richard Rogers' building. John Young points out that the office accommodation of the pounds 35m television building is, at pounds 125 per square foot, cheap to build and fit out. Imagination costs nothing. From its inception, Michael Grade and Lord Attenborough had, in the backs of their minds, a building that could be let or even sold if times got tough. Television has become an uncertain world financially. Channel 4 is not rolling in cash and is realistic about the need to be flexible in the years ahead. If it ever moves from Westminster, the building should sell on readily. Although special, it is not custom-designed to a degree that makes it usable only by another television company.

That said, it appears to be exactly right for Channel 4, both in terms of image - open, sophisticated, provocative to a degree - and in the way it works - a commissioning body and not a factory for the actual making of television programmes. Here is an approach to corporate architecture that will readily withstand a number of repeats without the rest of us switching off.

(Photographs omitted)