Don't knock it till you've seen it

The new British Library has certainly had its detractors, but Richard MacCormac believes those who actually use it will love it

Next year, the hoardings that both conceal and advertise the new British Library will finally come down and the occupation of the controversial pounds 450m building will begin. It will be an important event and, I believe, a surprising one. For the hoardings have been part of a collusion, a concealment and a frame of mind that has engendered a kind of phoney war of negative innuendo emanating largely from people who have yet to venture into this extraordinary building.

In this sense, the British Library continues a rich tradition of architectural abuse - St Paul's, the Houses of Parliament, the Law Courts and Nelson's Column were each the focus of incontinent spleen. At the opening of the Houses of Parliament, the building was deemed a "complete, decided and undeniable failure", and Disraeli suggested that if the architect were to be hanged in public, it would put a stop to such blunders in the future. Such psychology is not difficult to understand, for monumental works of architecture under construction are like lightning conductors for instant opinion ensuring the widest publicity - as, among others, the Prince of Wales has found.

Yet with buildings, as with people, it is easier to criticise from a distance and only possible to understand with curiosity and reflection. The removal of these hoardings and the opening of the building will invite another frame of mind, the exercise of what Sandy Wilson, the library's architect, calls the "natural imagination".

His argument, set out in his new book Architectural Reflections, is that the experience and meaning of architecture are embedded in its use, and although we largely obtain the experience through our eyes, our feelings are to do with our instinctive bodily experience of our surroundings, the extent to which we feel confined, protected, released or brought together with others to share a common experience of place. Arguably, these were the first intentions of architecture.

The interiors of The British Library are so large and so formed and lit that it calls upon sensations which are beyond the expected effects of building and which are more akin to the experience of landscapethan what we expect of architecture.

The journey into the building begins with one of four entrances into a courtyard currently hidden from Euston Road in London. Rather than one large space, it consists of a series of enclosures of various scales and at various levels. The main focus will be Paolozzi's astonishing statue of Newton after Blake donated by the Foundation for Sport and Art. This will be a place not just for sculpture, but also for booksellers, dramatic performances in a small amphitheatre and quiet places for visitors to relax. This is something entirely different to the massive, abstract and often windswept gestures of the Parisian grand projects and establishes a theme of adaptability and intuitive feeling for the psychology of place which permeates the building. Libraries, as the architect likes to remind people, are not places of public gathering and display, not like theatres or opera houses. Even by the classical tradition of great European libraries, entrances are modest.

So the entrance here is deliberately low and says little about the extraordinary hall beyond. This room is sensational, not simply because it is unexpectedly big, but because the impact, like that of entering a cathedral, is visceral - surprising and unsettling at first, and only then profoundly calm. Volumetrically, it develops up and away from the entrance doors in a series of great waves which appear to float on reflected daylight. This sense of natural expansion also finds expression in the broad flight of travertine marble steps which invite visitors to enter the reading rooms and other public areas of the building. It is also due to the divergent flanking walls of brick and stone which accelerate space away into the depth of the library, beyond the series of bridges linking the humanities reading rooms to the left with the science and technology libraries to the right.

What you do not see at this stage is the central feature and, arguably, the masterstroke of the interior. This is the King's Library, the leather and vellum-bound collection of George III, which is to be housed in a six-storey glass bookcase which will rise out of the basement bookstacks in the heart of the building, a powerful yet exquisite symbol of the library as a whole.

Architecture of this kind has a useful narrative; it tells you where to go, where to find an exhibition space, cloakrooms, lifts and so forth. But the text here is dramatic in its primary intent and also richly episodic. It is full of architectural sub-clauses - ramps, ranges of hanging lights, places for the disabled to wait for lifts - all seemingly carved out of the travertine, and internal windows like port holes which invite cross views between the reading rooms.

There was to have been another part of this narrative - the inclusion of commissioned works of art, including a great tapestry by the painter Kitaj, but in 1991 the Government refused funding for these works, an act of philistinism at the time that now seems just another sad symptom of an inability to judge what is of public value.

After the central hall one might wonder how the reading rooms can make an equivalent impression, particularly in the light of what many readers feel about transferring the library from Sydney Smirke's great circular reading room in the British Museum.

The answer to this is not simple, but expresses the crucial characteristics of the new library and the way in which the architect has exercised his idea of the "natural imagination". So, entering the rare book reading room, you pass under the upper reading gallery and experience a sense of temporary enclosure in a threshold between the volume of the entrance hall and that of the reading room you are about to enter. You arrive in a space which is lit by a kind of inverse lantern which you do not immediately see. The main reading area is furnished with leather-topped tables of American oak, and around the perimeter panelled doors give access to study carrels. So there are three distinct places to read: the raised gallery under the central lantern; the main area under the sloping roof with its comfortable peripheral feel; and the privacy of the carrels.

The psychology of all this is equivalent to that of the experience of landscape and the atavistic choices we make about where we feel comfortable, whether to enjoy a promontory or the fold of a valley or the seclusion of some natural alcove. Readers will find in this room a range of different places to suit their mood. Nearly complete, it combines an extraordinary quality of luminosity with the tangible effect of marvellous joinery work.

The humanities reading room, as yet a great hollow cavern under construction, is arranged in a similar way but on a more dramatic scale. The workers on the site refer to it as the "cathedral". Again, this environment for 500 readers, nearly twice as many as for rare books, will consist of a range of different kinds of space. Neither of these two reading rooms is intended to compete with the volmetric trumpet blast of Smirke's circular room, but each will combine the dramatic and unexpected with a humanity appropriate to the subjects they sustain.

Crossing the galleries from the humanities to the science and technology library, you enter a different kind of space, where open-access shelving predominates and readers are arranged along the edges of the triple height, side-lit volume which, with its solar shading, forms the elevation along the eastern edge of the entrance courtyard. In the distinctive "prow" of the building facing Euston Road, this rational, modular arrangement is transformed into a conference area with lecture room and seminar rooms. Here, changes of level create a series of specific, quirky spaces to stop, talk, sit or have a drink, reminiscent of the stepped terraces and changes of level of a hill town.

Having emerged from these wonderful and, to date, secret interiors, what of the exterior? A project like this which has been so long in gestation and construction becomes stranded in the tides of changing fashion, as did the Law Courts in their day. But this building was never intended to be fashionable and never will be. Historically, it is rooted in the English Victorian "free school" of Butterfield (Keble College, Oxford), Street (Law Courts, Strand, London) and Waterhouse (Victoria & Albert Museum), which allowed functions to precipitate form, and influenced Frank Lloyd Wright in the United States and Alvar Aalto and Hans Scharoun in Europe, whose works are its most direct precedents.

It is completely outside the orthodoxy of Modernism in its adaptive symmetry, reflecting arrangements within, and has close affinities, because of this, with Scott's Grand Midland Hotel, fronting St Pancras station, next door. It refuses rhetoric independent of the forms shaped by its use - so it is also outside the fashionable interest in Classicism.

Yet, like the buildings of the Gothic Revival, its unprecedented form and dramatic dissonances may have the special capacity to gain significance. What is now difficult to imagine is that the vicissitudes of construction will be wholly transcended once the building is successfully in use.

So the library will gain a symbolic charge as one of London's landmarks (even a national one) and as the world's leading resource for scholarship and research. As a monument it may continue to attract controversy, but I believe its efficacy as a library and the aptness and extraordinary quality of its interiors will win the affection of those for whom it has been built - its readers.

The author is senior partner in MacCormac Jamieson Prichard and an English Heritage commissioner.