From the outside, the Reid building at the Glasgow School Of Art resembles a half-turned green Rubik's cube, with monumental lines, severe, sheer cliffs of panelled glass and an air of unflinching modernism.

The interior, however, is one of space and light, of plain white walls, three huge internal chimneys of light, and studios and work spaces that aim to provide students with functional space and also ample light, air and a visual connection to other levels and studios within the building.

Built on a tight, restricted site with a clearly defined budget, the Reid building is a world and more than 100 years away, in terms of its appearance and especially the materials from which it is made, from the Mackintosh building - "the Mack" - it faces. However, the architects of the Reid building feel there is a clear relationship between the two. Chris McVoy, the senior partner at Steven Holl Architects, who co-designed the building as "design architect" with Holl, said the whole building looks to the Mackintosh building for inspiration.

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The architectural vision for the 11,000sq ft building was based, he says, on five concepts. The first, inspired by the huge studios of the Mackintosh building, was to base it around the main working space for design students: its studios and volumes of space they encompass.

Secondly, these studio spaces had to be adaptable and have "great proportions and life". So the plan of the building is simple: serried rows of large studios in a roughly rectangular pattern.

However, the third, and perhaps most important, guiding theme is also the building's most important but least tangible material: light. The architects analysed Charles Rennie Mackintosh's masterpiece across the road and calculated he had found 20 ways of letting natural light into the building. "And light is Steven Holl's favourite material," says McVoy. Just as the Mackintosh building is an "architectural symphony of light", the new building attempts to use light as its invisible superstructure.

So the most notable interior features of the building are the three mammoth "driven voids" of light, internal towers of space that run from the top of the building to its roots, poised at a discreet angle. These voids, made out of steel and concrete, bring natural light into the building, where, with the voids' epic tubular forms cut into by squares and rectangles, they radiate light into the studio interiors.

In addition to light, they allow ventilation of the whole building. The voids provide remarkable views, of course - sitting at the foot of a shaft, one can stare up and see the (let's be charitable) ever-changing Glaswegian sky, as well as staff and students on other floors. The precipitous view down, especially from the top floor, is also notable, as well as slightly alarming.

The design of the driven voids is McVoy and Holl's, but their inspiration came from Mackintosh, albeit from a detail that non-architects may have overlooked. "They actually come directly from the Mack," says McVoy. "Those three-storey windows Mackintosh designed in its library? They both push out of the building and come in, they hold the light. In fact, they are voids of light."

There is more to the light in the building beyond the voids: studios on the north side of the building have very large, inclined windows to maximise the "high-quality diffuse light" while those areas or spaces of the building that are less dependent on natural light, such as the refectory and offices, have been built on the south-facing side, and act "as a screen to control the light into the building", adds McVoy. The element of light will also be caught and refracted by perhaps Glasgow's first outdoor infinity pool.

The fourth key idea behind the new building was that the corridors, studios and other spaces inside should be open and permeable: that there would be few closed-off or discreet spaces not linked by a continuing pathway. They called this the "circuit of connection". McVoy says the students and staff inside had to have spaces where chance meetings could take place, where they could sit and talk and make connections as they move through the building. So the stairs are often open, and there are alcoves and seating areas. "Circulation space is one of the crucial spaces. Nobody in here wants to use the lift (although there are some); instead it is all about the stairs at the social spaces."

The stairs, and other stepped ramps, link all the major interior spaces: the sizeable lobby, the ground floor exhibition space, project spaces, the extensive lecture theatre in the basement, seminar rooms, studios, workshops (also in the basement) and green terraces, which are set into the south side of the building.

Finally, the fifth theme behind the design is that of maintaining a "connection to the street". The Mack is raised from the street, its entrance up a flight of steps, but the architects wanted the public areas of the new building - the lobby, the gallery - to be easy to access.

The building also incorporates some of the old: the GSA Student Association building and its Vic bar, housed in what McVoy says is a "fine thirties stone building" has remained, with an arm of the new Reid building reaching over it and down its side, as if it is being given a 21st century hug in glass and concrete.

"Initially perhaps, the idea was to tear it down but Steven looked at it. It is a fine thirties building, and because it is in stone, it creates an interlocking effect with the Mack over the street. And it provides the students with their own space:

"JM Architects of Glasgow really led on the renovation of the union, and we should say the whole design was a 50:50 collaboration."

There are design details inside the Reid building that McVoy is delighted to point out: the often mammoth wooden doors, some cut at remarkable angles. On the top floor, jutting out from the silversmith studio, there is an impressive large window that hangs in mid-air, providing views west as well as straight down to the south side of the city.

There is 1000 Future Skies, the specially commissioned work by Turner Prize winner Martin Boyce, which adorns the entrance.

There is, if you choose to measure it, the use of the Golden Ratio - the special number approximately equal to 1.618 that is found in geometry, art and architecture - throughout the proportions of the building.

"The technology we have these days - for the concrete, for the glass - Mackintosh could not have imagined and used in his day," says McVoy, "just as we could not use stone in the way he did. But it is reacting to the Mack throughout, and also, we feel, gives a new perspective on that building. You can see the whole facade of his building from our windows: it provides a new way of seeing his masterpiece." n

In tomorrow's Sunday Herald: GSA graduate Jonathan Saunders fashion shoot.