Lunch box: this addition to an Oxford College elegantly extends the historic continuum.

The centre of Oxford is a three-dimensional palimpsest. Many of the quadrangles and gardens date back to medieval times, when the colleges were religious foundations and all the dons in holy orders. Since then, the buildings have been altered and added to, generation by generation, often by the best architects of the day, so the whole intricate interlocked fabric is a commentary on English architecture from medieval to modern times.

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St John's is not one of the oldest colleges, but it is the richest. It was founded in 1555 by Thomas White, a London merchant, who left it very well endowed with property (among much else, it owns many of the pubs in central Oxford). It was formed on and around the Cistercian monastery of St Bernard, dissolved in the early 1540s as one of the last victims of Henry VIII's policy of seizing the assets of the great monastic institutions.

In the twentieth century, having wealth and a lot of land from having carefully looked after White's bequest, it was natural that the college should expand, and there have been several major building projects. MacCormac Jamieson & Prichard have much experience of building for Oxford and Cambridge colleges and in the early '90s the office was chosen to design the Garden Quadrangle, a reinterpretation for the late twentieth century of traditional student accommodation set round a raised secret garden over an auditorium and dining hall (AR October 1994).

The quad has worked well, so when ever increasing numbers of fellows caused the college to decide to extend its Senior Common Room (SCR) with new dining and social spaces, Richard MacCormac was given the job. While St John's has large grounds, they are precious, and the site for the extension was constricted, between the President's garden and the existing SCR building (parts are seventeenth century, and the whole is listed as a historic building Grade 1).

MacCormac's extension replaces one built in the early 1950s by David Booth and Judith Lederboer to the east side of the seventeenth- and eighteenth-century parts. Stone-faced over a concrete structure, the Booth and Lederboer building was a chokingly buttoned-up, po-faced compromise between neo-Georgian and Modernism (AR November 1957). Its disappearance can scarcely be mourned. The new piece could not be more different. At first floor level, it cantilevers eastward toward the President's garden as a simple and elegant glass box. All along the east side of the floor is an external slatted screen of oak shutters supported on a semi-independent frame of oak members flitched to stainless-steel splines.

This device serves two purposes: in the morning, shutters are closed and protect the east-facing glass box from the sun; later in the day, shutters are opened mechanically until they stand at right angles to the glass facade. From the inside, the arrangement, frames the medieval garden between fins, intensifying the relationship between the new lunch room and the trees over the ancient green space, while protecting (to some extent) the president's privacy--one of the reasons why the '50s extension was so buttoned up was that the then president was much less generous, and required fenestration of the east front to be kept to a minimum.

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The new lunch room is deep in plan, and the back of the space, away from the great east window, is illuminated in daytime by light slots along the north and south sides that pour luminance down the oak panelled walls a la Soane. (There is further homage to Soane in the SCR antechamber designed in 1980 with a shallow saucer dome by the distinguished architectural historian Howard Colvin, a fellow of the college.) The new lunch room is big enough to offer 36 new dining places, and its specially designed furniture can be reconfigured to provide a formal meeting place for senior members of the college. Joinery of the furniture and the room itself is immaculate. So is the wide oak balustrade that edges the room inside the glass wall, helps to provide gentle visual transition between room and garden, and prevents diners looking straight down into the President's garden.

The lunch room is the focus of the new addition. Existing stairs have been supplemented by new lifts, and new kitchens have been knitted in on the ground floor. Under the cantilever is a new sitting room, which looks out east across a slender garden and straight into a tall newly-planted, impenetrable evergreen hedge that protects the President's privacy at ground level. On the second floor is a terrace that serves another communal sitting room and rooms for visiting fellows. By being drawn back from the edge of the building, the terrace does not intrude on the President and his garden.

Such sensitive and nuanced understanding of geometry, locus, history and the craft of building gives the little place great subtlety, and makes it an enriching addition to Oxford's three-dimensional historic palimpsest.