Light lunch.

Bernard Desmoulin's new restaurant for the staff of the Ministere des Affaires Etrangeres occupies an elegant glazed extension of the organisation's nineteenth-century building in the heart of the seventh arrondissement of Paris, between the Rue du Bac and Boulevard Saint-Germain.

The site was a particularly sensitive one, for the restaurant has been inserted into one of those secluded green oases that you occasionally come across, hidden within the dense fabric of the city and made the more delightful by accidental discovery. This courtyard garden adjoins the eighteenth-century Hotel de Roquelaure, and surrounded as it is by old walls and trees and planted with apples and pears, it is the trace of an old kitchen garden, of an earlier rus in urbe. The area is in any case one of the most protected parts of the city and planning restrictions are stringent. Accordingly, in designing something as corporate sounding as a staff restaurant, Desmoulin sought to make as light and transparent an impression as possible.

The Affaires Etrangeres acquired the building from the Ministere de l'Equipement after the latter decamped for the Arche de la Defense. The former incumbents had nourished themselves in prefabs that disfigured the garden; and these having been cleared away, the space was planted with espaliered fruit trees.

Desmoulin's extension is a restrained and carefully detailed, low horizontal box, fully glazed across its width and shaded by a delicate mobile pergola of red cedar. The fine glass wall, 30 m long and braced by glass fins, barely separates the exterior from the interior; and the whole facade is rendered more diaphanous by being mirrored along its base by a horizontal plane of glass traversed by a simple wooden bridge. From the outside, if you did not know better, it can seem like a sheet of water. This second transparent plane is the means by which another subterranean level is illuminated, for in order to restrict the restaurant's encroachment on the garden Desmoulin has sliced it in two, sliding a lower floor underneath the ground. To render the subterranean as attractive a place as the upper garden room, he has taken care to give it an equally luminous but separate character of its own.

Instead of the dematerialised architecture of the ground floor, the lower level has a vaguely cloistered air, imparted by solid structure, rows of columns at the edge of a lightwell, shadows, and a 20 m long wall painting that suggests fragments of an old fresco. Through the long skylight there are perpetual glimpses of the sky and trees. The skylight which is assembled from glass panels, laid double with an air vent between, and an anodised aluminium frame, is self-draining and sturdy enough to walk upon.

Throughout the scheme, Desmoulin has used a few simple materials chosen to enhance the light: plain plastered walls, the glass and wood; and downstairs, textured concrete for the floor. Furniture and fittings are appropriately austere.