GRAND UNION.

Design of a canalside house in north London is a distillation in miniature of the architects' enduring preoccupations.

The house, by Benson + Forsyth in Islington, took shape over a period of about ten years, and as a result of its long gestation, it quietly evaded notice. But as an architectural essay by Gordon Benson and Alan Forsyth, it demonstrates in miniature many of the preoccupations they have shared since their AA days, and which have been worked out in various ways ever since. These architects are masters of light and space, of spatial layering and connection (with history, context, landscape). Their compositions are complex, each element distinct, interspersed with scholarly quotations, and woven into a coherent whole.

The clients, who are the founders and proprietors of Marico furniture, originally acquired the site in the late '70s for a new workshop. Islington then, with its terraces of eighteenth- and nineteenth-century houses, Georgian squares and canalside industries, was a much poorer and shabbier place than it has become (inhabited by Tony Blair and family before their move to Downing Street). Running along the north bank of the Grand Union Canal, the site contained the shell of a derelict factory and two brick-built cottages next to the towpath. To the west, serving a neighbouring housing estate, is a children's playground and small park.

Benson + Forsyth were employed to design a new workshop within the old factory shell, and only gradually did the idea take root of making a house out of the cottages.

The building takes its cues from the context. Connections, resonances, are established in quite complex and subtle ways. Scale, height and materials relate to buildings along the canal, to the factory and a neighbouring Victorian terrace. To maintain continuity with the existing fabric and memory of previous occupation, the architects retained the site's peripheral walls -- the brick boundaries of gardens to the north, and single-storey wall of the canal towpath along the south. Cottage walls on north and west were kept as a familiar external envelope and patched up, and these residual fragments, so obviously filled and mended, speak of historical passage. Insertion and the original are clearly differentiated: the soft old walls, pierced in places by hooded windows, like eyes, have hard edges of engineering brick. A projecting bay over the front door on the west side is clad in the same corrugated metal sheeting used on the exterior of the workshop, so unifying the canalside perimeter of the complex. Unifica tion of the two parts of the site continues in the structural system adopted for both. For technical and expressive reasons, the loadbearing function of the existing walls has been removed and a new steel frame inserted. As in the workshop, the insertion is made plain by disassociating new (metal valley-roof and walls) from old with peripheral glazing strips.

Echoing the traditional form of Islington roofs and that of the workshop (though rotated through 90 degrees), the roof is carried centrally on two pairs of steel columns. Internally it reads as a free-standing umbrella, similar say the architects to 'one half of the sine-curve canopy of the Heidi Weber pavilion in Zurich'; and its orientation establishes an axis within the house from the park on the west to courtyard on the east. Here a single glazed screen rises the full height of the building and mirrors the opposing face of the workshop.

Entering the house from the courtyard, you come to a luminous sitting room, a double-height void defined at the upper level by the balustraded edges of a gallery where bedrooms and more private quarters are to be found. A single-storey wing, containing a long dining table (designed and made by Marico in American cherry), links the main body of the house with the workshop, and supports a roof terrace overlooking the towpath. The wing too is glazed, with glass doors onto the courtyard so that it can become an extension of the ground floor, an external room.

Marico's spiralling wooden staircase (fitted together without a central column) takes you up to the gallery where privacy takes a different form. For whereas the ground floor is protected from public scrutiny by an external carapace, only minimally breached and given an internal focus, the gallery has been given glimpses of the external world through angled slots, reminiscent of medieval arrow slits. A secluded sitting room giving onto the roof terrace through glass doors has elevated views across and down the canal and over gardens. Boxes-within-boxes are devices that occur in complicated abstract form in the Museum of Scotland (AR April 1999); here, more literally, two cubes enclose most private functions. One of opalescent glass contains a bathroom; the other, suspended above the sitting room, is the main bedroom, a capsule under a translucent illuminated ceiling. Throughout this small house, connection and opposition - between exposure and shelter, public and private, light and twilight - is subliminally felt. A certain obliquity, which you associate with this practice, and which occurs in the angled glimpses and looking glass reflection, has a teasing medieval quality and is an occasional source of delight. On a sunlit day, ripples of light off water in the canal spill through the louvred clerestorey in the south wall and across luminous white planes of the interior.