Garden parti: a garden pavilion uses High-Tech vocabulary to create a transparent garden room, mediating between leafy exterior and nineteenth-century house.

The addition, by Simon Gander Associates, of a crystalline box to one side of a large nineteenth-century house in Canonbury, north London, transforms the ground floor confines of the old building and creates a new garden room.

The client owns the two lower floors of the house which faces south onto a tree-lined street. To the west of it there is a freestanding double garage. Between house and garage there was a gate and narrow path which led into a rough yard, rendered sunless by a tall hedge and large sycamore tree. Beyond the yard was a large mature garden, effectively screened from the street. There was little connection between the interior of the house and the grounds, for the main living room is one level up and at the front, street-side, of the building.

Since the client spends much of his time in the garden, particularly in summer, he asked SCA to design an extension giving directly onto the garden. In addition, he wanted a new utility room and entrance hall linking the new room to the existing building. The yard, which was next to the kitchen, was the obvious site. Its position, next to the kitchen, suggested easy links between a garden room, house and street, and building on it would leave the garden unscathed.

SCA's room is a sheer glass box, 3.6m wide x 7.2m long, and 3.0m high, built on a concrete slab underneath the sycamore. Initially, the architects wanted to glaze the roof, then they realized that in summer the tree would deposit an unsightly sticky glue on the surface. Instead, the roof is a composite steel and timber deck with a steel edge beam, supported on six 100 x 100mm steel columns. Flat and solid, and finished with concrete paving slabs, the roof adds a new terrace to the upper level of the house. Full-height, double-glazed sheets brace the structure and barely divide interior from exterior. Passage from the street, mediated by a series of three iroko screens, has been elegantly contrived to reveal the new building and garden by degrees. From the street, all you see of the new building as you approach is the first screen. It pivots to let you into a raised (iroko) deck that runs, striped with light, below a pergola to the front door.

This is a replica of the first screen and leads to a low hallway with the new utility room on the left. Beyond, is the third screen which swings open to reveal the secret garden, seen through the transparent walls of the new garden room.

Transparency and luminance were keynotes of the room's design. Glass walls are frameless (as are the iroko doors and ventilation panels), with the usual stops and seals being incorporated into the end of double-glazing units to reduce sightlines to a minimum. Cool north light is reflected off limestone flags paving the floor and off-white plastered walls and ceiling.