Red letter house: The inventive conversion and extension of a house in north London is one of an inventive trio of structures in a leafy garden.

The VXO house in north London, by Alison Brooks Architects, is the conversion and extension of one built in the 1960s with '70s additions, Inhabited by the clients for 15 years, it was a squarish block stepped around the periphery with a front door on the south and was set at the back of wooded sloping gardens off a steep lane.

As well as opening up the house and constructing a new bedroom and entrance, the practice was asked to replace an adjacent garage with a guest pavilion/gymnasium and to rebuild the car port at the entrance. Since the site is in a conservation area, the planners were keen that the stepped character of the house's exterior and its dialogue with the street be preserved.

The three elements, each one part of a unified composition, constitute a progression from open car port (the O Port) to transparent guest house (the X Pavilion) to transparent/solid house. Each structural essay employs playful supports in the form of giant scarlet steel letters.

The O Port at the entrance is closest to a piece of sculpture: an upside-down wooden deck (reversing convention is a recurring theme) is covered with pebbles and braced to the south by a stainless-steel picture frame through which cars pass. On the opposite corner, it is supported by the letter O which frames the garden and from which the eye travels to the brilliant X and V of pavilion and house.

As the intervening element, the pavilion faces both ways -- towards house and entrance -- with a transparent wall. A development of the simple carport, it has four solid planes conceived as a pair of folded plates, like cupped hands. The lower one of in-situ concrete studded with grey pebbles forms a wonderfully textured retaining wall and plinth; the upper one placed over the plinth is of wood. It provides a raised floor, hiding services and window frames, and screens a sunken shower and store. A sedum roof-a displaced piece of garden -- is lifted clear of the walls by X columns and is overlooked by the new bedroom of the main house.

To maintain perception of the house as a series of receding volumes, the front extension is a solid, wood-clad box projecting over a transparent entrance, supported on one massive V column and pinned back to the existing building. As a composition, the structure has considerable dynamic force because as the scarlet V thrusts upwards, the movement of the wooden cladding appears to be sideways, and the window, wrapping around, dissolves the south corner. An impression of lateral thrust is reinforced inside the entrance by Simon Patterson's inscribed blue wall, that shoots through glass at the side to screen a kitchen terrace outside and wc inside.

To most urban dwellers, the entrance would be daunting for it is a lens into the depths of the house. The interior is largely obscured, however, by an upside-down stair, magically suspended from the first floor and contained by a diaphanous veil of fine steel grating. (In the living room, a chainmail curtain hung from a metal rod is a fireguard for the aluminium clad fireplace).

Alison Brooks' originality, her continually intriguing detailing, have been seen before, most recently in design of the Atoll Hotel in Heligoland (AR March 2000). If the playful organicism that has permeated previous works is not so evident here, her preoccupation

With layering space and making it flow is. To dispel the rigidity of the existing plan, cross walls were removed or cut open and a double-height lightwell, created on the north, brings light down and through the house. On the upper landing, large occuli punched into the rood admit more daylight; and luminance washes over polished plaster walls in Tiepolo blue and pale pearly colours.