Rising to the occasion.

In one of the wealthiest and most congested parts of central London, a tiny mews house expands and contracts with elegant ingenuity.

Seth Stein's courtyard house for himself and his family in Kensington (AR October 1996) has been followed by another skilful exercise in urban evolution - the intricate design of a mews house in Knightsbridge.

But if the former spread itself expansively over the site of a Victorian stableyard, the latter has been extracted from the smallest conceivable footprint of land - an empty parallelogram, 6m wide and 8m deep, at the end of a mews. Even the prospect was cramped, for the site faced new bank offices of hard red brick across a tiny cobbled alley, and its back was pushed hard up against a rear garden wall, in the shadow of trees and a tall terrace of houses.

Difficulties of building anything more complicated than a garage on the site were compounded by the brief. The client, who collects old cars, wanted a pied-a-terre for himself and his wife and room for storing two cars when they were away.

In response, the architect's ingenuity has been remarkable. The supple architectural intelligence underlying design of this tiny house gradually reveals itself as you pass from its discreet presence on the street into its industrially wrought light-filled interior.

Restricted by the height of the mews' roofline, the site was excavated, providing another storey and a half under the building and further skylit space at the rear under a small paved courtyard. Not to waste an inch of valuable space - and this is the great surprise - they have called on the vocabulary of car repair. Behind the rolling shutter onto the street there is a two-storey pit which like those found in car mechanics' workshops is filled with a hydraulic lift with two car decks. But this contrivance has an upper car deck floored with green limestone, the lower one with translucent perspex panels each 30mm thick and calculated to take a two ton weight and point loading without shattering.

When empty of cars and raised, the lift provides a ground floor room with a translucent floor next to the entrance hall, and a basement living room with a translucent ceiling; when lowered, the upper room is floored in limestone and the lower one is translucent underfoot. The two decks are open on all sides, simply slotting into the building envelope at each level.

In cross-section you can see how the interior has been organised into levels stepped discreetly up and down to accommodate something above or beneath. The bottom of the pit is four steps below the adjacent dining room and kitchen so that when fully down, the lift does not disturb the furniture; at the top of the building, the roller shutter mechanism slots in beneath the bedroom floor, set above the level of the stone floor of an elegantly detailed skylit bathroom. The bed itself rests on a storage plinth, of the same warm wood as the floor.

Interpenetration of the various spaces and levels, as well as the diffusion of light through translucent floors and walls, creates an illusion of greater space. The principal continuous conduit of light through the building is the transparent flight of stairs, with glass treads and balustrading, that rises 12m from basement to first floor landing where a square window high in the facade gives views of the sky.

In pushing the spatial envelope to extremes, the architect may not have created the most restful of houses - the machinery and spatially movable feasts are entertaining but somewhat disconcerting. But this is an elegant austerely-finished pied-a-terre, miraculously open to light and greenery in congested central London; and you can only marvel at the amount of care and thought implicit in every detail of the design. The clients seem to relish living cheek by jowl with their cars - one after all is an original Fiat Cinquecento.

Architect Seth Stein Architects: Seth Stein, David Rusell

Steel fabricator G.T. Coulson

Glasswork Nero Glass

Teak floor Victoria Woodworks

Photographs Richard Bryant/Arcaid