A tiny apartment which uses new technology to utilise small spaces has been shortlisted for a major prize. Nonie Niesewand explores the revolutionary rooftop

Estate agents are targeting a new breed of international clients whose des. res. has to be a "lock up and go". Downsized, downtown, and smart enough to function without its owner. This rooftop apartment in Paris measures just 8m by 4m, little more than a walk-in cupboard.

But it has other mindblowing virtues: as its owner, the international human rights lawyer Chris Avery, Eurostars it to his Paris pied-a-terre for the weekend, a phone call from him activates the system which makes the limestone floors warm up, along with the water. This is smart architecture, and behind smart architecture there's always a smart architect.

Mark Guard, the designer of this apartment calls it "a flexible envelope in three inter-connecting boxes". Panels that slide or pivot can make three rooms or open out into open plan. Between the kitchen and the bathroom is an electro-chromic clear glass door which turns opaque when it is closed (an electrical impulse activates the coating). This kind of responsive glass gives privacy without cutting light from the core.

By singling it out as the winner of the RIBA housing award, to be shortlisted with six other buildings for the Stirling prize later this month, the institute is making a point about houses of the future. It uses the new technology to make the most of small spaces and shows how to pack in a lot of ideas within a few cubic metres.

Behind sliding doors which are hinged to fold back flat, are three distinct areas for sleeping, bathing and cooking combined with living (doubling as a guest room with a sofa bed hidden in a stretch limo of a white sofa). All the essentials of modern living are here and neatly concealed: TV, washing machine, fridges, and wardrobes neatly concealed.It uses limestone, glass and wood for a simple structural formality without sacrificing comfort. The sleeping area and the bathing area are at the furthest end of the rectangular apartment, separated by sliding screens.

"We needed to plan it to keep the full visual dimensions of the envelope" said Mark Guard who added that the brief was complicated by the client wanting his friends and family to have somewhere to sleep. So he devised the open plan to have private partitioning when needed.

The exterior is a bit Bladerunner, criss-crossed with walkways on the roof. It used to be the janitor's flat on top of an eight-storey Art Deco building in the 5th arrondisment, reached by taking a lift ride to the top floor. The front door opens rather disconcertingly on to the roof, rather than into an apartment, for a walk among the chimney pots to this little glass fronted house facing west among the satellite dishes.

Mark Guard opened up the view of the domes of the Pantheon and Val de Grase, the cathedral that is now a military hospital. Then he extended the limestone floor from within on to a terrace for al fresco summer dining.

The original was a very dilapidated cement structure with algae in the interior so it had to be rebuilt and fitted for a price of pounds 118,400. Mark Guard found it cheaper to buy French limestone slabs for the floors in Britain and to ship them to Paris where everything costs more. He's hesitant about calling himself a minimalist because he doesn't like the ``isms'' in architecture. He insists that his work is not about reduction, but more about addition as he adds layers of function within the existing space.