Le Roi des Belges: Set sail for the Southbank

A boat-shaped holiday let on the roof of the Queen Elizabeth Hall is a bold addition to London's landmarks, says Stephen Bayley.

Painfully slowly, really excruciatingly so, the external lift climbs a sheer concrete cliff face. Even my companion, an experienced global traveller accustomed to brain-warping delays, begins to look twitchy. The speed restriction is because the concrete belongs to the Queen Elizabeth Hall and bad vibrations might interfere with Janacek. Most auditoriums have an insulating double shell structure, but not here. By the lights of 1967, when the QEH was built, concrete was a material with peerless functional, aesthetic and acoustic properties. So, no twanging cables or rattling cars, but a solid rack-and-pinion drive delivers you, eventually, to this bleak Southbank roof.

London's roofs are terra incognita: vast unexploited areas, unseen, neglected and home only to manky urban pigeons, rusting water tanks, groaning lift-motor houses, high-level litter and sweating air-conditioning apparatus. It's undervalued territory that needs liberating either by artists or estate agents. Antony Gormley hinted at the possibilities with his 2007 Event Horizon, when life-sized human figures appeared on ledges and parapets, briefly enlivening dead spaces by drawing attention to them. But on top of the Queen Elizabeth Hall you now find something more interesting still.

When, relieved, you step out of the lift, you cross a jury-rigged walkway to find, precariously balanced over the precipice... a boat. And really quite a large one. Here, mysteriously beached above the concert hall, is a piece of startling architectural theatre.

"A mournful gloom, brooding motionless over the biggest, and the greatest, town on earth" was Joseph Conrad's description of a Thames scene in The Heart of Darkness. He might have been describing this February roof. Indeed, the boat is inspired by Le Roi des Belges, the quaintly imperial and ungainly vessel Conrad commanded on the Upper Congo in 1889. It's all here: the decking arrangements, the crow's nest, the portholes, the bulbous stern, the faint whiff of old-fashioned heroism.

This astonishing intervention on the skyline is the latest adventure in Alain de Botton's inspiring Living Architecture programme. Beginning with an audacious cantilevered metal-clad modernist "barn" in Suffolk in 2010 and continuing last year with houses on bleak Dungeness and in eccentric Thorpeness, next month Living Architecture will add a Norfolk house by Michael Hopkins to its portfolio of agitprop architectural assets.

Soon a Peter Zumthor "secular retreat" in Devon and, being conceptualised right now, a Welsh house by John Pawson, my companion in the glacial lift. The noble Living Architecture concept is to radicalise the holiday-let market by demonstrating that bold, unrepentant modernism can, like Marcel Proust (another de Botton hobby), improve your life.

But Le Roi des Belges, also known as A Room for London, is somewhat different. First, it's a temporary installation for only one year and available to rent for only one night at a time. Second, it's a collaboration with Artangel, an organisation that puts radical arts-based projects in motion. Third, the design is a joint effort by architect David Kohn and artist Fiona Banner (famous for installing retired and buffed-up RAF jets in Tate Britain). Fourth, it makes no attempt to be modern. In fact, on the BCDS (Bayley Critical Dogma Scale), it is somewhere between pleasingly whimsical and annoyingly cute. But this nagging duality is appropriate because when you step aboard Le Roi des Belges, you're not quite sure whether to experience vertigo or seasickness.

I admire Alain de Botton. Long before we met I had been fascinated by his ingenuity in using the graphic of a 767 seating plan as a device in a romantic story. The philosophical aspects of travel interest him greatly, although I have found he is sometimes less comfortable with its practicalities.

When we actually met it was by happy accident at a finca in the Sierra de Aracena north-west of Seville, where we were fellow guests. I fondly remember Alain having a very difficult time negotiating the gearbox of his Volkswagen hire car. But his theoretical enthusiasm for the modern world and its love affair with machinery remains pristine and intact.

It's in this context that Le Roi des Belges needs to be understood. The great modern architects of the early 20th century were all convinced that the functional disciplines of designing liners or aircraft should be an inspiration to architects who might otherwise be tempted by the snares of frivolous decoration and overstuffed sofas.

Perfection would be to live in a cabin with foldaway this and concealed that; everything disciplined by severe conceptual intelligence and an economy of means in the execution. All this with amazing first-time-ever views from a place a boat has never before been: it is "haze resting on the low shore running out to sea in a vanishing flatness", as Conrad put it. All this right on top of Waterloo Bridge with a view of the Savoy.

It's not really a boat at all. Le Roi des Belges was fabricated in three sections in a Brighton yard by an exhibition contractor and craned into place. Still, the illusion is impressive. You pass through a door into a saloon with heads on the left and galley on the right, secluded by concertina screens. This saloon is partitioned at its median and beyond it you find a double bed, itself stowable, facing the prow. A drop-down ladder allows access to a bridge, intended here as study space. And from this study another ladder leads to a hinged Perspex bubble. Above that, an openwork structure hosting at its summit three humming WTGs (wind turbine generators) that provide most of the power A Room for London needs.

It is amusing, strange and thought-provoking: exactly as intended. De Botton has written interestingly about the somatic links between actual travel and intellectual pleasure. Here you can experience them, if, since you are going nowhere, only vicariously. Nonetheless, a thrill of what might be called caravan complex is keenly available: that atavistic sensation of intimacy and security you get in well-designed small spaces. There are reminders of Conrad's Congo adventure in curios, books and maps, but you are not under threat from pygmy poison darts. Instead, you are excited by the sheer oddness of it all.

So what's for dinner? The galley arrangements are sailboat rudimentary : a microwave and an electric kettle. Bring a picnic or descend the wobbly gangplank and the sluggish lift to access the coruscating Babel of global resto-brands that is the Southbank. You need help at night? On the Upper Congo or atop the Queen Elizabeth Hall, no one can hear you scream. The isolation is authentic.

A fake boat roof-locked above an urban concert hall might seem an unsubtle metaphor of futility. But the experience of A Room for London directly stimulates unusual ideas about townscape plus the simple sheer excitement of occupying mood-changing space. Most new buildings do no such thing. That's the valuable lesson this playful piece of architectural theatre teaches.

Travel essentials: A Room for London

A Room For London, Southbank Centre, London SE1 8XX (aroomforlondon.co.uk). Public bookings are now closed, but each month a night is offered to winners of the Ideas for London competition (aroomforlondon.co.uk/ideas-for-london).