A FLAT, featureless door is set in the wall of what appears to be a warehouse. Mechanics work, with radio blaring, in a nearby garage; smokers huddle outside offices; workmen shout over concrete-mixers in the building site opposite. This is the heart of King’s Cross, London, and it’s ugly, raw and noisy. Yet behind that anonymous door there is one of the capital’s most ambitious pieces of Modernist architecture. It is called, appropriately, The Lost House.

“This is what I like about living here,” says its owner, Philip Bueno de Mesquita, 40. “It’s watching a visitor’s expression when he sees inside.” We are standing in the lobby — a narrow, high-ceilinged, shadowy space, which creates the sense of waiting in the wings of a stage.

We take a few steps and we are in a vast black chamber. Forget any concept of the home as a collection of neatly defined rooms. Imagine instead an exhibition space. There are five-metre-high black walls, black floors and, in the corner, a glass-walled gravel garden, open at the roof, inside which there is a single silver birch. The light plays tricks, shooting rays from light wells in the ceiling and slits on the walls, then bouncing off a glass-sided water garden. The zigzag shadows and reflections are disorientating, making you question distance: what is inside or outside, here or there?

This is the creation of the architect David Adjaye, whom de Mesquita commissioned, having bought the site for £500,000 in 1999 when it was a loft apartment overlooking a courtyard garden. He and his wife, Sandic, 30, lived there before the renovation, which took place from January to September 2004 at a cost of £750,000. Structurally the work involved knocking apartment and courtyard into a single, roofed unit. “I was very keen to get David on board, particularly after seeing his Dirty House in Hackney,” says de Mesquita, who owns the fashion company Acupuncture. “Initially he said he didn’t want to work on another residential property, but then we showed him how much space we had for him to work with — 2,000 sq ft of apartment inside, 2,500 sq ft of courtyard outside — and he changed his mind.” Just past the water garden, at the centre of the room, there is a sunken cinema decorated in a luminous light green, then there is another glass-walled garden, a kitchen area and a work studio. The hardcore industrial aesthetic is everywhere; even the kitchen work surfaces are made of concrete. “I think concrete is such a beautiful material,” de Mesquita says. “It’s warm and it’s so practical — you can make it whatever you want it to be. I love the way it weathers and wears with age. In fact, I want to build my next house entirely of concrete, even with concrete beds and chairs.”

To get to the upper level of The Lost House we backtrack to the lobby. But there’s little sense of “going upstairs” from here — we get to the mezzanine level via shallow, dimly lit ramps. The main bedroom is the showpiece. Painted a pale lilac colour, it’s starkly minimalist, with deep storage vaults under the floor. Nowhere escapes the hard-edged industrial look. Concrete slabs are exposed across the ceiling; from window slits set in the interior walls coloured lights glow, and there’s a sense of water.

Go up a few stairs and you find its source. This is the wet room, painted black, with a steam room adjoining it. And in the corner you duck your head to find a 15m-long swimming pool. “I have tried to create the impression of going into a cave,” says de Mesquita. “The lights reflect through the glass into the bedroom and you can also, obviously, watch people as they are swimming, which is fun.”

Back in the main living space, the afternoon sun has begun to dim. I notice for the first time that the street-facing wall of The Lost House is, in fact, a one-way window on the world outside. We can see the smokers from the offices opposite lighting up and the builders leaving their building site. But they can’t see in.

“I find this fascinating,” says de Mesquita. “Outside there’s all the noise of construction and people working, yet inside it is such a pool of calm. If you came here later you’d find this room has got a quite different kind of nightclub feel. It is changing all the time.” Before coming to live in King’s Cross, the de Mesquitas had lived in a fairly conventional mansion flat in Soho. They moved at a good time, just when the area was “on the up” — Sir Clive Sinclair used to live in a loft apartment next door, and the photographer David Bailey is still there. Why the urge to move again?

“It’s because I believe this is a very exciting time for architecture,” he says. “Only ten years ago people were happy to live in 15-roomed houses. Now we know that’s not functional, so young architects are being given the chance to be really creative with those spaces.

“I’m already looking for a suitable site in Central London, possibly Shoreditch. I’m keen to build a place with a concrete fascia similar to the houses built in São Paulo in the 1960s, possibly with a cantilevered pool on the roof. I love this kind of brutal architecture.” The Lost House, at 9 Crinan Street, London N1, is for sale with Cityscope for £2.85 million, 020-7830 9776

FACTFILE

The average property price in King’s Cross is £292,145 compared with the average for England and Wales of £211,168, according to the Land Registry.

Property in King’s Cross has risen in price by 26.1 per cent over the past two years, according to Hometrack.

Violence against the person in King’s Cross occurs to 42 out of every 1,000 people. The average for England is 16.5, according to the Home Office.

14 of every 1,000 of the population of King’s Cross are burgled. The average for England is 6.4, the Home Office says.

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