It's in through the outdoor

Nothing is quite what it seems at Little Thakeham, the country house Edwin Lutyens built in 1902.

The north façade is stonily mute, its windows as unrevealing as black vinyl. To begin with, the house gives nothing away. Standing across the drive under the small cluster of oak trees, the only lively looking thing on view is the pleasantly overgrown rustic wall fronting the courtyard.

The north façade is stonily mute, its windows as unrevealing as black vinyl. To begin with, the house gives nothing away. Standing across the drive under the small cluster of oak trees, the only lively looking thing on view is the pleasantly overgrown rustic wall fronting the courtyard.

There seems nothing to take in except weighty, symmetrical proportion - it seems but an inert bastion. All that is missing is Marianna, a moat and Tennyson to set the scene in iambic amber. But it's not inert after all. The property is a remarkable country house designed by Edwin Lutyens and built in 1902 for the railway and Madeira tycoon, Ernest Blackburn.

Why stand in the rain looking at it on a wet Wednesday? Little Thakeham, set in the very plushest Tudorbethan and turbo-Beamered folds of West Sussex, has just been sold for more than £2m; an architectural masterpiece for a bargain price. Blackburn, who gave his platoons of bricklayers, masons and carpenters a sovereign to mark news of the relief of Mafeking, was ensconced in the pile 18 months after work started on the site, having paid £16,000 for the land and £24,000 for the house. At today's prices, the land and building costs alone would have totalled £2m.

But the story of Little Thakeham is not about its monetary value. It's about layers of time; time and volumes of space which, despite the best part of a century's physical attrition, remain starkly surprising. Little Thakeham is also about virtual reality, or Lutyens' version - the realm of metaphysical beauty that was his Grail.

There is nothing virtual about the rain spattering down through the leaves of the oaks. The two Jack Russells who skitter past are not muddied algorithms. And from 30m away, there remains a nagging feeling that the north façade is deeply irritating. Why is it so po-faced? Why is the first reaction to wonder how thick the walls are? It must have been just the place for Hanbury Agg, KC - he, surely, of the sandstone chin and brows, lips as ashen and thin as pointing - who bought the house from Blackburn in 1919.

Scrunch across the gravel and into the courtyard, and the architectural game begins to unfold. At Little Thakeham, there are angels in the architecture. There, in the tonnages of masonry - the walls are indeed two and a half feet thick - are details that signal other intentions. The small packets of parallel tiles mortared into the masonry; the charcoal-grey plugs of ironstone in the pointing, like wobbly grids of blackheads; the hips and details of the chimney stacks; the scribed vertical lines in the masonry between the first floor window-sets. These walls reveal that Lutyens wanted to have his cake and eat it: he wanted mass, and succeeded with it because of the antimatter of his details.

Even so, there is no preparation for what lies in store. The front door lies within a towered porch whose low ceiling is slightly oppressive. The door gives into a long, transverse entrance hall - again, relatively low-ceilinged. A few more paces takes the visitor through the doorway into the great hall... and the game is up, the doves are released, the Big Idea snaps into focus.

The manipulation of space here is extraordinary, and even now belongs to the conceptually avant-garde. Two decades before Surrealism, the room - with its massive double-height oriole window and heavy oak doors set into external Palladian masonry portals - forces a slightly deranged physical and intellectual reaction. The visual evidence suggests that one has still to get indoors. But how can this be? The garden, very much part of the grand design of the whole project, is outside and glinting in the autumn rain.

And so the hall is the key to the house, the nub of Lutyens' concept, a set-piece Edwardian virtual reality that wonders if "inside" and "outside" might, in a certain state of grace, be one and the same thing. There is no quick-fix solution to it, either. To consider the precise relationship between the details - even the moulding on the huge display-case of a fuse box matches the profile on the mullions, for example - seems pointless.

The inside-outery is too delicate to stand rude investigation. But the fact remains: 90 years before planar glass and ever-lighter steel, Lutyens was broaching the idea of transparency and interrelated spaces; except that at Little Thakeham, the transparency is not literal - it is infiltrated into the charmed stranger by architecture that is apparently physically dense and opaque. In the great hall, nothing needed to be what it seemed, and it isn't.

Country Life magazine was more prosaic about it on 28 August 1909, noting that "it is a public rather than a private room. At the same time, though the arrangement might not suit all family habits and requirements, yet the whole arrangement and composition must be pronounced exceedingly attractive".

The writer, identified only as "T", seemed oddly uninterested in the subtle, liberating and eclectic contradictions at work, keener to liken the house to "a Quaker lady of two generations ago. She must be grey. Her dress must have neither flounce nor furbelow. Her poke bonnet must be untrimmed. But the silk shall be of the best, the tone choice, the cut and sewing skilled. Hers was a disciplined richness... it proves that our age has an architecture that will afterwards be recognised as its own - a product of its ethics as well as of its art". Not a metaphysician, then.

Artistry and artifice can be found in the rest of Little Thakeham, but it is of a different order. There are architectural and physical pleasures - the flexed oak, tile and masonry arches over the fireplace in the day-room, for example, and the fossilised winkle shells in the Horsham stone dredged from the nearby Adur river to cut into fireplace mantels. But there is the lingering feeling that the crucial statement had already been made and could only be carried off once.

Little Thakeham has one last ancillary glory - its furniture, ceramics and other objets d'art, which will be sold in situ by auctioneers Rupert Toovey & Co on 18 October. In addition to furniture designed by Lutyens, there are numerous pieces with Arts and Crafts and Art Nouveau lineages that the outgoing owners, Tim and Pauline Ratcliff, had brought in to maintain the original aesthetic and atmosphere. Until a month ago, the house had been run, unadvertised, as a decidedly select hotel. Nelson Rockefeller once turned up with two architects; more recently, Ralph Lauren liked the house so much he sent three architects in to measure up its details.

Standing again under the oaks, taking a last look at the house, a question bobbles to the surface: why write about Little Thakeham at all? The thought is momentarily troublesome, but can be stared down. Little Thakeham is still worth a few passing words because it is proof that, once in a while, there is some sort of golden mean at work in architecture, and that successful buildings - even if well past their stylistic sell-by dates - remain embedded with power and meaning.

Lutyens' virtual realities are still cutting-edge. And with the right software, Marianna could be hologrammed into the dense Indian ink shadows behind the leaded panes of the north façade, a pale face from the future and the past layered into the fantasia of Lutyens' creation.

There could be no room in this scene for a grey, unfurbelowed Quaker lady, whatever Mr T might say.