Yet, when you step, or more accurately stoop, inside Philip Cooper's shed, you feel like a character in a CS Lewis novel walking into the wardrobe and out into Narnia. One moment, you are among the rambling honeysuckle, rose bushes and mess of a suburban garden in Exeter. The next, you are in a tiny drawing-room of which Robert Adam would have been proud.
Dancing cherubs, moulded plaster work and gold panelling are set off by a cool Wedgwood blue background. Look up at the ceiling and Botticelli's Primavera gazes down, beside a chandelier that casts light on a frieze of Bacchean debauchery. Recalling the decadent opulence of Dangerous Liaisons, you can almost imagine John Malkovich and Glenn Close, like so many shed- seduced lovers, caught in flagrante among the bedding plants.
Perhaps the biggest surprise is discovering that the architect and master craftsman of this Tardis-like world (the shed measures 6ft by 5ft) is just 15.
Philip Cooper's peers prefer pop stars to plaster angels. They would rather ogle Pamela Anderson than enjoy a classical Venus. But that hasn't stopped him pouring as much energy into his shed (which he has had since he was five) as Jacques Gabriel devoted to Marie Antoinette's Petit Trianon at the bottom of the Louis XVI's garden in Versailles.
The detail is exceptional, right down to the curtains, made from muslin, edged with trim and pyjama cords at the top, and decorated with scrunched up wire ribbon, set against a gold decorated pelmet.
His father, Peter, sculpted the capitals, which Philip then moulded with latex. Much of the plaster work is Philip's own. What he has bought in is inexpensive and might, in a different setting, be dismissed as tat - the chandelier from Argos, costing pounds 14 raised from selling toys, the plaster niches from Wickes, the cherubs from the Christmas decoration counter in BHS, the Botticellis from Athena posters, the wall lights from a downmarket store in Exeter. But, placing them in the right context, he has created a diminutive stately home.
Philip's work has earned him a four-page spread in this month's the World of Interiors, an extraordinary accolade from a magazine which is to DIY what Capability Brown was to garden centres. The feature on Philip's shed is sandwiched beside a piece extolling the beauties of The Charterhouse of Padua, a fabulous medieval Carthusian monastery.
"Philip is amazing," said Min Hogg, editor-in-chief at the World of Interiors. "I've never seen anything like it from a boy of that age or a girl. Children are given those horrible little Wendy houses with their awful Wendy furniture and curtains. He's made his own thing 20 times better." Philip is not, however, at first sight, a particularly extraordinary young man. He remains, after all, half-boy, his bedroom still with a box full of teddies half hidden behind a curtain. He wears Doc Martens, likes surfy clothes and listens to indie grunge - Radiohead, the Manic Street Preachers, Garbage and the Cranberries.
But when he talks interiors you realise you are in the presence of a sophisticate. Why, I asked, is he so enamoured with classical style? "It's the drama of it," he said. "The plaster work, the draping curtains. There are nice curved lines, not necessarily symmetrical, but balanced. I love rococo, the leaping figures of clay on the ceiling all shouting at each other. It feels like there is movement in the room even though everything is still."
"Robert Adam is my favourite interior designer, the way he used complementary colours, like mint green and pale pink," he added as he reached under his bed for thick, colourful books to show me German examples of the styles he admires.
It's a hobby that wins little favour among the shaggier tendency at school. "They say, `Shut up, Philip, you're so sad. Shed boy,' they say, `we're going to burn it down. You're so gay.' So, these days, I keep quiet about it. They ask me, `Do you still do that gay little shed?' I say no. And they say, `Why don't you burn it down.' I just ignore them."
But he doesn't let his teenage critics get him down. Philip hangs out with his friend, Ruth Wilde, at the Picture House, Exeter's arty cinema. "We don't like going around swearing and shouting saying we snogged so and so. We like going to art galleries. We're interested in poems, adult things."
So is Philip Cooper just an extraordinary one off? His parents' 1930s pebbledash homegives no sign of spawning a design guru.
Philip, in that great tradition of children letting you down in public, describes it: "Beige walls, original carpet, carelessly painted with the cheapest emulsion, the furniture neither nice nor nasty, just practical. They say, `Philip, you'd better start on the house,' but when I say what I'd do, they can't afford it. They'd rather go on holiday to Venice."
His mother, Susan, who is studying for a history degree, struggles to find the source of her son's inspiration. Her maternal grandmother was a designer as was her father-in-law. Her husband paints, sculpts and is a photographer.
But then the crucial clue emerges from the family history. Sheds. This is a shed family, a home that has a thing about huts. Many of us have one of these little wooden structures lurking in our background. "I was in Uruguay," said Min Hogg, "where almost everyone lives in a shed, one more beautiful than the next".The Coopers have the shed-bug on an almost Uruguayan scale.
"I used to have a shed when I was a child," Mrs Cooper said. "I liked to make tea in there on a little methylated spirits stove. Being a woman, I had the idea of trying to work out what I could cook on it. I had bits of china, which eventually ended up in my daughter's shed." That is Philip's sister, Elizabeth, 22, whose shed adjoins his, but which became filled with garden tools once she left home for university. As children they pretended they were running a global business, using pre-First World War Bakelite telephones with a taut wire running between their two sheds.
Philip's father has annexed the sister's shed and also has another - or rather the garage. "He locks it up and doesn't let anyone in," Mrs Cooper said. "He brings out his paintings, but we don't see the progress. He's a private person, who likes to get on without intrusion."
Philip, unlike his father, is clearly a member of the exhibitionist school of shed person. Like Tony Goble from Roath, Cardiff, who complained about exhibits at the National Museum of Wales and was challenged to do better. So he lined up his favourite hammers, chisels and garden tools and displayed his shed to the public. Dozens turned up to view it.
Philip, likewise, has a steady stream of visitors constantly wishing to see his hut, an irritation sometimes because it makes redecoration difficult. And there is much to be done. Making a neo-classical palace out of a traditional, wooden slated shed creates problems that Robert Adam was spared.
"You have to battle with the elements," Philip said. "The wind and damp in the winter makes the wood rot, so you have to treat it in the spring. In the summer it gets too hot, so it cracks and you get wasps, spiders and wood lice and ants' nests under the carpet. You have to fill the gaps in with concrete every year so nothing can get in."
He nearly gave it all up for a new summerhouse, with lots of windows. And he is giving more attention to his bedroom, creating a more modern style. But he's staying faithful to his shed. "Some of my personality has been put into it. My flamboyance, all my pride. If it got blown down I would be devastated."
His advice to admirers? "Everyone should have a hobby. What else do you have to live for apart from your passions?"