Paul Trehin first noticed something peculiar about his son when he was 5: “Gilles drew a perfect chair. Perfect in every way. It had perspective. C’est incroyable.” But it was just the beginning. Soon after, Gilles drew the Empire State Building so vibrantly that the picture was published in The Washington Post.

As a child Gilles spent his waking hours alone sketching, sketching, sketching — buildings, landscapes, street scenes, all in the same mesmerising perspective.

All children have vivid imaginations. Many draw imaginary things. It’s a phase. For Gilles, though, the phase never passed. Aged 34, he’s still in it. And the city he started drawing as a child has grown and grown with him.

Autism was diagnosed early. Like Gilles, autistic children often show signs of prodigious non-linguistic skills such as a mastery of music, mathematics or art, along with the more common “negative” symptoms, such as a late development of linguistic skills and a lack of social skills.

The “discovery” of autistic savants is nothing new. In the 1970s the psychologist Lorna Selfe published her work on Nadia, a Nottingham child who, though severely retarded, had extraordinary drawing skills and an “innate” mastering of perspective similar to Gilles’. It petered out when her language skills improved. But Stephen Wiltshire — the autistic child artist first introduced to the British public in 1987 by Hugh Casson on BBC TV — is still drawing as incredibly as ever: his art gallery opened in Central London in the autumn.

Wiltshire’s drawings are more fine-tuned than Gilles’. They are faithful copies of what he sees or remembers, entirely compatible with another of autism’s common symptoms, extreme literality — the inability to fantasise, but the extraordinary ability to copy. Gilles’s drawings by contrast are more extraordinary for being entirely imaginary.

He first became obsessed with buildings, he says, when his family moved from France to the US as a child. “It was the skyscrapers,” he recalls. “I used to build skyscrapers in Lego, but then I realised that a city cannot only be a city of skyscrapers. It needs houses, a historic quarter, a population.” And so Urville, his fantasy city, was born.

He first drew street plans of particular neighbourhoods, then drew views from these maps. And slowly the city began to coalesce in his head and on paper in amazing systematic detail. But the city’s “system” extends beyond the page. Gilles has invented and documented not only a complete year-by-year history of the city stretching back centuries, and slotted into histories of France and Europe, but also enough minutiae about population changes and densities for every neighbourhood to satisfy the most hardcore nerd.

Such systematic detail is another common symptom of autism — what Professor Simon Baron Cohen, Professor of Developmental Psychopathology at the University of Cambridge and director of its Autism Research Centre, calls “hypersystematising”. Indeed, that Gilles should have been drawn to architecture and urban planning is no surprise. Engineering, many scientific professions, music and architecture, professions for which a skill at deducing or creating logical patterns and systems is a bonus, are populated, he says, by huge numbers who while not necessarily autistic, could be described as “autistic spectrum”, the theory that autism is not an isolated, extreme and discrete condition, but one that exists to various degrees throughout society.

There is a strong genetic pattern. Men certainly have autism spectrum “set” at a higher level than women. This might suggest why men overpopulate professions such as engineering, architecture and mathematics. It might also go some way in explaining why architects often prefer to explain their work by sketching rather than talking, their obsession with detail, their skill in conjuring up in their head entire spatial worlds, and indeed, their problem with empathy. “One question we are looking at right now,” the professor says, “is whether the better you are at systemising, the worse you are at empathising.”

What’s astonishing about Gilles is not just his imagination. Autistic people often display what’s called “weak central coherence”, an obsession with detail at the expense of the bigger picture. But Urville is detailed at many levels, particularly the bigger picture. It has a coherence, a context, a completeness of vision. But also an incompleteness. The detail goes only so far. The Lowry-esque stick people lack features. The architecture is low resolution, drawn in outline. But, to Gilles, though imaginary, it is also completely real. “I see it,” he says, “right in front of me.”

Like any city it has its good and bad sides. “Many of its population want to leave,” he jokes. Property is expensive, the traffic simply appalling. But it’s a damn sight better than Paris. “Paris represents the national state. Urville is the rebel,” he says.

So far, Gilles has drawn eight mammoth general views of the city, and about 300 smaller drawings, many of which have just been published in Britain for the first time.

The aim of the new book, says Gilles, is to educate the public about autism. “It’s a very complex condition. After 30 years of having it I still don’t understand it.” And to show the positive side to a condition commonly portrayed, especially in France, as negative. “Most autistic people have some hidden talent somewhere,” says Paul. “The hard thing,” adds Gilles, “is finding out what it is.”

Urville, by Gilles Trehin, is published by Jessica Kingsley. Visit Urville at: urvillecity.free.fr/ index.Urville-ENG.htm