Los Angeles Architect John Finton

John Finton is jumping over fist-thick cable bundles and skittering around insulation rolls, then striding at pell-mell pace through a hazardous three-storey maze of raw beams and unmarbled bathrooms that, by August, will be the new home of the actor Mark Wahlberg. It had better be: the party invites are in the post.

The architects and interior designers have pontificated and departed; now it is Finton’s task to transform this skeleton on six acres of muddy field into a living, breathing home for the Boston street thug turned Hollywood power-player. This is, after all, the dream house for a working-class boy who helped to create Entourage, the TV series that satirised the excesses of the real Hollywood, which sprawls below. And, on the ridge of Beverly Park, in the hills above LA, even behind gates within gates, you have to keep up with the neighbours. Especially when they include Rod Stewart (who has a disco with mirror ball), Samuel L Jackson (a golf range) and Sly Stallone (a Rambo prop room).

Here, says Finton, gesticulating, is the master bedroom, bigger than a Bournemouth bungalow. Over there is a wrap-around minstrel’s gallery overlooking what will be Wahlberg’s book-lined office, maybe with actual books. Beneath that will be the 3D cinema. Further away, perhaps 200 yards from the main house, will be a towering rock grotto with the filtered swimming hole never experienced by the young “Dot rat” from rough-neck Dorchester, whom Wahlberg authentically rekindled in his film The Fighter. There may be even a “safe room”, a smaller, more discreet version of the panic rooms that were favoured by the superwealthy in the 1990s. Finton is too discreet to say.

Tall and fit, with a good head of hair for a father of three passing 50, Finton has run his own construction company since the early 1980s, when he was a student at a local technical college and officially too young to hold a builder’s licence.

Today, as builder of choice for A-listers including Dustin Hoffman, Ben Stiller, Ben Affleck, Jennifer Aniston, Jennifer Lopez and Gwen Stefani, Finton is cashing in on his reputation. He must be the first hard-hatted construction boss — as opposed to “starchitects” and designers — to publish a compulsive page-turner that reveals what lies beneath the skin of some great houses.

Over 250 pages of properties to which you and I will never receive an invitation, Finton reveals the key to modern Californian style: to look that “whatever” effortless, that Zen simple, requires a lot of hard detail-obsessed slog.

None of the lucky residents is named in California Luxury Living: A Private Tour, because of tough nondisclosure agreements. Thanks to public records, however, some are known. Many of the homes are part-time residences — only in LA can you see your holiday pad on the beach from your workaday home up on Mulholland Drive — but they set the pace and dictate the material innovations that may one day trickle down to us.

Whether these people need such palaces is an open question. Bernie Ecclestone’s daughter Petra, 24, who paid $85m (£54m) for a 56,000 sq ft mansion, the biggest in LA, said she inhabits only a few of the rooms, and spends much of her time in bed, watching British TV. She could do that from a Kensington flat.

Finton: ‘My therapist says I am a pathological accommodator’ (Austin Hargrave)Yet for celebrities who cannot leave home without getting hassled, such an estate has to act as a world in itself. That is why Finton had to fit out a classroom for Eddie Murphy’s home-schooled children. Or why so many need ballrooms for entertaining — with great wealth comes great political fundraising. And for Hollywood types the cinema is tax-deductible. The gym, too.

On the book’s cover is the Beverly Hills home of Avi Arad, who, as the boss of Marvel Studios, brought us X-Men, Blade and The Incredible Hulk. For the hyperkinetic Israeli, who wears a Spider-Man pinky ring, the 22,000 sq ft, 10-bathroom chateau is surprisingly cool and calm — but building it was not that way, Finton reveals. First, he bought every antique oak floorboard he could find in France. That was still not enough, so he infilled with hidden beams from Pennsylvania. He employed French craftsmen to finish the walls with stuc pierre, a traditional plaster made from 85% limestone and gypsum, as an alternative to cut stone.

“Funnily enough, stuc pierre is all over Paris, throughout the Louvre, alongside actual cut limestone,” Finton recalls. “I never knew what I was looking at. But it works so wonderfully in California.”

Renovating Simon Cowell’s Beverly Hills home (depicted in the chapter called Now and Zen) was as tricky as you might expect, given the TV impresario’s reputation. He wanted a 13,000 sq ft mansion, as blindingly white as his megawatt smile, to be “tailored like an Armani suit”. So Finton flew to the Aegean island of Thasos, where marble has been quarried since classical times. But that was not enough: “Only 10% of the stone was unblemished pure white and acceptable for this home, and finding enough to cover the entire ground floor was tough. So, for a year, I had to keep flying back to Greece to oversee the harvest and inspect the super-slabs.”

Finton reportedly caused marble shortages in Thasos for the first time in 2,000 years. But that’s par for the course for the man known as the Indiana Jones of builders. (Harrison Ford started out in the trade, too, as a carpenter, and got his big break after building cabinets for George Lucas.)

Not all his tough customers dwell in Hollywood, though. “I was asked by a London client to smuggle in an ashera, a $100,000 hybrid cat that looks like a leopard. Luckily, they lost interest. I stay away from animals these days.”

His other nightmare is high-end fraud in the architectural salvage business. “We once imported an antique fountain, a beautiful European piece. During remodelling a few years later, the owner gave it back to us. In the removal, it broke, and we realised it was a modern fake. The client never knew about that.”

Finton says he cut prices by 25% during the recession, but now he is back at full capacity, running a $60m-a-year business. It spreads as far as the Moscow plutocracy, who are demanding 70,000 sq ft dachas with indoor lakes and slate from Vermont. “I ask them what they do, and they do not tell me.”

He can afford to turn away a few monsters, then? “There are household names with whom I will never work again, like the singer who dragged me away from my mother’s deathbed on my birthday. Or the actress I worked with every day who didn’t recognise me the next time I saw her in public. Or the wine fraudster who offered to settle his $400,000 bill in wine. You have to draw a line somewhere.”

So how does he deal with such people? “My therapist says I am a pathological accommodator. The people I work with expect that I am available between 4.30am and midnight, and I am fine to do that with 95% of the clients.”

Finton puts this thick skin down to a troubled childhood. At eight months he was given up for adoption by his 16-year-old mother, with whom he was recently reunited in rural Ohio, and he proved too troublesome for a series of foster parents. He became a ward of court, but put himself through college. It was there he found his purpose, as he walked round a house his girlfriend’s father was building — he knew he could do it bigger, better, brighter.

And he was right. He may not have been the only student driving a Porsche at Pomona College, in Claremont, California, but he was probably the only one who had paid for it himself. “People thought I was a drug-dealer. I made my company work like Disney — clean trucks and uniforms, no long hair. One deal led to another. Captains of industry who knew I kept my word led to the celebrities.”

His work ethic cost him his first marriage, he says, but it’s never been about the money. He lives in firmly middle-class Pasadena Old Town, in a 4,000 sq ft “English-style” cottage — some of the loggias his clients favour are bigger. “After my divorce, I lived out of a suitcase, and realised I did not need much more. I don’t need extravagance — but I would never tell my clients that. Whatever they want, however bizarre, it’s their money, so they are always right.”