Because I am a little early, and in true Paris fashion week tradition the morning’s schedule is already running a little late, I spend the first half hour of my visit to Loewe’s Parisian headquarters in the basement kitchen. This being Paris fashion week, however, this is a little better than it sounds: long, scrubbed, pale wood benches and a table piled with bowls of kumquats and slivers of lemon cake on a white platter. Fuchsia peonies in a glass urn are so artfully overblown that, when no one is looking, I stroke a petal to check they are real. (They are.) A charming French PR chats to me about how much she loves living in Dalston. A pair of male models, in dressing gowns and with alligator clips flattening their hair ready for the artless dirty-hair-don’t-­care look they will sport at the collection presentation later, spend 10 painstaking minutes trying to figure out the espresso machine; eventually, generous in triumph, they pass tiny paper cups of coffee around the room.

From the ground floor, the building blooms into grandeur: a double sweeping staircase, parquet floor, polished curlicue railings. Loewe occupies the lowest four floors of the building; someone confides, as we walk upstairs, that the sixth floor houses Catherine Deneuve’s apartment. On the first floor, LVMH executives in elegant black suits are drinking more espressos, up here borne aloft by waiters with silver trays. We climb another level, past photographer’s assistants gaffer-taping lighting cabling to the floor and makeup artists powdering more boys, to the office of Jonathan Anderson, the 30-year-old designer from The Loup, County Derry, on whose talent LVMH is gambling the Loewe brand, the man at whose whim the parquet has been painted wenge-dark and the models’ hair creased flat.

All of this – building, mood, floor, flowers, pomp – is as much part of Anderson’s Loewe as the avant garde menswear collection (think: cartoon-embroidered silk pyjamas for Japanese astronauts) which the boys on the stairs will start modelling for editors once their hair is flat enough. Anderson enters the room from the Juliet balcony where he has been smoking a cigarette, and immediately launches into his game plan. “When I started at Loewe, I took a year out before we did a collection, because I felt we needed to work out all the fundamentals. The pencils, the door handles, the style of the press release, the stone of the buildings, the choice of photographer. All of these questions had to be asked, because ultimately, you need to make people forget what the brand looked like before, and get them to believe that the brand was always like this.”

There are two Andersons, differentiated by a middle initial. JW Anderson, designer of London’s hottest label, leaves the W at home in Hackney when he gets on the Eurostar to become Jonathan Anderson, designer of Loewe. With or without the W, he doesn’t look or sound much like a fashion designer. He is handsome in a boy-next-door kind of way, with milky pale Northern Irish skin and sandy hair which he constantly scruffles in his hands. He is restless and fidgety. There is a full ashtray on the glass-topped table and a bumper pot of Extra mints. There are two empty espresso cups already, and he drinks another while I am there. He reminds me a little of the young Stephen Hawking as played by Eddie Redmayne. It’s something in the way he blinks faster when he gets animated, in the twinkly good looks semi-subsumed by an intense, nail-biting intellect.

His ambitions for Loewe are both bold and daring. Bold, because the scale of what he wants Loewe to become is almost disarmingly grand. The off-white colour he chose for its redesigned packaging was based on Portland stone, the material used for the British Museum and the UN HQ in New York, a reference he chose because he “wants to make Loewe about culture. I want the stores to be public landmarks, where you see things you might see in a museum, I want that credibility.” (“It’s just my nature, to set the benchmark high,” Anderson told the New York Times two years ago.) And daring, because he is cheekily dismissive of the theory on which most of the modern history of the luxury industry has been built. Brands founded in the 19th century (Loewe, 1846; Louis Vuitton, 1854; Burberry, 1856) have made big business out of dazzling modern customers with vignettes from their august heritage, namedropping history-book clients, pulling rank over newcomers.

“It’s like antibiotics,” Anderson says of the efficacy of this approach. “After a while, they stop working. I think people have become immune to that way of doing brands. To say, ‘We made things for the royal family’ – that’s not enough any more. It’s time for something new.” Anderson’s vision for Loewe is to “take a brand that operates in a luxury environment, and make it about culture. Make it a brand that articulates the period I am in now.” History is past its sell-by date in luxury, he believes. “Brands need to move at the speed the world does, and today that is fast. We live in the era of content. We put something on Instagram and it gets reposted and it’s everywhere and a minute later it’s gone, over. I don’t see that as a negative thing; it’s the way my mind works, too. It’s not just about consumers not getting bored of the brand, it’s about me not getting bored of it.”

Anderson is smart enough to hear how cocky this sounds. “People say to me sometimes that I’m too arrogant, too forceful,” he says – apropos of, I suspect, a reaction he reads on my face – “but that’s just how it is with me. I give 100% to whatever I’m doing. There is nothing held back.” He traces this to growing up in Northern Ireland in the 1990s. “It was a very difficult period. Car bombs, a town getting blown up. It was awful, and really confusing as a child to live through that, but it toughened me up. I don’t take anything for granted because I know that life is like a fuse.”

The now-you-see-it, now-you-don’t middle name comes from his father, Willie Anderson, a rugby international who was capped 27 times for Ireland in the 80s, then coached the national side. Willie did not pass on his rugby skills, but his son has inherited his fierce competitive instinct. “The other thing about my father’s job was that while I was growing up on a farm, surrounded by fields and cows, he was travelling and coming home with the treasures of the world. You know, a keyring, a koala bear teddy from Australia. I think in terms of drive and ambition, that was everything for me, the awareness that was all out there.”

At 18, Anderson moved to America, to study acting. He had just come out as gay, and “partied, drank, smoked, had a great time. Except after a while I realised I didn’t enjoy the acting.” After a brief regroup in Dublin, where he worked at Brown Thomas, he enrolled at the London College of Fashion and found a job assisting legendary Prada stylist Manuela Pavesi as a window dresser. “The first time I worked with her was in Harrods, and she had just got off a flight, and she was dressed in a crocodile coat with pyjamas and a wedge, and a crocodile bag and two Tesco carriers, and diamond earrings and a headscarf. And I was just… obsessed. She had the most incredible taste, and never compromised on anything. That was when I transitioned into wanting to be a designer, because I wanted to be her. I wanted to operate like she did, creatively but in a commercial context.”

Anderson’s ascent has been dizzying. Three years after graduating, he launched JW Anderson as a menswear label; womenswear was added two years later, in 2010. By 2012, the buzz around him was such that Topshop came calling; the resulting JW Anderson x Topshop collection was a sellout. A year later, Donatella Versace tapped him up to design a collection for Versus, the Versace diffusion line. By the end of 2013, he had inked a deal with LVMH for investment in his own label alongside the Loewe job.

His pitch for the Loewe job was a book of around 100 images, which he took along to his interview with LVMH’s Delphine Arnault. It opened with a beach scene photo of model Kirsten Owen, taken by Steven Meisel for Italian Vogue in 1997. “I was like, I don’t have a collection, but I have this image. This person, this beach, right now.” The image became the basis for his first advertising campaign for Loewe. It was a brave move, when you consider the clothes in the shot aren’t by Loewe, and the photo is 18 years old. But it chimed with a fashion industry currently obsessed with regramming vintage references on to Instagram, and it helped reposition Loewe as a brand that could spring a surprise, which is crucial. “The minute you can be predicted, as a brand, you’ve got a problem,” Anderson notes.

The beach scene also reflected how he is relocating Loewe in the public imagination from Madrid to Ibiza. The shift to the Balearics gives Loewe a personal connection to Anderson, who spent his childhood summers on the island, and still holidays there. (Two weeks in the village of San Carlos, last summer.) I suspect, also, that Anderson is conscious that Madrid, for all its history and grandeur, gets very little airplay in the global cultural conversation these days compared with Ibiza. My theory would be that he has sidelined Madrid for Ibiza as a tactic to buy Loewe more pop cultural bandwidth without leaving Spain altogether.

Anderson’s first menswear show for Loewe featured a T-shirt made from two silk scarves. It was a muted reference to the provocative ideas of fused gender that have made JW Anderson such a hot ticket at London fashion week. Anderson has been driving a catwalk conversation about fluidity in gender which reflects how gender ambiguity has become a cultural issue on every level, from Caitlyn Jenner to the rise of the unisex baby name (Taylor, Piper, Harper). That these are rapidly shifting sands is reflected by the fact that when JW Anderson showed lace shirts for men less than five years ago, the reviews were “so awful that the next day I was thinking, this is not working, I should give up fashion” – yet last month, at their blockbuster catwalk show watched by Samuel L Jackson and David Gandy, Burberry made lace shirts a key element of their menswear collection.

“A few years ago, maybe society wasn’t ready,” Anderson reflects. “Or maybe my concept was too hardcore and not refined enough. Or maybe both. But when I look back, that show was probably the most important one I will ever do. It was about gender confusion, because that’s an issue that’s around us, and I believe as a designer you have to reflect what’s going on. It’s fascinating to ask, how does this reflect into clothing? What does lace or silk mean, on a man or a woman?” Some of this, he says, is about the impact our hypervisual digital age has had on masculinity. “Facebook and Instagram have created these forums in which men are putting photos of themselves out there, and cultivating an audience and a response, and that’s something new.”

Every Sunday evening, Anderson takes the Eurostar from London to Paris, and spends two days at Loewe. On Tuesday evening he goes home, and spends three days working on his own label. He has a different phone and PA for each role; everyone knows not to raise Loewe issues on JW Anderson time, and vice versa. “It works. I’m sure if I am doing an interview like this in five years’ time, I will be saying, enough is enough.” He laughs. “But for now, I enjoy it. It’s a dream come true, for my personality.”

Pavesi’s best advice to Anderson, he says, was “to keep learning. About fashion history, cultural history. To learn in order to stay sharp, because you will never understand enough, but you will enjoy the view as you get higher.” The balcony of his office – presumably like that of fellow die-hard smoker Deneuve’s, a few storeys above – faces the immense facade of Saint-Sulpice, cathedral of the Left Bank, the church where Victor Hugo was married and Charles Baudelaire christened. Our interview time is up, and as soon as he has said his goodbyes, Anderson grabs his cigarettes and heads out to the balcony. The view is pretty good.