Nothing, as the designer says later in her glorious Franco-English, that would "make people peer at you in the street". Or at least, if they did, it would only be to admire your understated zhuzh-iness.
When Marant has tamed the sweater (she's also wearing laced leather trousers and worn-down ankle boots), we race upstairs to the mezzanine.
Her greying hair is pulled back and she's wearing no makeup. Her face is almost ferociously expressive; when she grimaces or laughs her face creases into great concertina lines that stay lightly etched in her olive skin. She's gorgeous.
She asks someone for a coffee and, crossing one leg over another, rolls a cigarette.
Really, she says, the demands of her job are so great that she should be like an elite sportive: early nights, swimming, yoga, no smoking. But the collection that she is preparing has caused her a lot of stress and she "cannot refrain from doing things that are not good for me", she says. "But I'm working on myself."
In the past decade Marant has gone from cult Parisian designer to international commercial big-hitter. Worn by key influencers - Alexa Chung, Katie Holmes, Victoria Beckham, to name a few - she is a fashion editor's favourite, but there are many, many non-famous women who regularly welcome Marant's clothes into their wardrobes.
Once you've become familiar with her aesthetic her signature is instantly recognisable: a skinny, leggy silhouette: feminine but grounded by the masculine, a little bit rock 'n' roll, a little bit global traveller - she likes her ethnic craftmanship. Simple but, as she says, "not very minimalist". In short, it's how many women want to dress now and, as a result, her designs have been much emulated by other labels and the high street. Even if you've never heard of Isabel Marant you've probably worn something influenced by her.
"There's not so many brands you can dress up in every day," Marant says, trying to explain her success. "For when you have to work, you don't have a driver, when you have to run, bring your kids to school, look good at the office and just after go to dinner with friends. I mean, that's our life."
She has always designed for herself, a strength in an industry where many womenswear creative directors are male. "It's true that I am my own muse. I don't like this word but when I studied fashion at Studio Bercot [the Paris fashion college] the director said, 'You shouldn't want others to wear things that you won't wear yourself', and that's something that never left me."
In the past she has described her woman as someone who starts getting dressed up and then decides she can't be bothered and puts on her jeans instead. "You want to look good but you don't want to spend too much time on it. I will always go to things that make me feel comfortable. Being a woman I've got a very honest eye on women, I don't have any fantasy. Sometimes I can create things that are beautiful but I say, 'When am I going to wear that?'. Every morning when I open my cupboard I need some clothes. I don't need clothes to go to awards or whatever, because it's once a year."
Her personal staples are "a pair of tight jeans, a pair of flat shoes and something that is a bit like a sweatshirt, a jumper or T-shirt. A good jacket, a good coat. I'm quite androgonystic. I'm very feminine but I always need to break it with something very masculine."
For evenings she'll just change her shoes to heels. She doesn't design eveningwear.
In fact, she says sometimes she would prefer a world without it. "It's so ugly when I see beautiful girls wearing these dresses because they have to. Sometimes they would look better wearing a well-cut trouser and a T-shirt. Much more chic than a ... cake dress."
As a child Marant didn't want to be a designer. Until she was 15, " fashion was all that I hated". Her mother was a successful model, but her father was keen to protect his daughter from the fashion industry, so the young Isabel "didn't really understand" what she was doing: "My mother was a kind of far-off angel."
Her parents separated when she was 6 and Marant lived with her father who remarried "a Caribbean woman, super-chic, like a model of Yves Saint Laurent in the 1970s". Marant was not impressed with her style. "A lot of makeup. It was during the 1980s so it was really 'waaagh!' And I was totally grunge. But even if it was too feminine for me, at least it printed in my brain this French chic. Very classic."
At the other end of the style scale was her nanny, the final of her "three mothers. She was crazy. She was drinking, smoking and had ugly clothes," she recalls, "but she was wearing them in such a cool way that, finally, she was a kind of icon for me." Perhaps it was inevitable, then, that, aged 11, Marant was going to school "dressed up in a very fancy way". She made dresses out of her father's old silk paisley robes and wore them with his checked slippers. "He had great cashmere sweaters and was quite tall and big, so on me they were looking super-nice."
She had a strong idea of how she wanted to look? "Oh, completely. I didn't want to look like the others and I hated myself so I was hiding my face with my hair. I was quite ugly." She shrugs. "I was not a pretty girl."
'That's how you felt,' I say, feeling protective of the little Isabel.
"I was not," she says, definitively. "No, because I was sucking my thumb and my teeth were all like that. I was a bit like Patti Smith. I was raised in quite a chic area of Paris and they were all wearing pleated navy-blue skirts and for me that was not ..." She gives up searching for the word. "Impossible." She laughs.
At 15 she started to notice that fashion was changing. "When [Jean Paul] Gaultier started, Comme des Garcons, Yohji Yamamoto, they kicked out all those Thierry Mugler shoulders, Montana, YSL." She went crazy for Vivienne Westwood. "She had a shop in Paris and I was babysitting for hours just to be able to buy something there." In a DIY, grungy spirit she started making tops from striped teatowels. Her friends coveted them.
"I was kind of earning my living at 16 selling my rubbishy things. I'd wanted to do economic studies but I discovered fashion."
After college she tried working in other houses but found the experience frustrating - too big and noisy in that 1980s' way she didn't like - and decided to go it alone, launching her label in 1994.
Although she started as small as can be with a fashion show in a squat and her friends as models, it never felt like a struggle, "because it was never a dream for me to be super-successful, recognised by all the fashion girls. I'm always surprised that I reached such a top level because I never thought about that. I built my company step by step.
"I was very lucky that I had great people working for me who pushed me a lot. It's not really pleasing me to have my name big. That's not what pleases me about my work." She grins.
Despite the international sales, the healthy turnover (€62 million ($103 million) in 2011, up 44 per cent from 2010), the two lines (Etoile Isabel Marant is her cheaper, diffusion line) and the 13 international stores, it still isn't. Although she works ferociously during the week, every weekend for the past six years she and her partner, accessories designer Jerome Dreyfuss, and their 9-year-old son, Tal, have escaped to a cabin in a forest outside Paris with no electricity and no hot water.
"What I love about it is that finally you [realise] you don't need a lot to be happy in life. And that's very reassuring to me," she says. "You know when you have all that pressure, pressure, pressure and sometimes I'm crying because I cannot achieve things, but when I'm in my cabin in the forest I say, 'finally I'm so happy'. I'm really somebody who believes less is more. I think we have too many things. It's just killing everyone."
She and Dreyfuss, who have been together for 16 years but have not married because they "don't have time" (she says they will do it when they retire) are very similar in their outlook.
"I think we found each other because we share that same point of view. We have the same taste, we admire the same painters, designers, singers or whatever ... but fashion is not only our life. It's what we love and what we do well, but we have many other things beside this."
WHY WE LOVE ISABEL
"Adorno began importing the label in 2009. Her success lies in her ability to blend ethnic bohemia and tomboy street effortlessly. Her ankle boots or wedge trainers teamed with one of her boyish jackets and flirty skirts achieve the effortless chic so often seen worn on the streets of Paris. A new shipment of Isabel Marant usually sells out within days."
- Jennifer Cole, Adorno
"What I love about her clothes is that they are slightly ethnic, but with a touch of rock 'n' roll. She captures that nonchalant, hip Parisian style that is so effortlessly cool. I think why her designs are appealing to women now is because they fit so comfortably with a modern woman's lifestyle - casual but chic. They are also ageless - you just need the right attitude."
- Helen Cherry, WorkshopDesigns from Isabel Marant's hip yet comfortable Etiole Fall Winter collection.
• Isabel Marant is stocked locally at Adorno, Workshop and Fabric.
