I am standing in the Old Bond Street headquarters of Stella McCartney’s fashion empire, waiting to interview the designer and wondering why there are massive wet rocks surrounded by moss on the shopfloor. I ask the publicity assistant and it is surely a coincidence that the fictional character Bubble from Absolutely Fabulous pops into my mind after she replies, in a voice imbued with significance and reverence: “Nature.”

‘I was like, Dad, this is weird… but can I have some rocks?’

Alongside the luxury clothing, there is also special clean air piped into every room to combat the pollution of central London, a ballpit for rich children to play in (at least, I don’t think any other kind come in here) and a three-hour jamming session of original Paul McCartney music playing on repeat, as Stella explains when I meet her in a private room on the top floor. Limited-edition versions of her clothes hang all around us, saved for favoured customers who make it into this locked zone.

Facebook Twitter Pinterest Inspired by the Beatles: highlights from Stella McCartney’s new All Together Now collection. Photograph: Daniel Benson/The Observer

Indeed, it turns out that the rocks have been shipped down from Paul’s farm on the Mull of Kintyre, where mist rolls in from the sea – the mist now rolls into his daughter’s garments, apparently. “I was like, Dad, this is weird…” she explains, “but can I have some rocks?”

Stella is bright-eyed and perky, quite frank, open and has the manner of someone not put on this earth to waste time. I ask if we are in a VIP room, but she groans and says that when somebody called it that during the planning stages she responded that they would not be working with her for very long if they used that word again. I think it is politically important to Stella to be seen as egalitarian, which must be hard when you’re selling fluffy jumpers for a grand.

Facebook Twitter Pinterest ‘Moss doesn’t really want to live on Bond Street. In a store’: the flagship shop in Old Bond Street. Photograph: Hufton & Crow

Anyway, the award-winning designer, 47, is dressed in layers of matching beige neutrals today, of her own design, of course. We are here to discuss a new collection she has created, called All Together Now, which is inspired by the 1968 Yellow Submarine film based on music by her father and his fellow Beatles. She has always been a proud bearer of her mother Linda’s vegetarian credentials, going to great lengths to avoid any use of leather and fur. But diving right into the Beatles legacy is something new.

But first we need to talk about the shop itself, of which Stella is equally proud. She was really specific about the rocks, she says, after her dad agreed to liberate them. “We had to reinforce the floors, to weight-bear them and all that sort of stuff, for my rock passion. And then, you know, they weren’t quite the right colour, so now we spray them.” Stella also wanted moss on them, but the problem with keeping that going, as she explains, sadly, “is that moss doesn’t really want to live on Bond Street. In a store.”

Kate Moss does though. She is living happily above us on the wall, in a framed photo with her arm around Stella, taken soon after they began working together as designer and muse, when Moss walked in McCartney’s graduating fashion show from Central Saint Martins College of Art and Design in 1998. Their friendship has continued, with Kate even cutting the ribbon to open this very shop in 2018 – it now being one of 56 stand-alone Stella McCartney stores around the world, which all stay true to their founder’s environmentalist roots by using only LED lighting, saving 75% of the power of traditional bulbs, and sustainable wood and paper. The UK shops are all fuelled by wind power.

“Oh yes the Moss is here, in many ways,” the McCartney agrees. When I put it to her that her friend is now a national treasure, she says Moss would never accept such a role, “because you probably wouldn’t like her so much if she did. You’d be like, ‘Oh, bummer, she’s not as cool as we thought she was.’ But I’ll tell her,” says Stella, clearly proud of her friendship. “I’ll say, Sophie says you’re amazing.” We both know she will do no such thing, but still.

Facebook Twitter Pinterest We all live in a yellow submarine: baby Stella and the McCartney family, 1974. Photograph: Bettmann/Bettmann Archive

The new Beatles-inspired range includes knitwear with ALL YOU NEED IS LOVE embroidered on it in various languages and Savile Row tailored jackets inspired by the marching-band suits in the film. There is a long, psychedelic Lucy in the Sky with Diamonds dress, and plenty of yellow submarine motifs throughout the collection, which is for women, men and children. “Look at that fucking Sgt Pepper’s Lonely Hearts Club Band bag for kids, how cool is that?” she says as she shows me her inventions.

The idea came to her after the film was remastered last year and her dad held a family screening. “Just a little one, but it was literally like all of the Beatles’ children and grandchildren there.” Influenced by the kids’ enjoyment of the film, she says she saw it through completely fresh eyes. “It just hit me really hard. I went far too deep into all the meanings. Even the line ‘All together now’ – I thought, that is so incredible that these four kids from Liverpool, at such a young age, came up with something so inclusive, which feels very contemporary to everything that we’re talking about today, in the now. I came away feeling really inspired, like I had to do something with it.”

Facebook Twitter Pinterest Models wearing clothes from the All Together Now collection Photograph: Daniel Benson/The Observer

I tell her that I recently showed it to my fascinated seven-year-old and it reawakened childhood memories of having my mind blown at a similar age, as if you could almost get drunk on the psychedelic imagery.

“I agree completely. It’s a gift. All hand-drawn with individual gels and, you know, who today in the world of music is going to create an animated film like that? Who on earth has created a body of work like that? I can’t name anyone.”

McCartney lives in west London (as well as in the countryside, where she rides horses) and is married to Alasdhair Willis, with whom she has four children. Her unwavering moral commitments have put her in an interesting position as an activist in the fashion business, which she describes as the second most harmful industry to the planet. “It’s my intention to stand shoulder to shoulder with the conventional houses and show that you can actually be respectful in your supply chain and manufacture.”

To which end, she’s created alternatives not just to leather and fur, but also to all sorts of other materials, such as PVC, because its chemical production is so harmful, “and they say it’s cancerous to the people who work with it, and then the residue runs into the rivers because the factories are built on rivers”. It has taken her 10 years of innovation to make a clear shoe without using it.

If everyone else was sustainable, we could have a level playing field, so it does feel unfair – but it’s my choice

Is it satisfying to have to work hard on the invention side of things? “No. It’s not like I go, ‘Oh, I’m not going to use PVC because the challenge will make me more creative.’ It’s like, ‘Well, that fucking sucks, and I’ve also only got three sequins that I can use in two colourways as opposed to 5,000 that everyone else will use.’ If everyone else was sustainable, we could have a level playing field, so it does feel unfair – but it’s my choice and I believe very much in my reasons for working in that way. You know what? It’s not like I’m here for an easy life.”

She says recent changes in the industry, with other designers waking up to the planet, are led by people power. “These recent changes are consumer-driven. I don’t think our industry would be doing that if the customers weren’t demanding it.”

Facebook Twitter Pinterest Dress to impress: Stella McCartney with her husband Alasdhair Willis. Photograph: Mike Marsland/WireImage

So it’s simply a capitalist response to the market? “I think so, yeah, and that’s OK, because that’s capitalism, you know, that’s what happens. But now is the time, as a consumer, to really understand. We’re going to have to push the people in power. And how fucking amazing is it that it’s 15, 16-year-olds who are doing that? Thank God for them.”

She recently bought back her brand in its entirety, having previously sold a 50% stake to French investors Kering. I wonder if she stays at work late into the night. “Oh fuck no. I’m very passionate about my art. And when you’re in it, you’re in it. But if somebody says, ‘Hey you could stay here until 2am, or you could go and ride your horse with your kids, bareback,’ I’d take option two. Any day of the week. Eating a bag of chips.” She found her own buyout “reinvigorating” and did it to “protect my name, my history – it was kind of about heritage and family and continuation”.

Does this mean that her two daughters and two sons, who are aged between eight and 14, will take over the business one day? “I don’t know. Of course one side of me is like, ‘I want the kids to do this.’ But then it’s like, is that my ego? I don’t want to put that pressure on them. My mum and dad didn’t go, ‘Right, you’re going to be writing all the next albums.’ The kids should just do what they want to do.” Indeed, she recently watched Succession, a TV drama about a media dynasty that battles over who will inherit the family business, and “found it a bit depressing, actually”.

Fashion means everything to her, though. She even wants to analyse my outfit. “Psychologically,” she says, “I find it incredibly interesting that you chose a vintage-esque, ethnic, quilted cotton coat to wear today. It’s a very feminine piece, probably women made it. It says you probably celebrate some kind of hand touch, heritage, some kind of travel.” The gold cuffs on my shirt, “also tells a lot about you, that little bit of Lurex peeping out”.

Facebook Twitter Pinterest ‘The kids should just do what they want to do’. Stella with models at Milan men’s fashion week SS20. Photograph: WWD/REX/Shutterstock

She’s not so keen on my leather boots, oops, (“People don’t bring leather into me, generally…”) but, tragically, it’s my face that says most about me, which will teach me not to make such an effort next time.

“And then you’ve got your red lip and… you know, you’ve got a lot of makeup on,” Stella McCartney notes, staring at my head. A pettier person than I might respond that she is wearing exactly as much makeup herself, only it’s all in neutral shades just like her outfit – but I’m not, so I won’t.

Facebook Twitter Pinterest Kidswear featured in the All Together Now collection Photograph: Daniel Benson/The Observer

The publicist tells me there is time for me to ask one last question, so I use my precious minute to ask what she thinks of dry cleaning. Do we actually need it in the world today? Stella bursts out laughing, almost barking with amusement that I would use my precious last moments on this topic.

“I love you,” she says, “this is such a great, random question. OK, so I went to St Martins when I was a baby and in my free time I studied on Savile Row to be a bespoke tailor. It was a very masculine world, incredible, obsessed.”

Were you the only girl in the room? “I was the only girl who had ever been in the room. I was there for three years and I barely learned how to set a sleeve head in a sleeve. It’s like architecture. It’s amazing. And the rule on a bespoke suit is you do not clean it. You do not touch it. You let the dirt dry and you brush it off. Basically, in life, rule of thumb: if you don’t absolutely have to clean anything, don’t clean it. I wouldn’t change my bra every day and I don’t just chuck stuff into a washing machine because it’s been worn. I am incredibly hygienic myself, but I’m not a fan of dry cleaning or any cleaning, really.”

Facebook Twitter Pinterest Menswear from the All Together Now collection worn on model Photograph: Daniel Benson/The Observer

And then her assistant whisks me away, urged by Stella to show me around even more of the shop, because building it “killed me. I had no interior designer and no architect, we did it all in-house.” So the assistant leads me into a downstairs lavatory, naturally, because “celebrities have signed their autographs on the wall,” she explains, trying to find some names to show me but finding only that of Keith Lemon, who has written a pee joke. And then I find one myself, a message signed with a name that looks an awful lot like Alasdhair. Is this Stella’s husband, I ask?

Oh yes, the assistant says!

“I JUST WANT MY WIFE BACK,” it reads.

All Together Now launches tomorrow, available in Stella McCartney stores, at stellamccartney.com and farfetch.com

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