NEIL BARRETT MAY HAVE based his fashion label’s sharp, geometric aesthetic on modernism, but his men’s and women’s collections retain the kind of timelessness that one might expect from a man who hails from a family of tailors.

“My grandfather was a master tailor,” says Mr. Barrett. “He was my hero when I was younger. He would be very willing to share information and tell me how things were made, and why things were made. I love ‘why,’ because you shouldn’t do anything in life unless you know why you’re doing it.”

After making his mark designing menswear for Gucci and Prada, the Milan-based designer founded his own label in 1999. Mr. Barrett’s rising star coincided with the dawning of the metrosexual age, when silhouettes narrowed and men’s fashion sense broadened.

“Men are more aware of trends nowadays, and for me that’s great. Because the more informed they are, the more likely they are to know about non-historical brands,” says the 50-year-old designer. “We’ve been going for 16 years, we don’t do that much advertising, and we don’t have that many stores around.”

With a clientele that has included names such as Brad Pitt, Justin Timberlake and Ewan McGregor, the red carpet has provided the largest billboard for his sleek suits, pristine biker jackets and stark, graphic sweaters. And while most of his store presence is in Asia, this week, Mr. Barrett opens his largest space in Europe, a shop-within-a-shop at London department store Harrods.

“I still find men creatures of habit,” he says. “Once they start buying your brand, they come back. You get on a roll and that roll gets larger and larger.”

The designer took time out from prepping his spring 2016 collection, just days before curtain call, to chat about his outerwear obsession, taking his new designs for a spin before they go into the store and why you won’t catch him wearing a suit every day.

The ethos of my design is: that I’ve always been timeless. I enjoy what I do, and so many of these things I can re-wear. You rediscover them and style them in a different way at another point in your life. Why throw things away?

I only shop in: Hong Kong and Tokyo. When I’m in Europe and have a moment free, I’ll go to the countryside or the sea and visit friends. I’m happy with my long hours, but when I have that moment after work when I would like to actually spend money, most stores are closed in Europe. In the Far East, the stores are open until 10 every night.

I try out: my collections all summer. I take things away with me and test them on my holidays, which makes them that much better when I put them into production.

I must have: at least 30 coats. Maybe 40 blousons between biker jackets and bomber jackets. Tailored jackets of all types of different constructions: 30, 40, something like that. Leather jackets? Ooh, 30 or 40. I have a lot. I’m a hoarder.

When I travel I’m: skeptical about bags arriving, so I always carry sufficient [clothing to] cover myself for the first three days in my hand luggage. Like that, I never get stressed if my bag doesn’t arrive.

It’s important to me to: spend time in Italy and work with mills. It’s a creative process that, if you’re living in New York or London, is going to go a lot slower and be a lot less inspiring. It’s not the same when you live in a country that doesn’t produce the majority of your fabric.

Milan is: a city that’s coming back up, and we’ve been having a great year. With Expo, I can really see a difference. Whole areas have suddenly come alive that were dead before. If you go down to Via Valenza, there are maybe 20 to 30 places to eat and they’re all new and open until 12 or two in the morning, and that never existed. The city is on a real high.

Wearing a suit every day would be: the most awful thing in the world. It’s a bit dramatic to say that, but for me that idea is the same as working in a bank every day—it’s something I can’t imagine. I’m very fortunate to work in a field that allows me to be creative. That’s the beauty of our industry: You can wake up in the morning and dress as you wish to dress. It’s very liberating.

My house is: very comfy but at the same time very graphic. It’s in a beautiful 18th-century building. Everything is black, brown or white, and very textural. I have a lot of 1930s and ’40s furniture, a lot of ostrich skin, brass and shark skin. I like modernity with a mixture of old and very cozy. I like things to feel very lived-in and masculine.

I love finding: stuff on 1stdibs. You can find some amazing furniture there. I would love to go to auctions, but I don’t have time.

We just opened: a store in Macau. We’re going for warm minimalism rather than the museum starkness I previously requested [of architect Zaha Hadid for other stores]. I think she’s done a very nice balance.

When I went to Central Saint Martins: I had a Saturday job at Yves Saint Laurent on Bond Street. I was working for a Lady Something. She was very British and old school and dramatic and fabulous. She took a shine to me, so that was nice.

I’m turning into: my grandfather. He would have many things of the same. When he died, he had racks of suits and jackets he hadn’t even worn. Everything was perfectly laid out.

I wear: my own clothes 99.9% of the time; the 0.1% would be Nike trainers, which I love. I wear old New Balance that I have from the late 1980s and early ’90s. I buy knitwear from other designers sometimes when I’m in the Far East. I like some Comme des Garçons T-shirts, but that’s it.

I like to: browse. I’m looking for something to excite me, obviously knowing I have my whole collection and don’t need anything. So I’m willing to try stuff on, but fortunately I’m usually disappointed. When I find something I love, I’ll buy two or three colors.