The lawn and terrace behind Michael Smith's home. Photographs by FrançoIs Halard Styling by Carolina Irving

For five years, designer Michael Smith lived in a gracious slate-roofed Bel Air house that he'd renovated in his trademark manner. Widely admired as a clean American approach to English country style, it was filled with pieces from different periods and highly personal collections, and became a place where Smith's clients and friends and dogs felt equally at home. Everything in it looked good: a great canopied bed, an antique Dutch leather screen, hemlock paneling, bits of Moroccan textiles, gilt consoles and Smith's own linen upholstery. Such was its seemingly magical ability to absorb everything into its chic but eminently comfortable aesthetic that when longtime friend and client Nancy Marks gave him some Doric cement columns she thought she'd tired of, she almost immediately came to regret it. Smith incorporated them into a pergola on his eucalyptus-scented back lawn, where, Marks later wrote, they looked "regal and perfect and right. I thought, 'Hmmm, is it too late to get them back?'"

Marks and her husband, Howard, are not the only friends and clients who've coveted what Smith has created for himself. In 2010, another couple Smith had worked with made an offer for the Bel Air house and pretty much everything in it, and, astonishingly, Smith accepted. "You really have to throw yourself over the falls all the time," he says. "Part of my own neurosis, which takes many forms, is the idea that to be good at what I do, I can't be complacent. I could have lived in that house for the rest of my life. I loved, loved, loved it." But: "I always want to be challenged."

The coffee tables are from CB2, as are the sofas, slipcovered in Smith's own Jasper fabric. The Tang Dynasty horse on the mantel was purchased at Christie's. Photographs by FrançoIs Halard Styling by Carolina Irving

Smith had originally looked at his current house, a distinctly modern space built in the 1990s, with the idea of recommending it to the same clients who landed in his former home. Across from the glitzy Holmby Hills pile that Candy Spelling sold to 22-year-old Formula One heiress Petra Ecclestone for $85 million, the new house sits on what is arguably the most expensive street in Los Angeles. Smith hastens to point out that he bought it at "the depths" of the market, but price wasn't the only thing that attracted him. "I decided I'd sell mine only if I could get this one, because it has such a different voice. With old houses, it's sort of one-size-fits-all. You can find a way to make them work for everyone."

Modern houses, on the other hand, are almost relentlessly "specific," he says. His own, which he shares with his partner, HBO executive James Costos, was commissioned to showcase an extensive art collection. "It's interesting to buy a house that's all about art when you don't have that much," he says, laughing. "We stretch what we can and use a lot of mirrors"—including an enormous Claude Lalanne that graces a living room wall. There's also his burgeoning photography collection. While his last house had a layered luxury that was easy to identify, Smith says he finds the sheer "volume" of the new house luxurious: "It feels heroic."

Smith is also fond of the "courtyard," a terrace and expansive green lawn onto which almost every window and door in the house look out, and where he and Costos entertain generous crowds—in both senses of the word. Last June the couple rounded up 350 guests to reportedly raise more than $1 million at an Obama fundraiser attended by First Lady (and First Client) Michelle. In March the president himself attended an event at the couple's Manhattan apartment. ("It's good to come to his home and critique," Obama said of Smith, who cracked up when the president proceeded to point toward some object or another and announce, "I don't like that.")

"Working on the White House made me really patriotic," says Smith, who was hired by the Obamas in 2010 to redo the private living quarters as well as the Oval Office. A critical New York Times piece on the color scheme of the latter, headlined "The Audacity of Taupe," hangs, rather cheekily, above the toilet in his sprawling bathroom. But Smith is serious when he talks about his newfound interest in fundraising: "It's great to be able to introduce people we know to people doing great things."

Still, there's no question that decorating is his first love. A graduate of L.A.'s Otis College of Art and Design, the California-born Smith worked for legendary West Coast antiques dealer Gep Durenberger and interior designer John Saladino before going out on his own at the brave young age of 24. Since then he's built something of an empire, including a design studio with a staff of 30; clients such as George Clooney, Steven Spielberg and Brian Grazer; three lines for Kohler (prominently featured in his most recent book, Kitchens & Baths); and a furniture and fabric line called Jasper. The latter, named for Smith's beloved Labradoodle, whose image graces the label, is housed in a stunning new 8,000-square-foot showroom on West Hollywood's Melrose Avenue where other vendors Smith admires are also on offer. Next up are a fourth book and a new furniture and fabric line, Templeton. "If Jasper is the old house," he says, "Templeton is the new one."

The new book, Smith says, centers on a house he designed in Malibu's Encinal Bluffs and includes interviews with everyone who contributed to the project—from "the plaster guy" to the landscape architect. A fan of Edie, the oral history of Warhol's famous "it girl," Smith says he thinks "clarity" will be gained through a variety of perspectives—and, besides, everyone loves a backstory. "Anyone can order a sofa," he says. "It's the process we're all obsessed with. It's like a Valentino movie. The clothes are pretty, but the process of making them is what's fascinating to watch."

While engaged in his own process, he takes inspiration from all quarters, including the French film Camille Claudel, saturated in his favorite greens and blues, and, most recently, Shampoo. "I have this idea for a project I'm doing in the desert, and it's going to show you how crazy I am," he says, launching into a description of Lee Grant's house in the movie, specifically the "kind of amazing" Indian print and bamboo bedroom in which Warren Beatty, the house-call hairdresser, sleeps with Carrie Fisher, who plays Grant's teenage daughter. "So, I'm thinking, if Lee Grant had a house in the desert, what would it look like?"

If anyone can make the premise work, it's Smith. "He's become best known for the English country look, but he can do anything," says producer Laurence Mark (Dreamgirls, Julie and Julia), who hired Smith years ago after "falling in love" with the house he designed for the late Paramount chief Dawn Steel. Mark (who describes his own house as "Cape Cod in L.A.") is a big fan of Smith's personal abodes, both old and new. "They're so different but equally tasteful and thrilling to look at," he says, adding that in any Smith house, the excitement comes from the mix. "He mixes—not matches—so beautifully."

In Smith's new house, his skill at mixing, not matching is on full display.

Given the space, he says, it would have been easy to take an all-modern route, complete with "Mies coffee tables and B&B Italia sofas." Instead, inspired by the living room's "villa-esque" proportions, he installed creamy upholstered sofas with clean, armless silhouettes and a set of gilded chairs. The mix, in this case, is not just about the look. Three of the chairs are copies of the 19th-century original, which came from the Villa Farnese, but the sofas, covered in Smith's own fabric, came from the supremely affordable CB2, as did the made-in-Vietnam concrete coffee tables.

Despite the scale, what is most striking about the house is its livability. Mark says that before leaping into a project, Smith is meticulous about learning how clients plan to use their space, and he clearly uses his own to the fullest. His rambunctious dogs (the two Labradoodles, Jasper and Sport, and a Bedlington terrier named Lily) take turns flopping on every conceivable surface. The "gallery," which he plans to transform into a dramatic library worthy of the Chateau de Groussay, currently houses a television and a treadmill, while a Nancy Lorenz screen hides "not great" built-in bookshelves. A pair of chrome and glass tables is usually divided between the living room and the sitting room, but the two occasionally come together in the dining room for larger dinner parties.

Smith and Costos entertain so much that they employ a full-time cook, Chris Kidder, whose latest creation, a lemon marmalade cake, is pressed into a visitor's hands. Smith says he became "obsessed" with marmalade during twice-yearly visits to a friend's house in Majorca, and the cake, embellished with homemade caramel ice cream, is indeed delicious. Kidder, however, has already moved on to preparations for a dinner for three that Smith is hosting in the sitting room.

Costos says such smaller, more relaxed gatherings are an example of how the pair keeps an "almost institutional space" real. "The house works really well with a lot of people," he says. "But as public as it is, it can still seem intimate, and it still has a sense of the casual." The sitting room's built-in bar, the popularity of which surprised Smith, is part of the more laid-back ethos. "It's the weirdest thing. I never had a bar before," he says, while pouring a Skinnygirl margarita from behind its limestone counter. "I was going to take it out, but people love it."

The bar, with its glass shelves filled with fine tequilas and a set of Lalique glasses found at auction, would have felt out of place in Smith's previous house. But it's part of the retro, 1970s chic that he has been drawn to lately. "There's a phrase used in vintage-clothing circles: 'It hasn't cooked long enough.' I love that. It's like, are we ready to look at Barry Kieselstein-Cord belts again?" The same thing, he says, applies to interior design. Among the items he currently professes to love most are "the weird '70s" John Dickinson lamps on the master-bedroom bedside tables. "When I first saw those lamps, I was lukewarm, but then you look at something long enough..."

"I get these crushes on things, on periods," he says. "You hear people say, 'I wanted to be an actor because you get to put yourself in another person's character.' Well, a house is both a set and a character." The main thing, he says, is not to keep repeating the same role. "I absolutely love English country houses and hope to have another someday. But this new house was like an intellectual and a creative exercise. Uniformity bothers me. If I were selling the same idea over and over, then I'd have to work harder every time to get the same liftoff. "