It works on two levels

The upstairs is hers; the downstairs, accessible only by an outdoor staircase and a private entrance, is his. Interior designer Ruthie Sommers doesn’t buy into any of that he-said-she-said dickering so popular these days on TV design shows. Why quibble, she wonders, when both sides can have their way? Or in her case, their own floors.

At her Santa Monica home, a two-story guest house hidden in the hills of Rustic Canyon, Sommers, 33, has taken full advantage of this philosophy. She shares a bi-level setup in a small but cleverly laid out clapboard cottage with fiancé Luke McDonough, 31, a media executive from New York who swapped his black tassel loafers for board shorts and flip-flops.

Their his-and-hers arrangement was a setup born partly out of aesthetics, (she didn’t want to overwhelm her grandmother’s delicate pieces with his bulky bachelor holdovers), partly out of a shared desire for separate turf (he likes to watch TV; she prefers to paint). But mostly out of a sense of propriety.

Born and raised in North Carolina, Sommers is an old-fashioned Southern girl, the kind who keeps fresh mint on hand in case the urge for a late-afternoon julep should hit and the kind who doesn’t take the idea of living with a guy before marriage lightly.

So when McDonough suggested they move in together last fall, Sommers was hesitant. They weren’t engaged yet, but after a year of dating and paying almost $5,000 in combined rent, she was willing to reconsider. The next hurdle was finding a place upon which the couple could agree.

He wanted Venice. She wanted Hollywood. He wanted surf. She wanted grass. He likes contemporary. She likes traditional. It had the makings of a knock-down designing-for-the-sexes episode, until they stumbled upon the ideal setting — Rustic Canyon, a provincial outpost that reminded Sommers of her Southern roots and afforded McDonough proximity to the waves.

The neighborhood, combined with a more reasonable rent, certainly made the property alluring. But the all-important bonus feature — two distinct floors separated by private entrances — made moving in a no-brainer.

“It was the perfect compromise,” Sommers says in a drawl slightly diluted by a seven-year stint as an interior designer in New York and a year of painting at Parsons School of Design in Paris. The couple, who had been friends for several years before romance set in, could live together but still have their own space. Most important, she adds half-jokingly, “my parents wouldn’t have to feel like we were living totally in sin.”

The challenge, of course, was creating two distinct zones that felt like they belonged to the same house. Sommers, a decorator known for her Hollywood-meets-Hamptons style, remedied the situation by tying the floors together with her signature preppy-chic look.

Sommers created a look she describes as “laid-back East Coast” — a mix of antiques and contemporary pieces with flea market finds she reupholstered in suiting fabric — far more affordable, she explains, than regular upholstery fabric. The effect is both refined and relaxed, as if Ralph Lauren were playing around on a budget.

If you enter through the main entrance on the top floor and miss McDonough’s family pictures — summers spent in Rhode Island and safaris in Africa — mingled among Sommers’ in the built-in bookshelves, you might not even realize a guy lives here at all.

Having sacrificed space for surf, Sommers set out to make her pad feel grander than its 800 square feet. Starting with a serene palette of seashell-inspired pinks and muted sands, she turned the wood-paneled living room into a sophisticated, unapologetically girly parlor.

“I wanted to make it feminine and boudoir-ish,” she says, fluffing the hand-painted botanical-print pillows on her Lulu DK-upholstered window seat.

The beach-meets-boudoir requirement includes such touches as a 19th century French chaise, recaned and topped with pink-and-cream-striped cotton pillows; a pair of coral-shaped sconces that look like they were plucked from a Caribbean reef; and a groovy glass coffee table of her own design.

Downstairs, in dude-land, the beach theme morphs as soft pinks and pale almonds ebb into rich chocolates, splashes of lime and varying degrees of blue. From navy to turquoise, royal to baby, blue is the link that allows a natural progression from feminine pastels to bold yet tranquil masculine hues.

But McDonough’s lair is hardly a frat-daddy cave. Rather, the den fits perfectly with Sommers’ vision for an upscale bachelor bungalow and his need for a casual place to hang out.

Sommers, who owns the La Cienega Boulevard furniture store Chapman Radcliff, covered the cold concrete floors with wall-to-wall brown carpet, “so he could walk barefoot and not worry about dirty feet,” then plopped a funky, chartreuse Greek-key rug by Jonathan Adler on top.

There are the guy requisites, of course: a centrally situated television, a mini-fridge stocked with Amstel and 2-week-old dip, a photo of Bob Dylan above the window and the dude’s destination of choice: the comfy chair.

This throne, however, is hardly an Archie Bunker La-Z-Boy with a pop-out footrest. Instead, it’s a shrine to the couple’s meet-you-in-the-middle lifestyle. “It’s as comfortable as anything I would want, but it’s much more attractive than anything I could ever have the sense to pick out,” McDonough says of the chair Sommers designed, then covered with $13-a-yard turquoise fabric and chocolate piping. Best of all, “it swivels so that I don’t even have to get up if someone walks in.”

Each lair comes complete with its own set of rules. “Going up to Ruthie’s place is like going to your parents’ house,” McDonough says. “The fridge is always stocked, but you can’t put your feet on the table.”

The opposite can be said for his digs, where a bachelor kitchenette doubles as a fully stocked bar, and crumbs on the corduroy couch are pretty much a constant. The law most often laid down in this land? “I’m not even allowed to think about laying a finger on the remote,” says Sommers, who keeps her own more petite television stashed under a table and only brings it out for the occasional Katie Couric fix. “If Luke’s down here, and I want to hang out with him, switching to HGTV is not an option.”

Still, there are no-fight zones where the two can meet on neutral territory.

The bedroom is common ground. But given its location on the top floor, design-wise, it’s still hers. White linen roman shades are trimmed with miniature seashells, and a delicate coral piping lines the upholstered linen headboard. Which isn’t to say Sommers rules with an iron curtain rod and McDonough is left out of the design decisions. “He insisted on having something fluffy for his feet,” Sommers says of the cream-colored shag rug.

“And he was very particular that the light from the sconces hit his book in exactly the right place.”

The teak wood patio is also shared turf, but since it’s downstairs, it caters first to McDonough’s No. 1 concern: comfort. A giant canvas umbrella shades an inviting cluster of cushy seats — a sea-grass couch topped with navy cushions, wicker chairs and a 1930s teak deck chaise.

Along one side, a more formal pair of carved Lutyen chairs Sommers picked up at the Rose Bowl flea market stand at attention.

“They don’t exactly fly with him,” she admits, “but they’re just so fabulous, I couldn’t resist.”

For the most part, the split is equitable. However, limited closet space posed a dilemma. She took the small, bedroom closet, while he was relegated to a dressing room downstairs. “His closet is actually way bigger than mine,” Sommers says of the tiny “second bedroom” she converted into a walk-in. To get dressed, though, McDonough has to venture outside.

Not an ideal situation, especially on winter mornings, but Sommers devised a simple solution. “I got him a fluffy Four Seasons robe, and he’s as happy as a clam,” she says in a lilt that could make an ice-cold morning shower sound like an enticing prospect.

McDonough insists he likes it this way too: “It’s just the right amount of separation, and, actually, I have the closet she desperately wishes she had.”

As it turns out, he’s even picked up a design trick or two. Back upstairs, a giant conch shell sitting in a sand-filled glass bowl takes pride of place on her wrought-iron console table. McDonough arranged it that way when he proposed to her in February, and Sommers hasn’t changed it since. “He stuck the ring in the shell and filled the bowl with sand from Point Dume, where we had our first date.”

The wedding date is set for October. Once they’re married, they could, with clear consciences and everyone’s blessing, merge, mingle and blend to their hearts’ content. But, Sommers says, “we’re not planning on changing a single thing.”

Except for maybe the remote control rules.

Where she shopped

Sources for some of the items used by Ruthie Sommers in decorating their home:

Black column lamp, $28 at Casa Victoria Furniture, 3202 W. Sunset Blvd., (323) 644-5590

Suit fabric used for wing chair, $8 per yard, at International Silks & Woolens, 9347 Beverly Blvd., (323) 653-6453

Carpet from Melrose Carpet, 7951 Melrose Ave., (323) 653-4653

Teak ship’s chaise, $120 at Casa Victoria, 3202 W. Sunset Blvd.,

(323) 644-5590

Pillow monogram, $150 at Embroidery Palace, 464 S. Robertson Blvd., (310) 273-8003

Cotton turquoise fabric, $13 per yard, and chocolate-brown piping, $4 per yard, at International Silks & Woolens, 9347 Beverly Blvd.,

(323) 653-6453

Upholstered ottomans, $40 each at Now/Again, 3815 W. Sunset Blvd., (323) 662-4338

Cream cotton fabric on the ottomans, $8 per yard, at Castle Fabrics, 432 E. 9th St., (213) 612-0646