Halfway through a pregnancy isn’t exactly the ideal time to buy a house. So after spending months scouting San Francisco’s Victorians and turnkey cookie-cutters—and almost defecting to the East Bay—Lorena Siminovich and Esteban Kerner decided to put the hunt on hold until after their baby was born. But then one afternoon Kerner, a design director with Old Navy, logged on to Craigslist on a whim. He saw a below-market listing for a single-family home in Noe Valley, their neighborhood of choice.

With crumbly brick cladding, peeling rust-brown paint, and rotting garage doors, the house lacked curb appeal. But the Argentine couple was drawn to the interior. “It was amazing and strange at the same time,” says Kerner of the 1,485-square-foot, multilevel, mid-century maze. “Mind-boggling,” adds Siminovich. “It was just a knot of doors and a series of insane stairs to nowhere.”

Owned by an elderly woman who hadn’t updated a thing since 1955, the house wasn’t staged in the slightest. “It had grandma’s furniture and musty rugs,” says Siminovich. Still, the couple recognized its potential. “We knew it was a diamond in the rough,” says Kerner. “But it was rough.” Such a fixer-upper, in fact, that despite the under-a-million asking price (a rarity in the neighborhood), the only other bids were from flippers.

For this duo, though, even with a baby on the way, it was a no-brainer. As the founder of Petit Collage, a line of vintage-inspired wall decor and accessories for children, Siminovich knew that she and Kerner were up to the challenge. “We’d finally found a house we could make our own,” Kerner says. “At Old Navy we strive to make the best clothes for the least amount of money; you know, give fashion to those who can’t afford it. That’s what simple mid-century modern like this is all about: quality design for the masses.”

Having sunk most of their savings into buying the house and with little money left for the actual renovation, the first thing they cut from their budget was a general contractor. “We sourced all of the materials ourselves, comparison shopped, selected every knob and paint color, and coordinated everything: the plumber, the electrician, the drywall guy,” says Kerner. “It was crazy! All-consuming.”

They did, however, need an architect. Most architects they interviewed struck them as standoffish—and even paranoid. “They didn’t want to offer any ideas. They seemed afraid we would steal them and then not hire them or something,” says Siminovich. Christi Azevedo made a different impression. “Christi was so down to earth. And she fell in love with the house immediately! She couldn’t sleep the night she first saw it. The next morning she sent us a sketch and we were like, ‘Wow, she is a genius.’”

Good thing, as it would take an adept spatial thinker to resolve the home’s five-level, one-bath, three-bedroom puzzle. “It was the craziest frickin’ thing,” laughs Azevedo. “It was like a Tetris game, putting it all together, trying to squeak out space wherever we could.” Which is exactly what they did—and in just three months.

Azevedo’s plan involved “blowing a hole in the center of the house” and creating a continuous stairwell to replace the multiple half flights that led to individual rooms. As a cost-saving measure, they picked up the existing staircase and rotated it 90 degrees. By working some stair wizardry and consolidating the laundry room, furnace, and water heater into a crawl space, Azevedo managed to carve out a small hallway, landing, and a relatively whopping four-by-nine-foot guest bathroom.

Meanwhile, Siminovich’s first priority was the kitchen, which she describes as “dark, dated, and Mad Men–like” (and not in a good way). So they gutted the room, knocked down a wall that had enclosed it, and replaced the original mahogany plywood cabinets and brown-tiled countertop with white cabinetry from Ikea. With an open plan, Siminovich and Kerner can now see all the way through the living room’s floor-to-ceiling grid of windows to the backyard and keep an eye on two-year-old Matilda as she plays.

The other major goal was to transform the dungeonlike laundry room into Matilda’s room, but there were obstacles. One wall was rough concrete, and waste piping protruded from another—hardly babyproof. Rather than box out the pipes and suck up space, Azevedo suggested they hire Norodd Wellman to build custom cabinets around them. They added insulation, drywall, carpet, and a long window overlooking the backyard. And, suddenly—accented by Siminovich’s cheery artwork—Matilda’s room went from scary to sweet.

In the new bathroom, installing the toilet sideways freed up more space for the vanity, an Ikea hack made of three chopped-up Akurum kitchen cabinets with Abstrakt doors. They also redid the 13-by-4.5-foot master bath, wall-mounting a toilet, installing a four-foot-long Kohler tub to maximize space, and splurging on custom cabinetry with solid walnut doors. “It’s easy to do a large master bath in a large house,” says Azevedo. “But in this house, to try and create a space for the baby to bathe while Kerner shaves was much trickier. To figure out this whole house, really…it was fun.”

“Fun” isn’t the first word Siminovich and Kerner would use to describe their breakneck three-month renovation. “I’d call our approach guerrilla-style. And I wouldn’t advise it,” Siminovich warns, smiling but not joking. Now that it’s all over, though, this busy family of three couldn’t be happier in their four-bedroom, two-bath home. “We still have so many ideas!” she says. “We just need to save more money. That’s what we do: Save, then renovate. Save, then renovate.”