Maisie and Pippa Hale possess the reckless abandon innate in most four year olds, but there are two things in particular that they aren’t afraid of: knives and power tools. Since I arrived this afternoon, the charismatic twins have been sitting on the yellow kitchen island countertop helping their mom chop vegetables, each taking turns playing assistant knife wielder.

Prentis Hale and Tracy Edmonds’s daughters grew up around moving blades. Hale served as both designer and general contractor for the family’s house in Seattle’s Mount Baker neighborhood, and the toddler twins often visited the construction site with lunch and Edmonds in hand. Back in the kitchen, Maisie turns from dicing onions: “We ran through the hole!” she squeals recalling scurrying through the opening in the timber framing separating their soon-to-be bedroom from their parents’.

But for Hale and Edmonds, planning the project came long before preparing for twins. In the 1990s, premarriage and prekids, the couple shared a one-bedroom apartment in Capitol Hill, just northeast of downtown. “We stayed there so long that we ended up being the old people in the building,” Edmonds shares with a laugh. The area became increasingly “too hipster and too loud” for their liking, and with friends starting families and buying houses, they started looking, too. After discovering that the lower-price homes on the market all required substantial work, Hale and Edmonds decided to build from the ground up. Hale was immediately thrilled with the idea: As a cofounding principal of SHED Architecture and Design, this was his chance to experiment with a personalized structure and a small budget.

Edmonds embraced the challenge of finding an affordable lot, scouring the Internet for properties priced less than $25,000. When a search unearthed one adjacent to a park on Lake Washington, she called the realtor immediately. The agent was initially brash, scoffing at the idea that anyone would be interested in the lot—but he wasn’t entirely without reason: The parcel was practically inaccessible. It was situated 20 feet beyond where the road dead-ended at a curb and lay another 20 feet below street grade. The plot lacked municipal hookups and plummeted down the hill at a 50-percent slope. But the neighboring property had caught Edmonds’s eye: Just beyond the steep pitch was the P-Patch community garden in which she and Hale had been growing salad greens for years. “We’d carry a trowel and bike over from our old apartment,” Edmonds remembers fondly. “It was our escape.”

Today, their home floats at the edge of the wooded park, nestled among the boughs and branches. Anchored upon 11 piers, it’s clad in colored strips of asphalt roll roofing, an affordable siding alternative that Hale chose to resemble bark. On the east and west sides, decks outfitted with hammocks and swings jut out Swiss Family Robinson–style. Last spring, Edmonds hauled beets from the P-Patch through a living-room window with a bucket and string, and she jokes about installing a zip line from the room directly to their section of the garden.

The Treehouse—despite conjuring romantic notions of a summerlong project—was anything but a single season’s effort. Hale and Edmonds purchased the property in 1999 for $15,000, but it took a decade to move in. First they had to test the soil to make sure the land could hold foundation piers (their only building option, since excavation was beyond their budget). Then, “we just sat on it,” Edmonds says. She moved to Italy in 2002 for a year of Montessori training; Hale joined her for a few months of photography. In 2006, Maisie and Pippa were born. Finally, after years of permitting and pricing—then repricing contractors and materials after the economic meltdown—construction began.

Hale (and Edmonds) quickly realized the downside of designing one’s own home: There’s no one to tell you when to stop. Hale’s flights of fancy took the design from a double-height art space to one including a wood and metal shop. For months, he came home with model after model of the house, each time hoping for an architecture school–like crit from Edmonds who just wanted Hale to pick a design and stick with it. “The hardest part was imposing discipline and coming to grips with reality,” Hale concedes. “I needed to figure out where the rubber met the road and start from there.” The resulting home is tucked quietly away at the end of the street, leaving just the top floor and skylight hutch visible from the road. Though the 1,644 square feet (built for just $162.82 each) are spread over three floors and stilted above the steep hillside, it feels more like a ranch house. “It’s like Frank Lloyd Wright’s Usonian homes: everyone in one main space,” Hale says. In many ways the house does resemble the iconic architect’s small, affordable homes, designed with simple materials, strong connections to the land, and the hope of building a better solution for living.

There’s rarely a moment when a family member is out of sight. The kitchen opens to a deck and the dining room, which anchors the living room on the other side. It’s in this great room (great floor, really) that Maisie and Pippa read Cloudy with a Chance of Meatballs together on Hale’s handmade couch, which is outfitted with built-in bookshelves. Hale often works at the dining table—a former banquet table repurposed by a friend—instead of in the loft, which houses his and Edmonds’s desk but has been commandeered by the girls and turned into their arts-and-crafts room.

The bathroom, a storage closet, and two equally sized bedrooms are located on the lowest level—for now. “The house is exactly the size we want it to be,” Hale says, “though I don’t know if Maisie and Pippa will think that in a few years.” If—or more likely when—the twins outgrow their shared room, the loft is shower- and toilet-ready, built with flexibility in mind to allow the space to be remodeled into an additional bedroom and bath.

“This house is Prentis’s ongoing project,” says Edmonds, adding, “that’s why I say designers shouldn’t live in their own designs.” Hale had hoped to build a bridge from the loft to the living-room roof, but that idea was placed on the backburner due to limited funds. He plans to clad the bedroom walls with plywood, one of his favorite materials for its texture and price tag, but he hasn’t gotten around to it yet. In the kitchen, Hale and Edmonds forewent installing Bosch appliances (though they hope to upgrade later), cashing in on a sale at Ikea that cut the price of the entire kitchen system and appliances to a mere $4,700. Outside, they bid adieu to building a slender driveway guardrail, as it would have required extensive, and thus expensive, testing to meet code. Parts of the house are still scrappy and unfinished—the bedroom doors stand unframed, the powder room pocket door doesn’t close completely—but the family makes do. “It was important that nothing in this house be precious,” Edmonds says. Tick marks running up a wall in the kitchen track Maisie’s and Pippa’s growth; their paintings and artwork are pushpinned directly into the wall.

When the sun begins to set, we gather around the dining table for the meal Edmonds prepared with her two tiny sous-chefs. When Maisie and Pippa start squirming in Edmonds’s arms, it’s time to call it a night. We make our way to the front door, where boots and sneakers spill out of the shoe-storage area. “This entryway is too small,” Hale critiques, “but on the upside, there are no long goodbyes.” Ours isn’t—but it’s clear that Hale, always the designer, is already reworking the space in his mind.