Lewis and Susan Manilow might just be the coolest grandparents ever: They actually hired an architect to build their grandchildren a fort. But this particular fort, a 700-square-foot modern prefab building-dubbed the weeHouse by its creators, St. Paul–based Alchemy Architects-is the sort that even adults find irresistible. "It’s a place to sit on the deck, have a picnic lunch, or cook dinner," says Susan.

The couple’s idea was to create a retreat on the property of their Burlington, Wisconsin, vacation home-something that would fit physically into a small clearing in a wooded area near a pond as well as stylistically with the main residence. That 6,000-square-foot house is an undulating electric-orange architectural marvel fashioned by Chicago-based architect Doug Garofalo out of an old farmhouse in 2001.

Well known for their generous support of the arts and for their own avant-garde taste, the Manilows were the perfect candidates for the playful weeHouse. The wood frame and fiber cement–sided (cement, sand, and a bit of cellulose fiber) structure is composed of two modules that were crane-lifted from a semitrailer and lowered onto a foundation, then assembled onsite by Alchemy.

A wood-burning stove and an Ikea kitchenette with eucalyptus-veneer cabinets are positioned at opposite ends of the winterized rear room. Walls of sliding glass invite the elements in or shut them out, depending on mood and weather.

The smallest weeHouse units are 330 square feet. The Manilows went with the doublewide: a winterized rear room complete with a tiny kitchen, and an open atrium in front. (No bathroom: outhouses are stationed nearby.) Separated from the winterized portion by a wall of sliding glass, the front room projects toward the pond with a cedar deck that descends in steps to the shoreline, a few feet away.

The entire structure looks like a gift box-royal blue on the outside, shocking yellow on the inside-lying on its side. Its bold geometry, vivid colors, and floor-to-ceiling glass give the structure a presence beyond its squat footprint.

Since it was built, the weeHouse has been used as everything from a summer reading haven to a winter warming house to a stage for impromptu family performances. (The ipe wood flooring inside, with its natural chestnut appeal, is famously durable, able to repel even melting snow from winter boots.) "Lewis’s eyes lit up when he imagined the theatrical potential-kids, grandparents, everyone getting into the act," recalls Alchemy’s principal, Geoffrey Warner.

Amusement is, after all, what this little house was meant for-as much for the owners as for their architect. "Most of the fun we have comes from getting to work with inspired clients like Susan and Lewis, who give life and personality to their projects," Warner says.

For information on resources, see Buyer’s Guide.