For architect Ivan Kostic, a caving-in garage struck him as an immediate opportunity—but he didn’t capitalize on it right away. “When my wife and I bought our house in Seattle in 2014, we thought the logical thing to do was take down the garage and build a detached accessory dwelling unit in its spot,” recalls Kostic. Several years and two kids later, after launching his firm, zeroblok, he finally got around to it. It’s now an ADU that’s half garage, half house, and, in Kostic’s description, 100 percent joyful. “As architects, we believe in joy,” he says. Luckily, the current tenants, Evan and Lenore, appreciate the home’s personality. “The stair is one of those things where I’m like, Wow, I get to walk through this every day,” says Evan, a fellow architect. As the couple continues to layer on details to make the ADU their own, the house’s personality continues to evolve. “In architecture, we ask ourselves, Who is the author of a space like this?” says Kostic. “Sure, the architecture i...