Painter Rachel Hecker purchased her 1920 bungalow in the Heights, northwest of downtown Houston, in 1987. It was charming and cheap—just $60,000—but uninsulated and teeny, with 1,100 square feet of space. She made do for years. Then, in 2019, she started researching options to expand. Adding on would cost $450,000, an astronomical sum more than seven times what she originally paid for the house. But a garage apartment? At roughly $200,000, it sounded a lot more doable. She gave her contractor a paper maquette of precisely what she wanted: a modernist structure with a pitched roof, clerestory windows, and gray siding, which would sit beside and “shake hands with” her existing studio, like a miniature backyard campus. It doesn’t seem like something that would fit with the…