Last year, 66-year-old Lauren Knoblauch sold or donated nearly everything she owned, from her two-bedroom home on a suburban Seattle lake to her furniture and many of her clothes. She moved everything else, two small carloads’ worth, into her new home: a downtown apartment that, at less than 150 sq. ft., is smaller than the average U.S. master bedroom. The move came as Knoblauch, who works in inmate rehabilitation, pondered her impending retirement. “I started thinking about what I was passionate about,” she said. “I wanted to see opera in Europe, to spend money on what was exciting to me.” Her new apartment, which costs $575 a month — less than half the $1,400 average for a Seattle one-bedroom — puts her about 20 minutes from Symphony Hall by foot and a short bus ride from the Opera House. With new financial flexibility, she’s traveled to Germany and Ireland to see opera performances. “I’m loving it,” she says. The burgeoning tiny house and micro-apartment movements, which ge...