This 128-square-foot "tree house" outside Baltimore was designed by architects Laurie Stubb and her husband Peter. "The outdoors here are a big playground," she says. "We had always wanted to build something for the girls that looked natural." So, in the summer of 2008, they designed this structure both for the children and for themselves. "We wanted it to have a use after they're gone—a place we can sit in and read or have a drink and entertain company."