An ADU in Seattle.Peter Bohler for CAST Architecture Small backyard homes are becoming increasingly popular in Seattle. One architecture firm designed a unit specifically for the city's unique climate. ADUs will likely become more popular after the city rolled back restrictions this summer. There have been a growing number of small flats, called accessory dwelling units, or ADUs, popping up in backyards in Seattle.An ADU in a Seattle backyard.Peter Bohler for CAST ArchitectureThis comes about four years after Seattle first rolled back restrictions on these units in 2019, in an attempt to ease the city's housing shortage.