From article by Zach Patton on Governing. com:
"Today, almost two-thirds of the city is zoned for single-family homes, so it's harder for Seattle to accommodate its growing population -- the city swelled from 563,374 residents in 2000 to 608,660 last year -- without spreading farther and farther into the forests of the Pacific Northwest. That's partly why the city saw backyard cottages as an attractive new alternative, a way to add affordable housing options without a wholesale redesign of the city's signature neighborhoods."
Patton explains that backyard cottages and granny flats are actually a throwback to common urban design practices of the first half of the 20th century. By the 1950s, however, Americans fled to the suburbs in pursuit of large single-family homes on large tracts of land. Further, "many urban zoning codes of the second half of the century essentially banned the construction of new backyard cottages."