Seattle was an American city.
Seattle was one of the cities suggested by Diana Goddard to leave Henry van Statten after she ordered for him to be taken away, his memory wiped, and dumped "someplace beginning with S." (TV: Dalek)
The Doctor had two friends in Seattle, old ladies who spent most of their time gossiping about their neighbours. (PROSE: Business Unusual)
Location information
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Seattle.
Nevertheless, Dalek does not in any way suggest that Seattle might be in Washington state, which this map clearly shows.