Harper Blaine's office is located in Pioneer Square at 103 Occidental Avenue South in Seattle, WA. This building is known as the Interurban Building and is a terra cotta-colored building on the corner of Yesler Way and Occidental Avenue South.
The building has a lion head directly over the entrance that the author loves.
Apparently Kat Richardson would have liked to have Harper's office be located in the Smith Tower at 506 2nd Avenue, but the rent was too high for Harper.
Kat Richardson explains the office location in chapter 21 of her Yodio recording of Greywalker locations in Seattle .