The location of Quinton's "apartment" on Seneca Street at Post Avenue. Photo taken 1 October 2013. Copyright Dave C. Jones. All rights reserved.
Quinton lives at the end of Seneca Street just past Post Alley in a cement "bunker" that is supposed to be off limits to the public. In Underground, Quinton takes Harper to his "apartment". She describes its location on page 44:
Quinton turned right onto Post. It's not officially designated Post Alley at the south end, but it's not much wider than if it were . . . [I followed] Quinton through the turns of the road until we reached a poured concrete wall under the Seneca Street off-ramp from the viaduct. A three-story retaining wall held back the tumble of the hill while a wide stone staircase climbed the side of the building perpendicular to it, creating a dark half room roofed by the roadbed above us. The other side of the street held the southern loading docks and dog-walking slab for the hotel tower of the Harbor Steps complex . . .
He motioned me into the darkest corner, where the retaining wall met the back staircase. A shallow, bunkerlike structure of concrete slabs poked out from the retaining wall. A rusted steel door had been set into the bunker wall and sported a triangular yellow caution sign with an odd symbol of spikes and circles and the words AUTHORIZED PERSONNEL ONLY.
Quinton . . . pushed . . . [and] the door swung in . . .
We were in a small cement vestibule that opened into a larger area bounded by old walls of stone and brick.
His apartment is apparently connected to the tunnels under Seattle, too.