The Armed Detective Agency's office sits at the top of a slope near Yokohama's port. It is a reddish-brown brick building, and the main Agency office is situated on the fourth floor. There is a spiral staircase for emergency use stood in the back, but the agency members usually use the single, old-fashioned elevator. Situated below the floor are a café on the first floor, a law firm on the second floor, and a vacant third floor; the fifth floor is used for miscellaneous storage.
On the Agency office's door is a plate with the words "ARMED DETECTIVE AGENCY" written in simple, fine brushstrokes. Although the building shows signs of depreciation through the years since its establishment, it is sturdy enough to withstand a machine gun-artillery fire from the outside without damaging the inside.
The interior of the office consists of an office floor, a reception area, a conference room, the president's office, an infirmary, an operating room, and a kitchenette. Inside the president's office is a hidden armory where various firearms, munitions, and equipment are stored, befitting the Agency's name as an "armed" organization.