The set for the holding cells was actually used in all three seasons, so that the cells in the 27th Precinct police station in Season One are curiously identical to those in the 96th Precinct police station in Seasons Two and Three. This is possible because the holding cells are a relatively small set that was set up separately from either of the much larger sets that represented the complex of squad room, interview rooms, and offices in which the detectives worked.
The holding cells are clearly in the basement of the station. The windows are placed high up in the outer walls, which are the typical cement blocks found in basement rooms. The interior walls are covered with plain tiles, rather like large bathroom tiles—which presumably makes them easy to clean.
The door into "Holding" (as it is usually referred to) lies at the top of a short flight of steps. It has a solid construction, with a peephole. Above the door is an illuminated exit sign. The hall outside is very similar in construction to the Holding area itself. Like the entrance corridor down the steps, the outer hall runs - for such of its length as we can see - along one of the outer walls of the police station. It presumably leads to a staircase coming down from the upper floors; and this staircase probably continues up to the central rear exit on the second floor (the one at the rear of the detectives' squad room), since this is the route taken by prisoners being taken back to the cells after interrogation.
Once down the steps, the corridor continues across the front of the Holding cells. There are two high windows along this wall. Each seems to have six panes of frosted glass. In some shots, branches can be dimly seen outside, suggesting that there is some form of shrubbery as a foundation planting on whichever side of the building is out there. From the exterior views of the police station, it is clear that there is no such shrubbery adorning the frontage on either the Queen Street or drive sides of the 96th Precinct building. The holding cells must therefore be somewhere at the back of the building.
The entrance corridor runs right along to the side wall of the room, thus bordering the cells at the top of the room. We've never quite seen it end; but there is certainly a rear wall to the cell on that side of the room. This appears to be an interior wall, since it has no windows.
There is a second corridor turning off the first, midway along the room. This runs down between the cells; and it is this corridor that has the doors into the cells. The Holding area thus has a T-shape, formed by the two corridors. There seems to be a water fountain at the junction of the two corridors.
There are two cells, one on either side of the room. If one walks down the corridor between the cells, one comes to a blank end wall - presumably one of the inner structural walls of the station.
A chair is often located at the far end of the corridor between the cells for the Custody Sergeant to sit on. However, the chair is sometimes shifted outside a cell if there is a prisoner inside who is considered to need more supervision.
Each of the cells has vertical bars on both corridors, and on the door. Inside, there is a single bunk bed against the back wall, and a toilet-and-sink unit (with no screening around it). In most episodes, the camera angle excluded the toilet; you can usually see no more than a small corner of it.
The presence of only a single bunk bed would suggest that each cell is supposed to hold one inmate. However, the actual number of prisoners held in a cell can vary by quite a bit. Although usually there are only one or two in the jail area, if the budget allows or the script calls for it, more may be put into each cell, despite the limited accommodation. This is only intended as a temporary holding facility.