The House of the Well was installed in a medianum-apartment from the Hadrianic period (opus mixtum). In the apartment a small domus was created, possibly in the second half of the third century. Windows in the west facade provided light to the apartment and the later domus. The entrance, in the centre of the west side, is flanked by two brick semicolumns. The floor of vestibule 5 (the original medianum, "room in the middle") is decorated with a black-and-white mosaic. A door with a marble threshold leads to room 4, a representative room of the medianum-apartment.

Room 1 had been a shop next to the apartment, but later it became part of the domus. The wide entrance from the street was blocked. A staircase leading to the first floor, set against the east wall, was demolished. The room was changed into a hall, at a slightly higher level and reached with two treads. The doorway between rooms 4 and 1 was widened, and decorated with two marble columns. Rooms 4 and 1 were adorned with a floor of opus sectile. In the centre of the floor of room 4 is a circular space, where a statue or ornamental basin must have been placed. The lower part of the walls of rooms 4 and 1 was decorated with marble, the upper part with paintings.

In the north-east corner of room 1 is a masonry podium. The upper part is smaller, and lined with marble. On top may have been a miniature temple (aedicula), a private shrine. Two small marble busts found in or near the house could then be related: one is of the child Commodus or Caracalla, the other of a deceased relative, or of the deified Antoninus Pius or Marcus Aurelius.

A staircase in the south-east corner of the same room leads to a large cellar below room 2 and part of room 3. It gave the building its name. It may have been a cistern for collecting rain-water. There are two terracotta pipes a little above the springing of the barrel vault, leading up, and the walls are covered with opus signinum. At the far end of the room are two masonry piers, one meter high, that may have supported a shelf. In view of this shelf and because cellars are extremely rare in Ostia, it could be that the room had a religious function as well, with statuettes of deites placed on the shelf.

Plan of the underground rooms. North is to the right.
Plan: Parco Archeologico di Ostia Antica.

In the other rooms are remains of mosaics and paintings. In room 10 was a single-seater latrine.

In the north-east corner of room 1 is a blocked doorway that led to building V,III,5. The two buildings were also connected by a (blocked) doorway in the north-east corner of room 9, near a rounded wall, facilitating the passage from one building to the other. Yet another (blocked) doorway at the south end of corridor 11 led to apartment V,III,4.