In this building a bust was found of the Greek fifth-century statesman Themistocles. The building was excavated during the 1938-1942 campaign. The north-west part was replaced by Guild Temple V,XI,1 at the end of the second century, the north-east part by shops V,XI,3. To the east is the Warehouse of the Annona (V,XI,4-5). To the west is an inner street, not a paved road. The building has a trapezoidal plan, resulting from the orientation of the streets in this part of Ostia. These streets led to the south-east, to a secondary city-gate at the south end of Via del Sabazeo. Some of these streets were blocked by the Baths of the Swimmer (V,X,3).

According to Gustav Hermansen the impact of the construction of the guild temple on the building can only be explained by assuming that the guild that used the temple (the builders, fabri tignuarii) also owned these premises.

The west wing

The west part of the west wing consists of a row shops and an apartment, built in the period of Hadrian (opus mixtum). The shops (1-2, 4-7) are interconnected, indicating that the same kind of goods was sold here. The thickness of the load-bearing walls is 0.60 m., so there were at least two upper floors. At the south end is an apartment (8) with five rooms. The interior walls have disappeared, but the size of the rooms can be deduced from the floor mosaics (white, with black borders). In the late second or early third century AD a new facade was built, c. 1.50 m. further to the west. A staircase in the third shop from the north (3) was now abandoned. Against the facade of the apartment a south-north running staircase was set, that probably led to a balcony supported by the walls of the new facade.

The east part of the west wing is badly preserved. It may have been a working or storage area.

The east wing

The east wing consists of a row of four apartments. They were built during the reign of Hadrian. The northernmost apartment (19-21) was reduced in size after the construction of V,XI,3. The thickness of the load-bearing walls is only 0.45 m., so the apartments probably had only one upper floor. Three of the apartments are medianum-apartments. The second one from the north will be described in more detail (see the plan below). In the centre of the west part is a door leading into a vestibule, with a travertine drainage lid directly behind the door. To the east is the medianum (room 24, the "room in the middle"). On the floor are small bricks in a herringbone pattern (opus spicatum). The floor and the lid suggest that the medianum did not have a roof. In the other rooms is a white mosaic floor with a black border along the walls. The vestibule is flanked by two small bedrooms (cubicula 23 and 25). At the north and south end are representative rooms (22 and 26). Room 22 is the exedra, the largest room (4.95 x 6.43). In front of the apartments is a narrow, unroofed alley (18). The preserved height of the west wall of the alley is c. 4 metres. It has two doors.

Wall-paintings were found in the apartments and in rooms behind the shops. They belong to the later second or third century. In rooms 11 and 12 three red and yellow aediculae can be seen on a white background (the yellow paint has almost completely disappeared). In between the second and third is a vertical groove, in which a light dividing wall must have been set. Between the aediculae are garlands, at the far left was a swan. In room 21 two horizontal zones can be seen, separated by a triple red line. Here too were (red or yellow?) aediculae and garlands, furthermore a bird and what seems to be a crustacean. A similar decoration was found in room 22, with red-and-yellow aediculae. Between the aediculae are garlands, while further panels are created by green lines. In the centre of these panels are masks.