The Guild Seat of Trajan was excavated in 1938-1939. It was built during the reign of Antoninus Pius, around the middle of the second century AD (opus latericium; a late-Severan date has recently been proposed). An obtuse angle in the plan was caused by pre-existing Trajanic buildings.

The seat was built on the spot of two domus, the remains of which were further investigated by a Swiss-French team in the late 1990's and early 21st century. The oldest house has been called Domus dei Bucrani, House of the Ox-heads. It belongs to the first century BC (c. 60 BC). The other house, the Casa a peristilio (Peristyle House IV,V,16) below the east part of the guild-seat, was used in the first and second century AD. In the House of the Ox-heads a painting with dwarfs was found. Various scenes are depicted: a dye-house, horse-races, customers of a bar, trumpet-players and soldiers. Some of the dwarfs carry weapons, and one has a strange head-covering in the shape of a truncated cone. This head-covering, the apex, was used by various priests: the flamines and the salii. The latter were priests of Mars Gradivus and celebrated a festival on the 1st of March and several successive days. In March weapons, trumpets and war-horses were purified. Later in the month, from the 19th to the 23rd, artisans - such as the dyers - celebrated a festival of Minerva. Mars and Minerva may lead us to the story of Anna Perenna, in which these deities occur. She had a festival on March 15th. Anna Perenna was related to the start and end of the year. The word "year", annum, is in her name. In the old Roman calendar March was the first month of the year.

The building was entered from the southern stretch of the Decumanus, through a passage more than 12 metres wide. In the passage were four marble columns, behind which is a semicircular exedra (A), 8 metres wide and 3.50 deep. In the back wall are two huge semicircular fountain-niches. In the back are holes for lead pipes, and there are remains of waterproof plaster (opus signinum). The exedra is flanked by shops and a staircase.

From the exedra, vestibule C (7.50 x 13.50) was reached. The vestibule is flanked by almost identical rooms, dominated by rooms D and E, with a rectangular front part separated by two columns from a semicircular exedra in the back part. The exedras open onto the rectangular rooms D4 and E4. To the north of D is a staircase, to the north of E a cistern. Rooms N and O had heated floors. Staircases F and K lead to cellars, staircases G and H to the first floor.

A passage with two marble columns leads from the vestibule to a very large peristylium. The peristylium (19.50 x 36 metres) orginally had a porticus on all four sides, with brick columns covered with plaster, and with marble bases and capitals. On the longitudinal axis is a long, narrow basin with many semicircular niches.

In the years 330-340 AD rooms were erected in the south part of the building (opus vittatum), and part of the peristylium now disappeared. A passage with two columns with spiral decoration leads to the main hall P (c. 8 x 10 metres). In the back wall is a semicircular niche, c. 1 metre deep. From the pattern of the mosaic floor can be deduced, that this was a dining room in which triclinia were placed. Around the hall are smaller rooms and a staircase, below which is a small semicircular wall-niche. Near the staircase, set against the western outer wall, is a latrine with four seats. Four rooms were set against the central part of this outer wall, but their relation to the building is not clear.

The floors in the front part of the building are of opus sectile, while the walls are covered with marble and plaster. In the rooms in the back part of the building are black-and-white mosaics with geometric motifs, animals and erotes. On the walls are marble and plaster with painted imitations of marble. A large statue of Trajan, which gave the building its name, was found in the cellar below room L. A plaster cast is today in room E4. A small head of Minerva was found to the west of the vestibule. Furthermore a statuette of Venus, two torsos of naked youths, a statue of wrestlers, and an altar with depictions of Anubis and Harpocrates were found in the building. A statue of Fortuna probably stood in the niche in room P.

The architecture, in particular the size of the peristylium and the absence of small, private rooms, suggests that this was a building used by an organization, not a house. Also, a large statue of an Emperor is not to be expected in a private dwelling. It has been suggested that it was used by a large and important guild, perhaps the fabri navales (ship carpenters), who had a temple across the street (III,II,1-2). Another possibility is that the premises were used by the navicularii Ostienses, but they are mentioned only in a fragment of an old inscription (not later than the period of Augustus) that was found in the building (EDR074049):

Marcus Pacceius is known from two other inscriptions, one from Ostia, the other from Tibur (Tivoli). The navicularii Ostienses are not documented otherwise. On another fragment of an inscription is a list of names, probably of members of a guild (album). However, for a guild-seat the monumental entrance is atypical.

A further inscription is a dedication to Lucius Volusius Maecianus, who taught law to Marcus Aurelius. He had been prefect of Egypt, Rome's main grain supplier, and prefect of the food supply. He was a patron of Ostia.

An attractive idea, developed by Christophe Bocherens, is that this was the office of the Procurator Annonae, an Imperial official stationed in Ostia, in charge of the food supply and of the financial and administrative organization of the harbours. The monumental facade of the building, with the two enormous niches, indeed strongly suggests a building with a public nature.