The excavations of Ostia have provided much information about the seats and temples of guilds, especially through the work in the years 1938-1942. A fundamental study was published by Beate Bollmann: Römische Vereinshauser: Untersuchungen zu den Scholae der römischen Berufs-, Kult- und Augustalen-Kollegien in Italien, Mainz 1998. A seat was called schola, a word originally meaning "a place for learned conversation or instruction, a place of learning, a school". The decisive evidence for the identification of a building as the schola of a guild is provided by inscriptions. The best example from Ostia is the House of the Triclinia (I,XII,1), the seat of the builders (fabri tignuarii). Here rooms are arranged around a colonnaded courtyard. In some rooms masonry dining couches were found, which gave the building its name. With this lay-out the building is quite similar to some of the wealthy dwellings (domus), and in the absence of further clues it is difficult to distinguish between the two. The House of the Wrestlers (V,III,1) is such a difficult case. A mosaic in the vestibule depicting wrestlers, accompanied by their names, points in the direction of a seat rather than a house.
In the House of the Triclinia a room on the long axis, behind the courtyard, was a shrine. The builders also erected a large temple further to the east, the Guild Temple V,XI,1. Another large temple was erected by the ship carpenters (fabri navales; III,II,1-2). Their seat has not yet been excavated or not been identified. The guild had many members, so that the large building V,VII,1-2 (formerly identified as seat of the Augustales) is a good candidate. Another sizeable temple was erected by the flax-workers (stuppatores; I,X,4).
From other sites we know that seats could also be simple halls, sometimes with an apse in the back wall. In Ostia the best example is the Seat of the Hastiferi (IV,I,5) on the Field of the Magna Mater. It was used by the religious guild of the "lance-bearers", worshippers of Bellona. Other good candidates are the so-called Basilica (I,II,3), the Hall of the Good Shepherd (I,II,4), and the Hall of the Group of Mars and Venus (II,IX,3). The grain measurers (mensores frumentarii) erected both a temple and a hall (I,XIX,1-3), as one complex.
The House of Bacchus and Ariadne (III,XVII,5), next to and connected with the Serapeum (III,XVII,4), has been interpreted as a seat of worshippers of the Egyptian deity Serapis. The identification of the House of Diana (I,III,3-4) and House of Mars (III,II,5) as guild seats is quite uncertain.
During their meetings in the seats the members could have communal meals. For the leading members special dining rooms were sometimes installed with couches on three sides, as can be deduced from the pattern of the mosaic floors. Deceased members would be remembered, but of course innumerable minor and major business-related issues were discussed: suppliers, prices, the effects of the demise of a member, and so on and so forth. The discussions will have become more intense when a guild was awarded the status of corpus, "body". Henceforth clients could conclude contracts with the entire guild instead of individual members. It was then up to the entire guild to meet the customer's demands. Presumably the seats of guilds in the construction business also served as a first point of contact for customers.
There must have been quite a few offices in Ostia. The painters (collegae pingentes) may have discussed the work that could be done with their customers in the so-called Inn of the Peacock (IV,II,6). A bar inside the building provided refreshments for the customers, a situation not dissimilar to what we experience when buying a car or a kitchen. The painters decorated buildings in the city and tombs in the cemeteries; the peacock, symbol of eternity, was painted in a little shrine in the courtyard of the building. Deaths had to be reported to the undertakers (libitinarii). They seem to have resided in the House of the Gorgons (I,XIII,6), named after large mosaic depictions of the peaceful head of Medusa, found often on sarcophagi. The Imperial government had stationed a procurator of the food supply in Ostia. It has been argued that his office was in the so-called Guild Seat of Trajan (IV,V,15), which had a monumental facade. In late antiquity Ostia was governed by the prefect of the annona. The building with Opus Sectile (III,VII,8) might have been intended as his office, but it was never finished.
To the list of offices we have added two possible schools. In the so-called Seat of a guild I,XII,8 a mosaic in the vestibule may have a depiction of the ninth sign of the Zodiac, Sagittarius. It could be associated with the wise centaur Chiron, and might point to a lecture hall or school of medicine. The building was accessed from the south side of the palestra of the Baths of the Forum. In the Baths of the Philosopher (V,II,6-7) two portraits of the third-century philosopher Plotinus were found. Again we may think of a school.