Why and when did Christians start constructing special buildings for worship?
The New Testament speaks of a large church in Jerusalem meeting together in a public space (e.g., the outer court of the temple in Acts 2:46) and in smaller groups in houses (e.g., the house of Mary, mother of Mark, in Acts 12:12). This practice must have been carried on in many cities of the Roman empire. For the most part, the church was dependent on members or supporters (patrons) who owned larger houses, providing a place for meeting. In Rome, there are indications that early Christians met in other public spaces such as warehouses or apartment buildings. Even when there were several meeting sites in a city, the Christians had the sense of being one church. They maintained unity through organization (from the second century on, beginning at different times in different places, one bishop in a city became the center of unity for orthodox Christians there) and symbolic gestures (in Rome, the eucharistic bread was sent from the bishop's church to other assemblies).
Before Emperor Constantine recognized Christianity as a legal religion in 313, corporate ownership of property by the church could be legally ambiguous. It seems that the first property owned by the Roman church were the catacombs. These were not places of meeting, however, but burial sites.
Unless claims for recent discoveries of early Christian meeting places are confirmed, the earliest building certainly devoted to Christian use is at Dura Europos on the Euphrates River in eastern Roman Syria. It was a house that came into Christian possession and was remodeled in the 240s. Two rooms were combined to form the assembly room, and another room became a baptistery—the only room decorated with pictures. Dura was destroyed by the Sassanian Persians in 256, ...