Where the church should gather?

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By way of answer, I refer you to this article on the history of Christian meetings. I'll summarize some sections.

In the first century, Christians generally met in homes, especially the homes of more prominent members.

The Acts of the Apostles portrays the first Christian community in Jerusalem as gathering in the temple colonnades and “breaking bread in their homes.” As the Christian message gained a wider hearing in eastern Mediterranean cities, early believers commonly met in the homes of the community’s more prominent members. Though houses came in various styles and sizes, an atrium in a Roman villa (or a spacious dining room of a Greek house) would accommodate the needs of the small Christian communities.

By AD 150, even though the church had grown significantly, the pattern had not changed much:

Justin’s defense before Rusticus also suggests that although the Christians in Rome were becoming fairly numerous, they did not abandon meeting in homes, even if that meant the Christian community could no longer assemble in one place. Thus, the house-church pattern, first articulated in the New Testament, continued for the first generations of the church’s expansion in the Roman world.

However it does not appear that this was done for theological reasons.

Then the prefect, Rusticus, demanded: “Where do you meet?” “Wherever it is each one’s preference or opportunity,” said Justin. “In any case, do you suppose we can all meet in the same place?”

100 years later, around 250AD, Christians were meeting in buildings converted specially for the purpose. One example was unearthed from the siege of Dura-Europos in 256AD.

[One of these houses] had been altered to become a Christian church. The private residence was modified by removing one interior wall in the dining area, creating a larger room for Christian services. A small dais at the eastern end of the hall probably served as the worship center. Benches were installed around the walls of an interior courtyard, perhaps to mark off a place of instruction. In yet another room, a canopied baptismal font was erected, flanked by frescoes of Adam and Eve, and the Good Shepherd—perhaps signifying the Fall and Redemption.Though it began as a private house, after renovation all domestic use ceased, and the building became property of the church.

Very shortly after that, churches were being purpose-built.

Unlike the house churches, which looked outwardly like private dwellings, the churches erected [around 260AD] were large structures. They were designed to accommodate the throngs of new believers who swelled the church’s ranks. In some cases, Christian communities acquired property adjacent to the renovated house churches, tore down or modified their properties, and built spacious new prayer halls.

Another fifty years and church building were becoming highly visible.

Church buildings attracted the ire of Diocletian and his colleagues during the last and greatest persecution of early Christianity (303–311). It was left to the patronage of Constantine (312–337) to rebuild these churches in an even more splendid manner.

To answer the question specifically, by 400AD church buildings were commonplace, and had been around for at least 150 years. Some examples of these buildings still exist today.