We see how the church began in Acts 2:
40 And with many other words he [Peter] testified and exhorted them, saying, “Be saved from this perverse generation.” 41 Then those who gladly received his word were baptized; and that day about three thousand souls were added to them. 42 And they continued steadfastly in the apostles’ teaching and fellowship, in the breaking of bread, and in prayers. 43 Then fear came upon every soul, and many wonders and signs were done through the apostles. 44 Now all who believed were together, and had all things in common, 45 and sold their possessions and goods, and divided them among all, as anyone had need.
46 So continuing daily with one accord in the temple, and breaking bread from house to house, they ate their food with gladness and simplicity of heart, 47 praising God and having favor with all the people. And the Lord added to the church daily those who were being saved.
Now think about what this actually says. This is in Jerusalem just seven weeks after the death and resurrection of Jesus. About 3000 were added that day. So now imagine 3000 people. Try to picture that. Then note what else it says in verse 44:
Now all who believed were together, and had all things in common,
So how were they together, all three thousand?
46 So continuing daily with one accord in the temple, and breaking bread from house to house, they ate their food with gladness and simplicity of heart,
So "together" didn't mean all in one big building. Yes, it does say they met in the temple, but the temple was the center of the religion of Judaism, and there were restrictions on who could be in what part. We don't really know very much about what "meeting in the temple" actually was. The outer courtyard of the temple was the largest area, and that was "outside." I think it is unlikely that they met in the temple building which was composed of the Holy Place and the Most Holy (or Holy of Holies.) Only the High Priest could enter the Holy of Holies, one time each year. And the Holy Place was where the Judeans (Jews) offered their sacrifices.
The part of the temple where they met was probably Solomon's Porch. There are three other places in the New Testament this porch is mentioned:
And Jesus walked in the temple, in Solomon’s porch.
(John 10:23)
Perhaps even Jesus was only in that part of the Temple, I don't know. Were the apostles following that example?
Now as the lame man who was healed held on to Peter and John, all the people ran together to them in the porch which is called Solomon’s, greatly amazed.
(Acts 3:11)And through the hands of the apostles many signs and wonders were done among the people. And they were all with one accord in Solomon’s Porch.
(Acts 5:12)
Whether they met in only Solomon's Porch or some other parts, it was only for a relatively short transition period. It wasn't long before the Jews decided that they didn't want to tolerate this new thing. And remember what Jesus had said:
See! Your house is left to you desolate;
(Matthew 23:38 and Luke 13:35a)
And we know the Temple was destroyed about 40 years later.
But even from the very start it says "and from house to house."
Recently I came across a better understanding of this phrase in the Greek "from house to house." "From house to house" is not a very literal translation, although it does correctly capture one aspect of the idea. The Greek words are κατ οικον (kat oikon). Now bear with me as I dig a bit. κατ is shortened from κατα where the ending vowel is dropped when the next word (oikon) starts with a vowel.
So we have two words "kata" and "oikon." Oikon is the Greek word for "house" and kata is a preposition. This preposition in its first basic meaning is "down." Prepositions are all those little words having to do with position, like "in," "by," "on," "with," "under," "upon," "through," "beside," "up," "away," and quite a few more. Both English and Greek use prepositions as prefixes to other words to make new words. Greek does this even more than English. An example in English is "understand" that is "under stand." I will come back to that in a bit. Many of the Greek preposition words have additional meanings depending on the form of the words in the context.
Kata has an extended meaning of "according to." So a very literal translation of "κατ οικον" would be "according to houses." And that brings another facet to the meaning.
But there is more about this word κατα that we can see from how it is used in other places in the New Testament. There are dozens of places where kata can mean "every". When kata is followed by the Greek word for year, day, city, one, church, or month, it means "every year", or "every day", and so on. So "kat' oikov" could also be translated "every house." Now I would not necessarily translate it that way because, first off, it wouldn't include the houses of non-believers in the city, and I don't think anyone took it as a law that every believer had to have a meeting in their house. But putting the three ideas together strongly points to the idea that there was a lot of mingling among the many houses.
One of the most common meanings of kata is "according to." So to take that as a primary meaning of kata in this context, you could think of it as "aligning under" a guiding principle. Now as I mentioned earlier our word "understand" means that once you have learned something well, you are then aligned under it, and this fits with the meaning as "down."
So kata oikon quite certainly means that the early church didn't just meet in houses because they didn't yet have big buildings, but rather this was the guiding principle of how they met.
This was normal, and they didn't have "church buildings" for some 300 years. Buildings didn't enter the picture until after Constantine, the Roman Emperor made "Christianity" the official religion of the Roman Empire and mixed it in with all the pagan religious rituals and festivals. The pagan "temples" then became the first "church buildings." And, sadly, the rest is history!