I’m sorry I don’t know what questions to ask you to find out what you need.
If you don’t know enough about multiplayer then it would be a good idea to stop this project and learn how to do very basic simple multiplayer things first. Once you can do that, then the other things will make sense and people can help you better. Kind of like prerequisites for a class in school.
First, learn to just make a simple multiplayer where two Characters can run around and see each other move correctly.
Then learn to spawn new Actors from both the Client and from the Server, and have them appear correctly.
If you already know those two things then you can start thinking about how to make a building system.
For a building system you will need at least:
- the building pieces
- a way to select them
- a way to choose where they will appear when they are created.