This may clarify the difference: In this use, the word "just" is time-related. It expresses recentness, as in "I have just arrived -- 5 minutes ago!" So, "I have just come to my house," tells me that you have been at your house only a short amount of time. "I have *already* come to my house," does not have that same kind of temporal meaning. It expresses only that you completed the activity of coming to the house, and now you are at the house.

"Just" implies that something happened very recently. "Already" means that something happened in the past, but not necessarily recently." For example, "I have already been to France" -- the speaker might have been to France last month or 20 years ago. But "I have just been to France" means that the speaker went to France very recently, maybe weeks or months but certainly not years ago.