In my Father’s house are many rooms. If it were not so, would I have told you that I go to prepare a place for you?
What people don’t realise Jesus was speaking about the temple.
And he said to them, “Why were you looking for me? Did you not know that I must be in my Father’s house?”
As such he was referring to a new temple since the temple in Jerusalem was being destroyed at that time.
A few verses after that he said this.
Jesus answered him, “If anyone loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him.
You are that mansion with many rooms. You are the new temple.