Greetings again Hobie,
The word “mansion” usually conveys the idea of a large, luxurious dwelling and most modern translations have changed this word to “rooms” or “dwelling places”.
John 14:2 (NASB95): “In My Father’s house are many dwelling places; if it were not so, I would have told you; for I go to prepare a place for you.
John 14:2 (NIV84): In my Father’s house are many rooms; if it were not so, I would have told you. I am going there to prepare a place for you.
John 14:2 (ESV): 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?
The phrase “my Father’s house” is not speaking of a house in heaven, but the following shows that it speaks of God’s spiritual house, composed of Jesus and the faithful:
Hebrews 3:1–6 (KJV): 1 Wherefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus; 2 Who was faithful to him that appointed him, as also Moses was faithful in all his house. 3 For this man was counted worthy of more glory than Moses, inasmuch as he who hath builded the house hath more honour than the house. 4 For every house is builded by some man; but he that built all things is God. 5 And Moses verily was faithful in all his house, as a servant, for a testimony of those things which were to be spoken after; 6 But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end.
Jesus was not going to heaven to prepare a mansion, rooms or dwelling place for the Apostles and believers:
John 14:2–3 (KJV): 2 In my Father’s house are many mansions: if it were not so, I would have told you. I go to prepare a place for you. 3 And if I go and prepare a place for you, I will come again, and receive you unto myself; that where I am, there ye may be also.
Jesus was going to be crucified, and by his perfect sacrifice he was this preparing the way for the believers.
The phrase “where I am” is not particularly speaking of physical location but speaks of spiritual status:
John 12:23–26 (KJV): 23 And Jesus answered them, saying, The hour is come, that the Son of man should be glorified. 24 Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. 25 He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal. 26 If any man serve me, let him follow me; and where I am, there shall also my servant be: if any man serve me, him will my Father honour.
As far as a physical house is concerned, it will be the Temple in Jerusalem on earth during the 1000 year reign of Jesus on the Temple Throne of David during the Kingdom of God.
Kind regards
Trevor