John 14:1-3 Set your troubled hearts at rest. Trust in God, trust also in Me. There are many dwelling places in My Fathers House…..and if I go and prepare a place for you, I shall come again and take you to Myself, so that where I am you may be also.

He has built those “mansions” on earth. The “Father’s House” , where the “rooms’ are prepared, is a reference to the temple’ of the Lord’ s Christian believers during this Church age.

1Timothy 3:15 But if I tarry long, that thou mayest know how you ought to behave yourself in the house of God, which is the church of the living God, the pillar and ground of the truth.

God dwells in the church in the person of the Holy Spirit. Before he departed, Jesus said that he must go away (to heaven) so that he could send the Holy Spirit to us.

John 16:7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.

In essence, Jesus had to go away to heaven, so that the Holy Spirit could come and dwell in his House on earth, which is now the Church. And as soon as this process began, Jesus ‘returned’ back to us via the Holy Spirit. If you will read beyond vs 3 in John 14, you will see this is what he meant. The ‘mansion’ is again mentioned in vs 23, and it’s whereabouts is identified. See the word “abode” underlined below in vs 23? It is the same greek word G3438 rendered as “mansions” in vs 2.

John 14:23 Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.

The abode/mansion is inside the believer now. The believer is a part of the church/Father’s House. Jesus ‘returns’ to dwell inside each believer via the Holy Spirit. The many rooms are inside the believers, one room per believer, which corporately add up to be the Father’s House, or the Church, as plainly stated in 1Tim 3:15.

The word “mansions” sends the imagination of the modern reader fluttering. Yes, the idea of a large palatial home in Heaven sounds wonderful, but that is not at all what is being promised here. In Christ’s time the word “mansion” was used to mean a tent. Like the ones the Roman military would use when they were marching as they moved from place to place. A very small, easy to carry dwelling place. A tarp with a stick really. Not very grand. A “mansion” actually became a unit of measurement based on how far the army could march before they set up this tent at night.

In the time of the translation of the KJV a mansion had come to mean an apartment house. A house with many dwelling places in it. Thinking that they recognized the message being conferred here, that the Father’s house has many dwelling places in it, like an apartment house, they interpretatively translated mone to mansions. But really they just butchered the text to now read as if to say “The Father’s House” (an apartment house with many rooms) contains many mansions (apartment houses with many rooms).
A mansion as referring to a large palatial single family house is a fairly modern definition. This is classic example of how meanings have changed over the last 400 years. We should ignore ‘mansions’ and substitute the word ‘rooms’ in vs 2. Then read the rest of the chapter soberly to understand what Jesus is saying. He does a long Q & A on what was said in vs 1- 3 in the rest of the chapter. We don’t have to interpret what he says, he plainly tells us the interpretation.   Ref: Crush; Bible forums