The popular belief today is that Jesus had gone to heaven to undertake a massive construction project, building mansions for all believers.
Then, at whatever time that construction is finished, Jesus would return.

However, Jesus was not speaking of actual buildings in heaven. The place he went to prepare, was a Spiritual Relationship between God and the Believer.

The Father’s House is a Spiritual Relationship to God in Christ, not Buildings in Heaven

Jesus told his disciples, “In my Father’s House are many dwellings; but if it was not so, I would have told you. I go to prepare a place for you.” (John 14:2)
Jesus understood that the Temple at Jerusalem symbolized his Father’s House. We remember when he said to the Jews, “Do not make my Father’s House a house of trade!” (John 2:16)

Jesus was the True Temple of the Father

Jesus also knew that the reality of that Temple was himself. After he had chased the traders out of the Temple precincts, Jesus boldly stated that he was the fulfillment of the Temple. He said to them, “Destroy this temple, and in three days I will raise it up.” According to John, Jesus was speaking of “the temple of his body.” (John 2:19-21)