Incidentally not mansions within a house. Besides the Greek idea (unlike the Latin version’s mansiones (which can be temporary)) being the simple μονη/monē, only used again in Jhn.14:23 (to room in us), the point was living with Jesus & his father (so Jhn.17:3), not the grandeur of the accommodation.
As C S Lewis said, “happiness or misery beyond death, simply in themselves, are not even religious subjects at all. A person who believes in them will of course be prudent to seek the one and avoid the other. But it seems to me to have no more to do with religion than looking after ones health or saving money for ones old age. The only difference is that the stakes are so very much higher” (Reflections on the Psalms 1961: 38). He went on to focus on desiring God, and God’s response that he is available for eternity. Thus room in the house is a means to an end, namely to be with God eternally. I could live with a dingy grotto if the company is so good, but would feel a mansion more compensation if the company is dire. I suspect God’s rooms means space, space for all who wish to be around him – one could say, all who wish to hang with his son. I imagine we’ll have the universe, by God’s grace never ending, to explore.