MANSION (Gr. monē, an abiding place). An expression that occurs only in John.14.2 (kjv; niv “rooms”), where the plural is used. It is correctly rendered “abiding places,” the plural form denoting the great extent of God’s house.

MANSION, KJV for μονή, G3665, John 14:2, meaning a permanent dwelling place of which there are many in heaven, appointed by the Father for His children in Christ. Mansion is derived from Lat. mansio, but this does not mean a palatial house. A room set apart for each person is connoted. Derived from μένειν, “to abide permanently,” the word indicates the believer’s eternal, unchangeable and final home in the [[House of God]].

International Standard Bible Encyclopedia (1915)

In Joh 14:2, the word is used in the plural: "In my Father’s house are many mansions," the [[Revised Version]] margin "abiding places." The ideas conveyed are those of abundance of room, and permanence of habitation, in the heavenly world.