Objection 1. It would seem that the degrees of beatitude should not be called mansions. For beatitude implies the notion of a reward: whereas mansion denotes nothing pertaining to a reward. Therefore the various degrees of beatitude should not be called mansions.
Objection 2. Further, mansion seemingly denotes a place. Now the place where the saint will be beatified is not corporeal but spiritual, namely God Who is one. Therefore there is but one mansion: and consequently the various degrees of beatitude should not be called mansions.
Objection 3. Further, as in heaven there will be men of various merits, so are there now in purgatory, and were in the limbo of the fathers. But various mansions are not distinguished in purgatory and limbo. Therefore in like manner neither should they be distinguished in heaven.
On the contrary, It is written (John 14:2): "In My Father's house there are many mansions": and Augustine expounds this in reference to the different degrees of rewards (Tract. lxvii in Joan.).
Further, in every well-ordered city there is a distinction of mansions. Now the heavenly kingdom is compared to a city (Apocalypse 21:2). Therefore we should distinguish various mansions there according to the various degrees of beatitude.
I answer that, Since local movement precedes all other movements, terms of movement, distance and the like are derived from local movement to all other movements according to the Philosopher (Phys., liber viii, 7). Now the end of local movement is a place, and when a thing has arrived at that place it remains there at rest and is maintained therein. Hence in every movement this very rest at the end of the movement is called an establishment [collocatio] or mansion. Wherefore since the term movement is transferred to the actions of the appetite and will, the attainment of the end of an appetitive movement is called a mansion or establishment: so that the unity of a house corresponds to the unity of beatitude which unity is on the part of the object, and the plurality of mansions corresponds to the differences of beatitude on the part of the blessed: even so we observe in natural things that there is one same place above to which all light objects tend, whereas each one reaches it more closely, according as it is lighter, so that they have various mansions corresponding to their various lightness.
Reply to Objection 1. Mansion implies the notion of end and consequently of reward which is the end of merit.
Reply to Objection 2. Though there is one spiritual place, there are different degrees of approaching thereto: and the various mansions correspond to these.
Reply to Objection 3. Those who were in limbo or are now in purgatory have not yet attained to their end. Wherefore various mansions are not distinguished in purgatory or limbo, but only in heaven and hell, wherein is the end of the good and of the wicked.
Source: Summa Theologica (New Advent)