This was explained in the Live Letter update thread.
Quote Originally Posted by http://forum.square-enix.com/ffxiv/threads/356388-Letter-from-the-Producer-LIVE-Part-XLI-Update-Thread
For example:
If you have two characters (character A and B) on one World, and A is enlisted in a free company which owns an estate, then B will not be able to purchase a free company estate.