http://forum.square-enix.com/ffxiv/t...=1#post4541756

Players will be permitted to own only one private and one free company estate per World per service account.

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.

Further details will be announced on a special site which will be released later.
Owning a room in an apartment won't be considered as owning an estate, so you may purchase without any concerns.