When you remove the estate you get the housing permit back, the one you had to buy in order to build a house after you buy the land. There is no sort of deed to the land itself that I can see. There are very few options available once the house gets stripped back down to just the land. You can build an estate, or you can relinquish land. If anyone has first-hand experience with what Relinquish Land does, or what happens when an FC with a land plot disbands, that would be very helpful.