Ultima Online had a good solution to this a long time ago. Originally a single character or account could have as many houses as they were able to buy and place. Then they changed it so that an account could only have one house (per world, iirc) but grandfathered in those players who had multiples. The solution? Tie this to the housing decay timer. Accounts with one house that was placed after the change were immune to having their house disappear as long as they maintained their subscription. Players with multiple houses could manually refresh all of them, or opt in to the primary-house immunity at the cost of becoming unable to refresh the rest of them.
