There is a much cheaper way to do it, unless there are reasons behind it that dont make my proposal interesting:
You can do a private contract and do a official copy to keep the date officially recorded. This would keep the house registered in your name but they would have a contract that in the future can be registered, if need be. No taxes today; maybe taxes tomorrow.
Also keep in mind inheritance taxes in Spain are likely to dissapear (they have already in some regions like Madrid)
In any case, the main thing is to know WHAT do you want to achieve with this transaction and perhaps another solution can be found.