Hi
I think most people would say that it depends on your local adjuntament.  We have land in Garcia and arranged for someone to have a meeting with the local mayor for us to find out if we could have a mobile home or a log cabin on our land.  He said we could as long as it was screened and blended into the surroundings and no one complains!  It seems that anything "mobile" is OK, but you will not get a stamped piece of paper giving you permission.  Again, that is only Garcia, it could be different where you are.  If you want the details of the translator who went to see the Mayor for us, I can let you have details, he is based in L'Ametlla.
Regards
Eddie