Times was when anything log cabiny was ok.  Now however watch out.  The new definition of what is mobile is that it has wheels and can be literally hitched to a car at the drop of a hat and towed away.  That doesn't need any planning permission.  Everything else does.  Please be careful here as the builders of wooden homes are still advertising them as planning free, and they are NOT.  With the new law even local councils will giver permission for things as they have always done, meaning whatever and wherever but all planning applications have to go up to Tarragona and there they will not approve them if they don't comply with the new planning laws.  What does this mean?  YOu can build it and no one will stop you but you will never legalise it (unlike before) and no one will ever be able to buy the land with the house mentioned on the deeds.  So they will be unable to stop the council running a road through it and no bank will ever give anyone a mortgage on it as it doesn't exist.  

There are a lot of sharks out there, watch out.