Building permit is essential, they can tell you what they require to grant one.
You want to talk to the local government (kommunen), call the swithboard and ask about building permits. They are likely to be fairly generous, but toilets may be a sticky issue. You probably could get away with a separating outhouse, the urine to a buried "rock-pile". Compost the solid matter fully in the barrels and use for the garden that you may plant (or at least that is what you tell them that you would like to do with it).
Also check all rights of way (servitut) that may apply to the property.