I have just begun learning about this. So far it isn't easy or obvious. I've spoken to two counties and both have said that any structure, no matter the size, requires a building permit. And neither knew what a cob house or an Earthship is.
The two counties are Highlands and Hendry.
They follow the Florida Building Code, so there was little difference between the two counties. They both also require you to get a permit to live in an RV while building your house. (In both cases I specified that I wanted to buy land zoned as agriculture.) One of them explained that this is because people have just dumped their sewage from their mobile homes on the ground, creating an environmental issue.
I've sent an email to Highlands county asking about
compost toilets, but I don't have my hopes up.
Anyone else learned more?
Kevmo