Hi, I'm contemplating buying land in Spain (will be traveling there in a few weeks) and am eager to learn how regulations there differ from the U.S.
While it varies from county to county, here in the U.S. you generally need to install very expensive equipment in order to legally sell dairy products. Raw milk cannot be sold legally in most places. It's common for local authorities to shut down or fine those who do not follow the rules. There are ways around it, for example by selling a monthly 'cow share' instead of selling milk directly, but I'm not sure if that protects you completely.
Similarly, if you want to sell meat products here you need to have a government approved butchering facility on your farm, or you need to ship the animals to a government certified butcher who puts a stamp on it and then ships it back to you so you can sell it.
As far as earthworks, the local government can prohibit changes that would interfere with water drainage, some states do not allow you to collect rainwater. I've been on farms that were fined by the EPA for inviting people to drop off their woody material, they said it was 'public dumping' and could be hazardous.
Are there similar laws in Spain regarding food or earthworks? Do the local authorities enforce those laws? A lot of these things are not a problem if you are simply selling or trading with friends, family, or neighbors, but how about in a farmer's market setting, or with a CSA?