Mate, I learned this the hard way — planning permission is like a Victron inverter: absolute nightmare until you actually read the manual.
Ring your local council's planning department directly rather than relying on their website (which is usually written by someone who's never actually lived anywhere). Ask specifically about "permitted development rights" for your situation — that's where the magic sometimes happens.
Also worth knowing: building control and planning permission are two separate beasts. You might sail through one and hit a brick wall with the other. And if you're in a conservation area? Yeah, they'll want a say in your solar panel colour.
The real kicker is that most councils don't have a clue about off-grid systems anyway, so sometimes it's less about rules and more about how you present it. "Energy-independent dwelling" sounds better than "I'm unplugging from the grid" on a form, if you catch my drift.
Document everything though — if you ever sell, the next owner will thank you for having the paper trail sorted.