Your local city codes will most likely tell you yes. Basically you need a permit to change out toilets in most areas. Obviously most people don't get a permit for every little thing. They then have to weigh the odds when selling the house that the next buyer will call them out on it. The other thing is if something were to go wrong like a fire or something your insurance may not cover it because a permit wasn't pulled. I have a tankless and it's outside my house. I could be wrong but one in an attic sounds odd. I'd pull a permit to be on the safe side.