In the United States, you cannot fly "completely" without falling under FAA jurisdiction, but there are strict altitude ceilings for unlicensed recreational activities. For unmanned drones (UAS), the legal limit is 400 feet above ground level (AGL) in uncontrolled airspace; flying higher requires a Part 107 professional license and specific waivers. For manned ultralight aircraft (which do not require a pilot's license under Part 103), the FAA requires you to remain "below the floor of controlled airspace," which in many areas is 700 or 1,200 feet AGL, though you must still yield to all other aircraft and avoid congested areas. It is a common misconception that you can fly as high as you want if you own the land; the FAA owns the "navigable airspace" starting from the ground up. If you exceed these limits without proper authorization (like LAANC for drones), you risk significant fines. Essentially, if you are an unlicensed hobbyist, you should generally consider 400 feet your hard "safety ceiling" to stay out of the path of commercial and general aviation traffic.