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You want to be higher. Altitude gives you more time in an emergency (like waiting for that chute to deploy). Once you’re above the altitude that will kill you on impact, more altitude won’t make you more dead. Also, wires are at low altitude, and are responsible for many rotorcraft accidents.

The real limitations come from Part 103, which limits ultralights to uncontrolled airspace (Class G). These aircraft will be limited to 1200 ft. AGL in most of the country, and 700 ft. AGL near many airports and their associated approaches.