The LO 120 S is of unusual design. The fuselage resembles that of an ultralight trike, but instead of a hang glider flexible wing, it mounts a rigid, straight wing with a span of 14.940NaN0 in the motor glider role, with optional shorter wings for powered cross country flying. Unlike conventional trikes, the aircraft has a tail, of twin-boom configuration with an inverted v-tail. Later models enclosed and faired the cockpit into the wing, making it a high-wing aircraft.
The LO 120 S is of mixed construction, using plywood, metal and composite materials. The specified engine is the Hirth 2704 of 300NaN0 mounted behind the cockpit as a pusher powerplant. The landing gear is tricycle gear.