A custom built aircraft requires a pilot license to fly in addition to an airworthiness certificate for the airplane. An ultralight does not require a pilot's license or an airworthiness certificate - generally speaking. Basically, there are five different types of materials used in building amateur-built airplanes.

Yes, it is entirely legal to build and fly your own aircraft, provided you adhere to specific government regulations. In the United States, the Federal Aviation Administration (FAA) governs this under the "Experimental Amateur-Built" (EAB) category. The primary requirement is the "51% Rule," which states that the major portion of the aircraft must be fabricated and assembled by persons who undertook the project solely for their own education or recreation. Once the building process is complete, the aircraft must undergo a rigorous inspection by an FAA inspector or a designated airworthiness representative to receive a Special Airworthiness Certificate. Additionally, the builder or pilot must hold a valid private pilot certificate (or higher) to fly the aircraft, and the plane must be registered with its own unique N-number. Operating limitations are typically issued, which often include a mandatory flight testing period—usually 25 to 40 hours—conducted in a restricted flight area to ensure the aircraft is safe before it can be flown over populated areas or with passengers.