https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 20-27G.pdf
b. We recommend that before you build your aircraft, you contact your local FAA MIDO or FSDO. Discuss the type of aircraft, its complexity, and its materials. Provide a three-view sketch, drawing, or photograph of the proposed aircraft project, and an approximate date of construction completion. Notify your local FAA MIDO or FSDO if you intend to use commercial assistance to build your aircraft. Keep in mind that a determination of major portion will be made by evaluating the amount of work accomplished by the amateur builder(s) against the total amount of work necessary to complete the aircraft, excluding standard procured items.
Note: The major portion of the aircraft is defined as more than
50 percent of the fabrication and assembly tasks, commonly referred to as the “51-percent rule.” For example, an amateur-built kit found on the FAA List of Amateur-Built Aircraft Kits has 40 percent of the fabrication/assembly completed by the kit manufacturer. In order to be eligible for an experimental amateur-built airworthiness certificate and per the major portion rule, the fabrication and assembly tasks that may be contracted out (for hire) to another individual (or builder/commercial assistance center) needs to be less than 10 percent.