Technically the NAS begins at ground level, however there have been court cases at the state and local level that put it starting at the height of the tallest structure an a property. Right now Senator Feinstien has a bill proposal that would require drones to operate no lower then 200' AGL. Personally I avoid private property and unless I have a dedicated operating area, I like to stay above 100' AGL.
For the record - the lowest fixed winged aircraft could operate is 500' AGL in sparsely populated areas. In urban areas 1000' AGL.
You are incorrect. The lowest that fixed wing airplanes can operate in sparsely populated areas is on the deck; zero feet.
§ 91.119 Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no
person may operate an
aircraft below the following altitudes:
(a)Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to
persons or property on the surface.
(b)Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the
aircraft.
(c)Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the
aircraft may not be operated closer than 500 feet to any
person, vessel, vehicle, or structure.
(d)Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to
persons or property on the surface -
(1) A
helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each
person operating the
helicopter complies with any routes or altitudes specifically prescribed for
helicopters by the
FAA;