It is generally regulated by the local Aviation authority.
For example, in USA, a non profit organisation, United States Ultralight Association supports ultralight aviation and ultralight aircraft.
The requirements on operating Ultralight are:
Among the restrictions of FAA's FAR Part 103:
Used only for sport and recreation
Daylight operations only
No operations over congested areas
Used by a single occupant
Maximum five gallons of fuel
Yield right-of-way to all other aircraft
Operation in controlled airspace and restricted areas requires prior permission
Two-seat ultralights, except for those used for instruction, are regulated by other portions of the FAR's (see question 1.4.) See FAR Part 103.
Similarly, in Great Britain, Civil Aviation authority regulates the requirements for flying any kind of aircrafts and classifies types of operations either as:
** General Aviation:
Private flying, aerial work and recreational flying involving all types of aircraft, together with Corporate or Business Aviation where aircraft are operated by companies or organisations but do not carry fare-paying passengers.)
** Commercial Aviation
So, I would say, you must speak to local Aviation Authority / Aviation Regulatory Body is the one who could be of utmost help available at your disposal for such queries.
Hope it helps.