Rotorcrafts, like aircraft, have to be certified with specific airworthiness requirements.
You will find here the airworthiness standards for rotorcraft, the United States Airworthiness Standards (FAA FAR) and the European Aviation Safety Agency (EASA).
These Airworthiness Standards should be used as a reference only, as in actual designs the requirement is frozen in a certain date and amendment.
The Airworthiness Standards available for download:
Airworthiness Standards for Normal Category Rotorcraft
Airworthiness Standards for Transport Category Rotorcraft
Certification Specifications for Small Rotorcraft
Certification Specifications for Large Rotorcraft
Many other countries have their specific regulations. From an engineering point of view, an aircraft that complies with the FAA or EASA requirements is basically qualified for compliance with all other requirements.