Certain single pilot aeroplanes are designated HPA in Appendix 1 to JAR-FCL 1.220. In order to fly such aircraft you are required to pass additional written exams to ensure that you know how to operate in the environment they are designed to fly in. ATPL holders are deemed qualified.
These include:
Beech 90/100/200/300/1900 Series
Cessna 406/425/441/501/500/505/525 Series
Mitsubishi MU 2B
Piper PA31/PA46
Rockwell AC680/690/900 SeriesFairchild 226/227 Series
Pilatus PC7/PC9
Walter Extra 400
Aerospatiale MS760