Take a wander over to the BHAB site at
http://www.bhab.flyer.co.uk/sitenew.htm
A little bit from that site.
"Group A/Class 1 helicopters, on take off or landing, are required, in the event of failure of a power unit, to be capable of rejecting safely onto the take off/landing area available or of flying away on the remaining power unit(s) avoiding all obstacles by a vertical margin of 35 feet."