None, unless there happened to be any persons, vehicles, man made structures or, at a long stretch, vessels within 500 feet. Unless of course there is a 'hard to uncover' rule governing the operation of aircraft above such 'reserves'? I should imagine that in the case of engine failure and subsequent damage to any protected parts should spark an inquiry as to the pilot's actions.
Interesting question though - I remember when I was younger walking along the promenade at Hunstanton, a chappie flying his motor powered paraglider at about 10' above the beach, about the same distance from the promenade with lots of rubber necked tourists gawking at him like myself! I should imagine that would perhaps attract the attention of the CAA more than aforementioned example!
Cheers, Jack.