Sill heights with no doors?..

[robotvillage]

Ok, so I've sorted my seat belt issue(!!), As I'm designing car with no doors, is there a minimum height that the sills or side protection must be from the floor? I've looked at the regs but can't really find anything.. I was thinking what the Moke does and cars like that.. I did hear that if the top of the sill was at a certain height there needed to be some sort of step incorporated, is this true.. Many thanks in advance..