Horsebox Regulations?

[Sessle]

Am I right in thinking that theres new regulations regarding the height in between the ground and the bottom of the box? If so can someone explain this in more detail and what is the actual minimum/maximum amount of space that you are allowed between floor and box?
Just want to know as looking at horseboxes! Apologies if I'm talking nonsense!!
Thanks

 

[Shipley]

the maximum "space" from the floor is 55cm, The rule also states about what the sides should be like