I suppose they might in theory prefer a surface that water can drain through, but it might mean that whereas now the water flows to the specific drains in place now via the surface of the Tarmac and drains, if that's replaced with a surface that lets water through all over, that might not work so efficiently any more.
It may be more a building regs issue than PP, I think PP would be granted for the new surface given what's there today, it will be the building regs for drainage that determine whether what's there now will work with the new use and surface.
Depends on how official you would want to be and whether you think anyone will be affected (eg someone beyond the garden) or dib you in with the planners
One option might be to leave the existing Tarmac in place with the existing drainsand shutter up around the area you want with scaffold boards and.then use pea gravel to a good depth of 6-8 inches. Pea gravel is the best surface for horses hoof health and tho it looks un comfortable to us they like it and will lie on it. Drainage would be unaffected and although you've added a surface it's just an extra one and a free draining one. Just a thought.