What is the strength of the permethrin in the spray? 0.5% is what most people use. If you are in the UK, it can be hard to get the insecticides used to kill mites and lice. Neem oils is said to be fairly effective on the roosts and coop materials. Getting rid of the mites before the chicks are added would be easier that treating an infestation. Be aware that some people bring in mites on straw bedding where they can hide inside the blades of straw. Spraying around the coop itself may be needed.  Mites hatch every 5-7 days, so a  second or third treatment maybe needed to kill the newly hatched mite eggs. Mites can hide in tiny nooks and crannies. Good luck.