Hi Ludi,
Its a time honoured tradition around the country to breed greyhounds and sell them.
Its almost never been the subject of plannign permissions or indeed rates as fare as I am aware but its small scale stuff.
I think he will require planning as its not an agricultural use and the potential scale of it is enormous.
It sounds like a commercial breeding kennels and he will need a license I expect and properly set up buildings
The way the exempted development schedule works is that if its not stated explicitly it probably isn't exempted development.
A lot can be built under Agricultural or Industrial Uses without permission.
If this is agricultural land you should talk to the local planning officer.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.