I'm not really sure why you need to be classified as a chicken farmer.
If it's for planning permission (I notice you're from the UK) then as I remember the criteria is something like being able to demonstrate that you have a viable agricultural based business capable of supporting you and your family, which used to be around £10,000 per annum when I was attempting to go through the process. I
think you'd need an awful lot of chickens if you're going to try to earn that much money. Maybe you could think of diversifying and looking for several different ways of raising money from land?