You will need planning permission for a caravan (for living in) on any land, be it your garden, agricultural or any other land.
Do you have many neighbours, do you know them, will they complain, are you visible from any roads, do you have any shelter in the field so your not spotted by satellite when the powers that be do there mapping? Maybe worth just putting a caravan on there and seeing if anyone objects

If its not agricultural land you could possible get away with having a caravan on there and moving it around?
Someone posted a link on here the other day in another thread which I found very interesting:-
http://www.fieldtofarm.com/I may have to get the book just out of interest.
It explains (more so in the book I guess) the procedure for - well - turning your field into a farm.
From what I can remember you will need to have a little more acreage (12.5) and a good agricultural business plan along with a need to stay on the land to care for your business needs/animals.
I wish you the very best of luck.
Ta
Baz