I think it depends where abouts you are, and what your council is like regarding developments for agriculture. Its virtually impossible to get ANYTHING where we are, unless you have hundreds of acres, when it seems ok to put up countless barns for animals and feed storage. We have two farms adjoining each other, one 8 acres, the other 14. Even a mobile field shelter seems to be frowned upon, and it has to be sited well back from a road, away from neighbours, you have to put on the plans how you intend getting rid of foul water/slurry. Like I say, it may be ok where you are. But from past experience, its better to ask, and work with the planning. If you go ahead, and then have to get retrospective planning to keep the building, they can refuse it, and make you take it down.
Nearby residents, and also the highways department can comment - if it affects traffic in any way, or is over developed for the area etc.