I think this depends on the area you live in, different local authorities have different policies. I do feel sure you will need planning permission, neighbours can get quite sniffy about caravans so I wouldn't try it without at least asking the planning dept.
If you can show that you need to be there for a good deal of your time and especially if the location is some distance from home you might just get it approved. It might help if the location is sheltered i.e. hidden from view. Security might also be a consideration, if horses in the locality have been stolen or attacked then they are more likely to allow an application. Finally you might consider getting the backing of your councillor on the basis of - tain't what you know it's who you know.