You definetly need planning and it is very hard to get it.
Speak to your local planning department first if you do find a piece of land. Enquire whether they think it is likely to get planning for what you want to do.
Most offer a free service, or if not a small fee to pay for their advice.
Most "fields" are countryside so probably tell you unlikely, as contadicts the councils planning polices, but best to ask first rather than speding thousands on a field.
Also sometimes you can offer on a field but subject to planning permission, so don't exchange till then, bearing in mind may take you months even years to get permission, if you do even.