You only need to apply for planning permision if you are making permanent structural changes to the field ie-: constructing a hard base or hard surface runway (obviously not necessary in your case) or if you put up a hangar for the sole use of aircraft only. But assuming the field in question is part of a farm, then you can construct a permanent hay shed/machine store which can then double as a hangar
. The rational behind that is the land does not change and the field can easily revert to farming again if and when flying ceases.
Very good advice from BDoyle and Stovepipe about the neighbours. Make sure you let them know exactly what your planning for the field. Most non aviation people will think a grass strip WILL
become an international airport in a couple of years, if it's not stopped. And good use of the spare seat will go a long way.