In most cases, garden rooms do not usually require planning permission as they fall within permitted development rules. This is a 'pre-approved planning permission', as long as the garden room meets certain criteria, including that it should be no higher than 2.5m from the floor to top of roof if within 2m of a boundary, that you do not take up more than half of your garden, and that it's not to be used for living or sleeping in.
There are a few other things to consider, but Harrogate Garden Rooms can talk you through the dos and do nots during your design consultation.