Assuming you live in England or Wales (the law is different in Scotland and I know diddly squat about it), I'm pretty certain that you will not require planning permission if you merely want to change the use of your outside loo. If, however, you want to extend it, then that is a different kettle of fish and you may well need to seek planning permission if, for instance, you have exhausted your permitted development rights (i.e. you've extended your property previously) and/or the proposed extension is a certain distance away from your property or a highway. This is a general guide though, as different councils have different rules. It would be a good idea to have a word with your local planning office for advice.