what are the chances of getting planning permission in my back garden

Hi, I was wondering if anyone might be able to help me out with this one. I am currently considering building a log house in my back garden for my son and his fiancé to live in. This structure would be one story and about 60 / 70 m2 , We have a rather large garden and this structure would not be likely to infringe on any neighbours land or view. I have heard that planning permission may be difficult to come by considering the only access for emergency services would be through the house, or over a back wall. To people in the know this may seem like a silly question but I really havent got a clue so any help at all would be great.

I really dont think there would be any complaints from neighbours as most have built something over the years, my next door neighbours have built on a 2 storey “granny flat” and also a stand alone house to the side of their semi detached.

Thanks