Hi a friend of mine is also seeking permission to build beside her parents house. She has tried several times before and been shot down, she needed to do percolation tests, get the land signed over to her from her dad, and have a separate entrance made. She also had to put in separate sewage treatment - this may not apply to you. She is building a 2700 sq ft 4 bed dormer so the size of the house will also impact the planners decision.
Stick in a planning appl for outline planning asap and they will come to inspect the site, you will most likely fail on a number of grounds but this will give you an idea of some (but not necessarily all) of their requirments. Talk to an engineer too then before making the second submission, or even before your first application.
One of the considerations is that in say years to come, if your parents house was sold, would your property be completely separate or if you wanted to sell could you without issue?
A couple of neighbours of my parents have built rather large granny flats in their parents back gardens - 2 bed decent little bungalows so not sure what applies here, with one shared entrance, but it seems to be a little more lenient. Their kids are using them as starter homes and I guess that the idea is that they may swop with the parents in a few years time.
Might be worth considering.
Hth