NOTE: 'besides' means 'in addition to', 'also', 'on top of that, or 'apart from' and is different from 'beside'. 'beside' has a physical connotation and would indicate that something is next to something else or on the side of something, e.g. 'at the side of', 'next to'.
About your question: 'outside' refers to that part that doesn't belong to what is considered to be inside. Outside the house, there is a road. Here the road could be next to the house without any space in between or with space in between (physically); the road could be beside the house, in front of the house, behind ...
I would recommend you to look such things up in e.g. Cambridge Advanced Learners Dictionary:
1 at the side of, next to:
Come and sit here beside me.
Our school was built right beside a river.
2 compared to another person or thing:
Those books seem rather dull beside this one.
beside the point
not important or not related to the subject being discussed:
The fact that he doesn't want to come is beside the point - he should have been invited.