This question is piggybacking on this old post because there doesn't seem to be a question for the undergraduate level.
Should the aim of the undergraduate thesis be different than the PhD thesis? Specifically, should it be more accessible to the average reader? At the end of the day, I share the same concerns as the PhD student from the other question:
I don't want to look lazy by not including material, but at the same time I don't want to waffle on.
My rule of thumb -- whether for Bachelor, Master, or PhD theses -- is to assume precisely the level of knowledge the student had when starting to work on this topic. (I.e., "How would you like to have had this explained to you?")