Bachelor's thesis can i use wordings from my supervisor I am in the middle of writing my bachelor thesis and recently submitted a paper that I wrote with my supervisor about the same subject to a conference. If it makes any difference, I should mention that I am the first author. 
I would like to use the paper (15 pages) as the basis for my thesis (about 40 pages), i.e. I want to re-use most of its content without changing the wordings. 

My supervisor told me that it is okay and I have read that it is even common practice among some institutions to reuse your own previous work for a bachelor, master or PhD thesis.

However, I am not quite sure if it is acceptable to reuse paragraphs which my supervisor wrote as co-author to that paper. In the Statutory Declaration that has to be attached to the thesis it is stated that I wrote the thesis just by myself with only the help of the cited material. Do I have to reformulate the ideas in this case?
Do you have a supervisor for the BA thesis? If so, that's the person to talk to. Assuming you get to choose a supervisor, it'd be sensible for you to pick the person you co-authored the paper with. Presumably, they will have no objection to using the text in full for your thesis (including the parts they wrote). 

There's no issue with plagiarism, unless your institution has an unusual interpretation thereof. The point of rules against plagiarism isn't to make you pointlessly rewrite your previous work and in any case, co-authored papers don't allow for distinctions between the paragraphs you wrote or the ones someone else wrote, even if you personally can tell them apart.

It's also expected that a supervisor provided input to the thesis and (at least in the US) it's not unusual for that to include substantial editing or clarification. Especially if the thesis is good enough to be published down the road: might as well get started on the process early. You address this by thanking your supervisor in an acknowledgment section, along with other people who read your thesis or provided ideas.

Taking plagiarism rules literally would lead to absurdities like citing "personal communication" over coffee or drinks. If someone provides a really cool idea and you use it, it's polite to thank them in a footnote; but that's the extent of it. The point of the rules is for you not to go take an idea from someone else's paper, then claim it as your own insight.