I need advice please!!!!

I am almost 18 and living with my parents but plan on moving out pretty soon after I turn 18. I rescued a rabbit and haven't told my mother. My father knows and is ok with it but she will not be. I didn't ask her permission because she just deflects whenever I ask about a pet. I have a good job, enough money saved up for medical visits and have personally bought all the supplies and plan to continue fully paying for it myself. I do all the cleaning in the house so she wouldn't be cleaning up after it. She has been gone for a few months and I have fully litterbox-trained the rabbit and acclimated her to my room and I am not willing to give her up. How do I tell her when she gets home? I don't want to cause a big argument. How do I start and handle this conversation?

I recommend, when your Mum gets home, first step is to stay calm don't be nervous, sit your Mum down and tell her that you have something to tell her and that you hope she'll save her thoughts and questions until the end. This way hopefully you'll have time to explain everything. Explain to her that you'll take care of the rabbit and pay for everything, and that you don't expect any help from her. From the sounds of it you already have one parent on your side, before your Mum gets home talk to your Dad, ask him for some help with talking to your Mum.
I do want to ask is there a reason for adopting already? Since your still living with your parents, and you know that your Mum doesn't want a pet. And while you might not like to hear it, it is your parents' house, what they say goes. If after talking to your Mum she doesn't agree to keep the rabbit you'll have to either give it to a relative/friend or give it up. (I'm not saying this to discourage you, I just want you to keep this in mind and to remember no matter the outcome, you have to respect your Mums' wishes, even if they upset you.)
Hope all goes well! I know how hard it is to give up a pet after getting attached.

thank you for the advice, the thing is she has never said no to a pet she just says the will think about it and never responds. she loves animals and we have a dog already.