@cal In our home with our 1w year old daughter.. we set expectations and make them clear.
You can live here BUT its expected that you will contribute somthing. You can have your space BUT its expected that you will maintain relationships within our family (aka.. no hiding out in your room for hours on end..coming home and grabbing food and shutting your bedroom door ect). The expectation is that she is responsible and will contribute toward the functioning of our household.
What does this look like?
Chores. She gets (clean room, make bed, feed dogs 1x per day, take dogs pee 1x per day, make lunch for school). If she completes her chores, she can expect to receive an allowance on every Friday.
If she makes a mess in the kitchen-she is expected to clean it up.
If she wants clean clothes- she is expected to do her laundry.
If she wants to hang out with friends. She is expected to be home by curfew and inform us of where she is going to be.
If she pulls an "FU" face or she whines or complains or argues. I respond casually with " if you decide you want to behave this way, then I will have to remove x privilege.". This is a choice you can make for yourself. Do you want X or X?
One thing i have learned is this...
Kids need structure. Teens need structure. They may not ask for it, but they need it. Especially if they feel out of control. Girls tend to feel more out of control because of the PMS mood swings and hormones. At this age, they are 12 going on 20. They want their complete independence but lack the maturity and understanding of responsibility. Without structure.. they feel more out of control.. being out of control leads to behaviours like self-harming because its somthing they have control over themselves.
Giving them an opportunity to have control over their own life by phrasing things as a choice, empowers them to take control and develop independence. You also take away that element of you being their jailer. The tendency is for the teen to blame the parent for the "thing you took away". They are less likely to view it that way, if you casually talk to them about how its their choice that determines the outcome- positive or negative.
The second thing to realize is that somtimes these behaviors(belligerence and self harm) ae attention seeking. They are for the "shock factor". How can i shock my parents or get them to react? Our daughter kept cutting herself superficially and the counsellor at the school kept calling us to report this and so she came home and I said to her "hey can i see your wrist? I heard from the counsellor that you are cutting yourself and i havent seen it yet..could i see your wrist? And then i promise i wont ask about it again ok?". So she shows me her wrist and i say "okay. Thanks for showing me" and i carry on as if i hadnt seen it at all. I dont treat her any different. I had a meeting at the school and asked her counsellors at school to try and reduce the amount of attention or reaction they give to her when she shows them.
Sure enough a week later..she stopped. But why? Simple. She wasnt getting the reactiom she wanted or attentiom she wanted from it.
Some may say "oh well your kid will find somthomg else to do then". Maybe she will and maybe she wont. Kids push boundaries and want to know just how far they can push and just how we will react. If we stop feeding them the reactiom they want.. they will stop doing it because its no longer fun for them. They are losing the game.