Back in the old days when I was 17 I moved out of home into my own rental place.

I didn't share, I was dealing with the difficult fall-out of a dreadful family situation and desperately wanted my own space.
I ended up renting a granny flat - best decision ever! Not too $$ but I had my own space and no more flatmate dramas. If you rent privately (like a granny flat) you'd have even more leeway. The important thing would be to have good refs and be able to prove you can afford it. If a landlord is going to be ultra-picky, they probably aren't worth renting from anyway.

Moving out at 17 was a great decision for me, not just because of my family issues, but in terms of giving me space to grow, managing my own life and really becoming an adult. If she knows you are there to support her I think she's got a great safety net and she'll probably do much better. GL!
I was in a similar situation, although i was kicked out of home without a reason found my belonging in garbage bags on my parents front porch and was told by my own Mother i was to never under any circumstances to come home, not even for birthdays, Xmas ... nothing !! So my parents last saw me as teenager then the next time they saw me i was a 30years old grown woman. Very sad for them missing out on me growing up but worse that anyone's parents could abandon their own child so Fiona i couldn't speak highly enough of the way you are dealing with this, in sharing such compassion & support ... it's truely wonderful. And as Jennifer suggested why not a granny flat as that's what i first moved into, it was furnished even right down to pots & pans and rent was cheap (paid cash), they had advertised in a local paper. I felt safe living on the property of a lovely family, i had my own personal space but never felt lonely which you shouldn't feel as a teenager !!!