It ultimately depends on your schools policy. State run schools (UC & CSU system) require you to have a permanent residence continuously for a year with the intent to make California your permanent residence. Thus, if you are from out of state and move to California to attend a state run school, you do not necessarily become a resident after your first year since you were in the state for the sole purpose of attending school. How do you show intent to permanently remain? The key factor is financial independence. If your parents are paying for your living expenses, this does not demonstrate an intent to permanently remain. Of course, you don’t have to actually have money to pay expenses. You can get student loans in your name, which will indicate financial independence. Also, employment in California will help show financial independence. But note: living in dorms will normally not count as a permanent residence. Many dorms on California campuses kick you out during school breaks (with the logic that you will return to your parent’s home). In other words dorms are temporary housing for school, and therefore cannot be considered a permanent residence. On the other hand, school run apartments could count for residency as long as you have a standard leasehold estate. Non state schools may have different requirements, but usually are the same. Whether or not you have in-state tuition ultimately depends on what you can demonstrate to the school. If you live off-campus for a year and pay for school with personal loans or by actually working, you will qualify for instate tuition the following year. In fact, even if your parents are planning on paying for school, you still may want to pull loans out in your own name in order to help establish residency requirements.

You asked: "Would you think a regular apartment (say living with someone as a roommate even) would count as financial independence?" It would certainly count as a permanent residence. Financial independence is more about who is paying for it. When you fill out a petition for in-state tuition, you will be asked for all of your sources of income. Even if your parents give you some money, so long as you have enough independent income to cover your expenses you will qualify. However, there is not magic number or ratio that defines “financial independence.” Remember, the purpose is to show that you intend to remain in the state, and are NOT just there for school. Financial independence is most often how people prove this, but not always. For example, if you have already accepted a job offer in California that starts in the future, this could show an intent to remain. Your safest bet would be to completely pay for your first year of school with loan money, income from work, or other personal savings. Your parents could pay off all those loans at the end of the year, which would not matter. As for having a roommate, it does not affect residency. In fact, the school will never know if you had a roommate or not.

Come into the country illegally.