California law doesn't make it easy to evict a roommate. In some cases it may be impossible. The exact rules depend on whether you're dealing with a co-tenant, a subtenant or a guest.
If your roommate co-signed the lease with you, she's a co-tenant, with the same right to stay there that you have. If her name isn't on the lease but she pays rent directly to the landlord, she's probably still a co-tenant.
You have no right to evict a co-tenant. Only the landlord can do that, and even he can't remove her without cause. For example, if she's not paying the rent or she's damaging the apartment, you can notify the landlord or property manager and ask him to step in. However, your roommate may be able to stave off eviction by fixing the problem — making up the back rent, for instance.
The exception is if your roommate is actually using or threatening violence. You can ask a court for a restraining order, including a residence exclusion order requiring the roommate to move out.
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In California, a subtenant is a roommate who pays rent directly to you, not your landlord. For example, if you find you need help affording your apartment, you can sublet to a new roommate. You may need your landlord's approval for this.
In a sublease, you're the landlord and the roommate is your tenant. Under California law, you can evict him, but you have to follow the same procedures as your landlord would with you. Harassing your subtenant, changing the locks or otherwise trying to force him out is illegal.
You must give your subtenant advance warning before evicting her. If you simply decide not to renew a month-to-month sublease agreement, she gets 30 or 60 days' notice. If she's late with the rent, damages the apartment or breaks the terms of the agreement, you must still give her three days' warning. Like a landlord, you can't go through with the eviction if she fixes the problem.
Going to Court
Notification doesn't do the trick by itself. Your subtenant may choose to stay after the deadline without fixing the problem. To remove her, you'll have to go to court and file an unlawful detainer lawsuit. If you win, you can ask the county sheriff to forcibly evict her.
Living With Someone
If you simply invited someone to live with you — your new boyfriend, or a family member, say — and didn't sign an agreement or put him on the lease, he has no legal right to stay if you don't want him. Hopefully he'll up and leave when you ask. If not, you or your landlord may be able to remove him as a trespasser.