Legal Question in Real Estate Law in California

This is a tough one. Normally, when two or more people become cotenants on a lease or rental agreement, there is no legal provision or right regarding excluding X from Y's room. Both X and Y, generally at least, have an equal right to possess, occupy and use the entire rented unit.

However, beyond that, so-called "roommates" (who aren't really roommates at all; they are perhaps more correctly called "apartment mates" or something like that) usually have some kind of agreement or understanding, generally oral but perhaps occasionally written, that modifies the basic condition that each has full run of the place. Typically, there'll be an understanding that this is my bedroom, that is your bedroom, and we'll share the kitchen and den -- or something of this ilk.

Such understandings would probably be enforced in court, if their existence and terms could be proven to the satisfaction of the judge or jury. Proving the existence and terms of an oral agreement regarding private bedrooms would generally be pretty easy. However, proving that there was any understanding regarding something as abstract as access to a circuit breaker could be very difficult. I'd suggest that the two of you try to work out something to "de-fuse" what appears to be mistrust between the two of you. It's pretty rare for roommates to feel the necessity to put locks on their doors -- maybe from the inside, but seldom on the outside.