At Berkeley, this depends on the major. If you're accepted to the College of Letters & Sciences (which houses degrees such as Physics, CS, Math) with an intended major, you're actually undeclared when you start out.
Once you complete your lower division prerequisites passably (typically after two or so years), you can then declare your major as a CS degree.
This isn't the case for College of Engineering which houses the EECS program (while sharing the same department as CS, confusing I know), wherein admissions are more selective, but you don't need to declare your major. You also have the advantage of already being in the program, which helps with registering for courses (especially upper-division courses, which typically only have a small number of slots reserved for non-majors).
Once you complete your lower division prerequisites passably (typically after two or so years), you can then declare your major as a CS degree.
This isn't the case for College of Engineering which houses the EECS program (while sharing the same department as CS, confusing I know), wherein admissions are more selective, but you don't need to declare your major. You also have the advantage of already being in the program, which helps with registering for courses (especially upper-division courses, which typically only have a small number of slots reserved for non-majors).