Here is the UC campus alternate major summary:
UC Berkeley:
If interested in applying to a High Demand major in the College of Letters and Sciences, that major needs to be your 1st choice. Alternate majors will be considered in there is room available usually during the waitlist process. UC Berkeley only guarantees review of your primary major.
UC Davis:
Applicants are encouraged to list an alternate major, but not in the
same area as the primary major (e.g., Computer Science Engineering with alternate Computer Science in the College of L&S). Occasionally, applicants are admitted to the alternate major.
UC Irvine:
In the case that UCI is unable to accommodate all qualified applicants in their first-choice major, those students who indicate a valid alternate major may be offered admission in that major or Undeclared. Nursing and Business Administration would not be considered as alternate majors.
UCLA:
UCLA only guarantees review of an applicant’s first-choice major.
UC San Diego.
Alternate majors are considered and selective majors are highly competitive. Also note: Selective majors require additional pre-req courses and specific GPA to be able to qualify if changing majors. Also if applying to a capped/selective major, it is recommended to select a non-capped major as an alternate.
UC Santa Barbara:
College of Engineering: Students are selected by major for all engineering and computer science majors. Only applicants with a solid background in advanced high school mathematics will be considered for admission to engineering. This includes high grades in all math courses through grade 11 and enrollment in pre-calculus or higher in grade 12. A student not selected for their first choice major will be reviewed for admission to an alternate major outside of the College of Engineering if one was selected (College of Letters and Sciences). CCS applicants are also encouraged to list an alternate major in the College of Letters and Sciences.
UC Santa Cruz:
Important Note for Prospective Engineering Students: Choice of major does not influence the selection of first-year students, except for those applicants interested in a major offered by the Jack Baskin School of Engineering (BSOE). Freshmen who are interested in a BSOE program should be sure to indicate a BSOE proposed major. Students who do not indicate a BSOE program or who apply as undeclared might not be able to pursue a BSOE program.
UCR and UCM:
Admission by major but alternate/2nd choice major will be considered if applicant does not need their first choice admission standards.