All owners and income boosters are credit checked. A fully submitted mortgage application will leave a hard credit footprint.
Any individuals who are associated with the owner(s) through continuing linked finances, marriage, or civil partnerships should be added to the application. If the owner(s) prefers for these persons to not be included, they'll need to get independent legal advice. Proceeding in this arrangement is subject to underwriter discrection.
Each customer is subject to the same checks, whether contributing to the mortgage or not.