Where the line usually falls
A serious habitability problem affects your health or safety — like no heat, a sewage backup, or dangerous wiring — while a minor problem is mostly cosmetic, like a small scuff on the wall or a slow-draining sink. California habitability law focuses on the serious kind.
Serious problems usually involve essential systems (water, heat, electrical, structure), health hazards (mold, pests, sewage), or safety (broken locks, unsafe stairs). These are the conditions that can make a home legally unfit to live in. Minor issues are still worth reporting in writing, but on their own they generally do not rise to a legal violation.
There is a gray area, and it often comes down to how bad the condition is, how long it lasts, and whether the landlord responds after you give notice. Several "small" problems together can also add up to a serious one. If you are not sure which side of the line your situation falls on, we are glad to take a look.
This is general information about California tenant rights, not legal advice. Every situation is different.