Where the proof comes from
Proving a unit is unpermitted usually comes down to public records and the unit's physical characteristics, combined with your rental documents. You often do not have to prove it alone.
A key source is the city or county building department, which keeps permit and property records showing what was legally approved for the property. If the records show no permit for the unit you are renting — for example, a garage or basement that was never approved as a living space — that supports the unit being illegal. A code-enforcement inspection can also formally document violations.
Physical clues help too: makeshift kitchens or bathrooms, no proper second exit, low ceilings, unpermitted-looking wiring, or a unit that clearly was not built as a home. Keep your lease, rent receipts, photos of the unit, and any communications where the landlord discusses the unit's status.
You generally do not need to become an expert — officials and, if needed, a lawyer can help establish the facts. If you suspect your unit is illegal and want to understand what it means for your rights, a free, confidential review can walk you through it.
This is general information about California tenant rights, not legal advice. Every situation is different.