In California, there is no such thing as a non-refundable deposit. However, an owner who has taken the unit off the market and held it for the prospective resident (presumably turning away other applicants), can deduct a reasonable amount from the deposit to cover costs of keeping the unit vacant (usually in the form of a daily rental charge) or costs associated with advertising stops and starts.
Caution should be used when deducting from holding deposits, however, since the potential amount withheld must be stated up front, and if challenged, the onus falls on the owner to prove deductions are reasonable. If owners choose to take holding deposits, the transaction should be put in writing (Form CA-017, Holding Deposit Agreement).