Bus. & Prof. Code, § 21713.5

(a) The owner of a self-service storage facility may assess a reasonable late payment fee if an occupant does not pay the entire amount of the rental fee specified in the rental agreement, subject to the following requirements:

(1) No late payment fee shall be assessed unless the rental fee remains unpaid for at least 10 days after the date specified in the rental agreement for payment of the rental fee.

(2) The amount of the late payment fee shall be specified in the occupant’s rental agreement.

(3) Only one late payment fee shall be assessed for each rental fee payment that is not paid on the date specified in the rental agreement.

(b) For purposes of this section, a “reasonable late payment fee” is one that does not exceed the following:

(1) Ten dollars ($10), if the rental agreement provides for monthly rent of sixty dollars ($60) or less.

(2) Fifteen dollars ($15), if the rental agreement provides for monthly rent greater than sixty dollars ($60), but less than one hundred dollars ($100).

(3) Twenty dollars ($20) or 15 percent of the monthly rental fee, whichever is greater, if the rental agreement provides for monthly rent of one hundred dollars ($100) or more.