Will my late fee survive?
California sets no ceiling on a residential late fee, so nobody can tell you yours is under the limit. There is no limit. The test is whether the fee is a genuine estimate of what a late payment costs you, written into the lease before the tenant was ever late.
This shows you what yours works out to as a share of the rent, which is the shape courts look at hardest, and the one notice it must never appear in.
How much can I charge as a late rent fee?
A late fee is a charge added when rent arrives after the day it was due. California sets no cap on the amount. What it sets is a test: a late fee has to be a fair advance estimate of what a late payment actually costs you to deal with, rather than a punishment, and the lease has to say the fee exists before you can charge one at all. This tool covers the City of Los Angeles RSO, unincorporated Los Angeles County, Santa Monica, West Hollywood, Pasadena and AB 1482. It shows what your fee works out to against the rent, the test it has to pass, and the notice it must stay out of.