In California, any late fee must be detailed in the lease agreement and the amount of the late fee must be reasonable. There is no statute or case law that defines a line between reasonable and unreasonable, but you should expect courts to look skeptically at any late fee that appears in any way punitive as opposed to being intended to reasonably approximate actual costs and losses associated with the late payment of rent. Also, as adjusterjack indicates, when a landlord does not enforce a lease provision for three full years of tenancy, a court is very likely to deem the provision to have been waived by the landlord.

