I don't have the time or inclination to go state by state. However, I found this on the NOLO website. From what I could see any late fee has to be outlined in the rental agreement. If it is not there, the landlord cannot legally charge one.
"Limits on Late Rent Fees
Laws in a few states restrict the imposition of late fees, both by amount, and whether the landlord must wait until you're a certain number of days late before he imposes them. See your state rent rules for details (and you're local rent control ordinance, if your rental is covered by rent control or regulation).
Most states, however, do not put dollar limits on late fees. Does this mean your landlord can charge whatever he wants? No. Under general legal principles, your landlord may not charge an unreasonably high late rent fee.
Here are some guidelines for judging what’s unreasonable:
The fee shouldn’t begin immediately. Normally, a late fee should not apply until at least three days after the rent due date.
The fee should be within a certain percentage of your rent. Your landlord is always on shaky ground if the late charge exceeds 5% of the rent. That’s $38 on a $750-per-month rental. Of course, if the rent is extremely late—say, ten days—a higher late fee, such as 10% of the rent, might be reasonable.
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