12 C.F.R. § 160.33
A Federal savings association may include in a home loan contract a provision authorizing the imposition of a late charge with respect to the payment of any delinquent periodic payment. With respect to any loan made after July 31, 1976, on the security of a home occupied or to be occupied by the borrower, no late charge, regardless of form, shall be assessed or collected by a Federal savings association, unless any billing, coupon, or notice the Federal savings association may provide regarding installment payments due on the loan discloses the date after which the charge may be assessed. A Federal savings association may not impose a late charge more than one time for late payment of the same installment, and any installment payment made by the borrower shall be applied to the longest outstanding installment due. A Federal savings association shall not assess a late charge as to any payment received by it within fifteen days after the due date of such payment. No form of such late charge permitted by this paragraph shall be considered as interest to the Federal savings association and the Federal savings association shall not deduct late charges from the regular periodic installment payments on the loan, but must collect them as such from the borrower.
12 C.F.R. §160.33