Massachusetts General Laws Part II. Real and Personal Property and Domestic Relations (Ch. 183-210) Ch. 183, § 59

A mortgagee, assignee or holder of a mortgage note secured by a first or subordinate lien on a dwelling house of 4 or less separate households or on a residential condominium unit occupied or to be occupied in whole or in part by the mortgagor shall not require the mortgagor to pay a late charge or late payment penalty unless the penalty is specifically authorized in the loan documents.

A mortgagee, assignee or holder of a mortgage note secured by a first or subordinate lien on a dwelling house of 4 or less separate households or on a residential condominium unit occupied or to be occupied in whole or in part by the mortgagor shall not require the mortgagor to pay a penalty or late charge for any payment paid within 15 days or in the case of a bi-weekly mortgage payment, paid within 10 days, from the date the payment is due.

In no event, in assessing a penalty because of the delinquency in making all or any part of a periodic payment under a mortgage note, shall the penalty or late charge exceed 3 per cent of the amount of principal and interest overdue, and in calculating the penalty or late charge, any amount of the periodic payment representing estimated tax payments required by the terms of the mortgage note or deed shall not be included.

A late payment penalty or late charge may not be charged more than once with respect to a single late payment. If a late payment fee is deducted from a payment made on the loan, and the deduction causes a subsequent default on a subsequent payment, no late payment fee may be imposed for the default. If a late payment fee has been once imposed with respect to a particular late payment, a fee shall not be imposed with respect to any future payment which would have been timely and sufficient, but for the previous default.