District of Columbia late fee law

6%per year

Attaches to the debt by operation of law when you agreed no rate, so it accrues before any lawsuit and can be stated on a demand letter.

"The rate of interest in the District upon the loan or forbearance of money, goods, or things in action in the absence of expressed contract, is 6% per annum." Verified on the official D.C. Council code site. (Judgment interest on non-District judgments is separately set at 70% of the IRS underpayment rate, § 28-3302(c).)

Parties to a written instrument may contract for interest not exceeding 24% per annum (D.C. Code § 28-3301); contracting for more forfeits the interest (§§ 28-3303 to 28-3305). Exemptions exist, including federally insured banks/savings institutions (§ 28-3308) and certain regulated loan classes; no general blanket commercial/corporate-borrower exemption was verified in this research - verify with counsel for large commercial loans.

public+private-construction

Quick Payment Act of 1984: District agencies must pay proper invoices within 30 calendar days (7 days for meat, 10 for perishable agricultural commodities) and owe an interest penalty on late payments at a rate set by Mayoral regulation of not less than 1% (D.C. Code § 2-221.01 et seq.). Private Contractor and Subcontractor Prompt Payment Act of 2013 (D.C. Code §§ 27-131 to 27-136): private construction owners must pay contractors on statutory timelines and pay interest of 1.5% per month on undisputed late amounts; same rate applies down the subcontract chain.

No DC statute specifically governing flat late fees on commercial invoices was found; agreed late charges are generally enforceable subject to the liquidated-damages/penalty doctrine, and interest-type charges on written contracts should stay within the 24% ceiling of § 28-3301.