Late fees on invoices in California are enforceable only when the fee is a reasonable estimate of what a late payment actually costs the business, not a penalty designed to punish the customer. Civil Code Section 1671 sets that standard, and it applies whether the fee is a flat dollar amount or a percentage of the balance.1Civil Code Section 1671 If a contract is silent on late fees or interest, an unpaid invoice accrues interest at 10% per year by default under Civil Code Section 3289.2Civil Code Section 3289 The rules tighten further for consumer transactions, where the business has to affirmatively justify the fee rather than simply collect it.
The Reasonableness Test
California has no statute that authorizes a specific late fee percentage. Section 1671 treats any pre-set charge for breach as liquidated damages, and the test is whether the amount was a reasonable estimate of probable harm at the time the contract was signed. Costs that can support a late fee include the administrative time spent chasing payment, the lost use of the money, and additional bookkeeping or collection expenses.
The size of the invoice matters. A $25 fee on a $500 invoice looks very different from a $25 fee on a $50,000 invoice, and a flat fee that produces an enormous effective rate on a small balance is the kind of thing courts strike down. Percentage-based fees pegged to the overdue amount tend to survive scrutiny better because they scale with the harm.
Many California businesses use 1.5% per month, or 18% annualized, on overdue balances. That rate is widely used in commercial practice, but common use is not a legal safe harbor. If the fee is challenged, the business still has to show the rate bears a reasonable relationship to actual losses.
Consumer Versus Commercial Transactions
Section 1671 flips the burden of proof depending on who the customer is, and this is the single most important distinction in the law.
In a business-to-business contract, the late fee clause is presumed valid. The party challenging it has to prove the amount was unreasonable when the contract was signed. That gives commercial parties considerable freedom to negotiate late fee terms, as long as the number isn’t wildly disproportionate to any plausible estimate of harm.
In a consumer transaction (goods or services bought for personal, family, or household use), the presumption reverses. The fee is presumed invalid. The business collecting it has to prove two things: that calculating actual damages from late payment would have been extremely difficult at the time the contract was made, and that the fee was a reasonable estimate of those hard-to-calculate damages. Failing either prong makes the fee unenforceable.
In practice, that means a 1.5% monthly fee between two companies is unlikely to be successfully challenged, while the same fee on a residential customer faces a much higher bar.
When the Contract Says Nothing
Silence doesn’t leave the creditor with nothing. Under Civil Code Section 3289(b), a contract entered into after January 1, 1986, that does not specify an interest rate bears interest at 10% per year once the payment obligation is breached.2Civil Code Section 3289 No special invoice language is needed. That 10% is a floor, not a ceiling: a contract can specify a different rate under Section 3289(a), but any specified rate still has to survive Section 1671 and California’s usury limits.
The 10% Usury Ceiling
Article XV of the California Constitution caps interest at 10% per year on loans and forbearances of money. Late fees that push the effective annual carrying cost above that number can trigger usury problems, especially in consumer contexts.
Banks, savings associations, credit unions, and most licensed lenders are exempt, and many transactions between sophisticated commercial parties fall outside the cap as well. A small business invoicing a consumer, or a landlord billing a tenant, does not have those exemptions. If the late fee plus any stated interest produces an effective annual rate above 10%, the interest charge can be voided as usurious. Businesses that aren’t exempt lenders should work out what their late fee translates to on an annualized basis, particularly for smaller invoices where a flat fee can produce a surprisingly high effective rate.
What the Contract and Invoice Have to Say
The late fee has to be agreed to before the obligation arises. A business cannot send an invoice, wait for the payment to run late, and then add a fee that was never part of the deal. Tacking on undisclosed charges risks being treated as an unfair business practice under the Unfair Competition Law, Business and Professions Code Section 17200.3Business and Professions Code Section 17200
Civil Code Section 1657 fills in a related gap: when a contract doesn’t specify a time for performance, a reasonable time is allowed, and a pure obligation to pay money is due immediately once the amount is determined. If your invoice doesn’t state a due date, a court won’t assume your preferred 30-day window. Spell out the terms.
For consumer transactions, the California Consumer Legal Remedies Act adds another layer. Charging fees that weren’t clearly disclosed before the transaction was finalized can be treated as a deceptive practice. The workable approach is to put the late fee policy in the original contract or service agreement, reference it on every invoice, and specify three things: when the fee starts, how it’s calculated, and what balance it applies to.
Compounding, Stacking, and Mid-Stream Changes
Charging a late fee on top of a previously unpaid late fee is risky. Unless the contract explicitly authorizes compound interest and California law permits it for that type of transaction, courts generally expect late fees to apply only to the original invoice amount. Stacking fees on fees can push the total charge into territory a court will read as a penalty, which is exactly what Section 1671 prohibits.
Raising a late fee rate or adding a new late fee provision to an existing account requires proper notice and mutual assent. For accounts subject to electronic fund transfer rules, federal Regulation E requires at least 21 days’ written notice before a fee increase takes effect. Even outside that federal rule, California contract law generally requires agreement to material changes. Slipping a new fee onto an invoice without advance notice is a reliable way to make it unenforceable.
What It Costs to Get This Wrong
An unenforceable late fee doesn’t just get struck down. The Unfair Competition Law authorizes civil penalties of up to $2,500 per violation in actions brought by the Attorney General, district attorneys, or certain city attorneys.3Business and Professions Code Section 17200 A business that systematically overcharges across hundreds of invoices faces exposure that multiplies quickly.
Courts can also order restitution, requiring the business to refund every unlawful late fee collected. The Attorney General’s office investigates unfair billing practices, and consumer complaints can trigger cease-and-desist orders and mandatory corrective measures. Widespread harm can produce class-action exposure with mandatory refunds and injunctive relief.
Two Cases That Anchor the Rule
In Garrett v. Coast and Southern Federal Savings (1973), the California Supreme Court voided a late charge on a consumer loan because the lender had made no reasonable effort to estimate what a late payment would actually cost. The court called the charge “punitive in character” and designed to coerce timely payment rather than compensate for real losses. Garrett predates the 1977 amendments to Section 1671, but the core principle still governs: the fee has to reflect a genuine attempt to estimate actual harm.
Twenty-five years later, in Ridgley v. Topa Thrift and Loan Association (1998), the Supreme Court applied the same principle under the updated statute, striking down a fee-retention provision in a loan agreement as an unenforceable penalty. The court rejected the argument that recasting the charge as part of loan pricing should shield it from scrutiny. Courts look past the label to see whether the charge actually functions as a penalty.
How Long You Have to Sue or Seek a Refund
Under Code of Civil Procedure Section 337, the statute of limitations on a written contract in California is four years from the date of breach.4Code of Civil Procedure Section 337 For an oral contract, the window is two years. The clock starts on the date payment was due, not on the date the business first tried to collect. Once it runs out, the right to sue for the unpaid amount, including any late fees, is gone.
A customer who paid an unlawful late fee and wants it back faces a limit too. Claims under the Unfair Competition Law generally must be brought within four years. Waiting too long means losing the ability to recover the charge even if it was clearly unenforceable.
Resolving a Late Fee Dispute
Most invoice disputes end short of a courtroom. Mediation, often available through county bar associations and dispute resolution programs, lets both sides negotiate with a neutral third party who helps but doesn’t decide. If negotiation fails, small claims court handles most invoice disputes: individuals can file claims up to $12,500, businesses up to $6,250, and the process is designed for people without lawyers.
Some contracts include arbitration clauses that require disputes to go outside court. California enforces these when they were clearly disclosed and mutually agreed upon, but courts have struck down provisions buried in fine print or heavily tilted toward one side, particularly in consumer contracts. An arbitration clause added after a dispute has already started is almost certainly unenforceable.
Disputes above the small claims limits move to superior court, where formal litigation rules apply and both sides typically need attorneys. That path is worth pursuing mainly when the amount at stake justifies it or when the case raises broader issues, such as a pattern of unlawful late fees across many customers.