Can I Add Interest to a Demand Letter? California Interest and Fee Rules

Can you add interest to a demand letter in California?

Yes, when a statute or your contract gives you a right to it, and only at the rate the law allows. The key questions are whether the amount was fixed or easy to calculate, when it became due, and whether your contract names a rate.

Real estate loans secured by a deed of trust fall outside section 3289(b). Which rule applies can itself be disputed, especially between the 10 percent contract rate and the 7 percent constitutional rate. Loans also raise usury questions under article XV of the Constitution, which limits contracted interest rates and exempts certain lenders; if your claim involves interest you charged on a loan, get advice before demanding it.

Example: adding 10 percent interest to an unpaid invoice

Example (hypothetical): a graphic designer's contract with a client, signed in 2025, sets a fee of $8,000 due March 1, 2026, and says nothing about interest. The client never pays. The designer writes a demand on October 1, 2026.

Showing the formula matters more than the total. A recipient can check every number, and a judge can see you claimed only what the statute allows. This illustration uses simple interest; if the contract or facts are unusual, have the method checked.

Can you charge a late fee on an unpaid invoice?

Only if the contract provides one, and only if it is enforceable. A late fee agreed in advance is a form of liquidated damages, governed by Civil Code section 1671.

• Business contracts: a liquidated damages clause is valid unless the party challenging it shows it was unreasonable when the contract was made (1671(b)).
• Consumer contracts and residential leases: an agreed damages figure is void unless, from the nature of the case, fixing the actual damage would be impracticable or extremely difficult (1671(c) and (d)).
• Any contract: damages must be reasonable, and grossly oppressive amounts are not recoverable (Civil Code 3359).

If your contract has no late fee, you cannot add one after the fact. Ask for the statutory interest instead.

Can you add attorney fees to a demand letter?

Only when a contract clause or a statute allows it. California follows the general rule in CCP section 1021: each side pays its own lawyer unless a statute or agreement says otherwise.

• Contract fee clauses. Civil Code 1717 makes a one-sided fee clause work both ways: the party who prevails on the contract can recover reasonable fees, whichever party the clause names. The right cannot be waived in contracts covered by the section.
• Wage claims. Labor Code 218.5 awards fees and costs to the prevailing party in nonpayment-of-wages actions if requested at the start, but an employer recovers them only if the employee sued in bad faith.
• Consumer claims. The Consumers Legal Remedies Act requires fees and costs for a prevailing plaintiff (Civil Code 1780(e)).

A demand letter can state that fees may be sought if a clause or statute applies, and cite it. Do not demand a lump sum for attorney fees you have not incurred. Also note that under 1717(b)(2), there is no prevailing party for fee purposes when a case is voluntarily dismissed or dismissed under a settlement, which affects how a settlement should address fees.

What about court costs and collection fees?

Court costs come later, in a lawsuit. Under CCP 1032(b), a prevailing party is entitled to costs as a matter of right unless a statute says otherwise, and section 1033.5 lists allowable items such as filing fees and service of process.

Collection charges are tightly limited for consumer debts. California's Rosenthal Act bars debt collectors from collecting their own fees or collection expenses from the debtor unless the law allows it (Civil Code 1788.14(b)), and the federal Fair Debt Collection Practices Act bars collecting any amount not expressly authorized by the agreement or permitted by law (15 U.S.C. 1692f(1)).

How to build the amount in your demand, step by step

1. Start with the principal: the exact unpaid amount, backed by the invoice or contract.
2. Find the due date, and check the contract for an interest rate, a late fee and an attorney fee clause.
3. Choose the interest rule that fits from the table above, and calculate it to a specific date.
4. Add a contract late fee only if the clause exists and appears enforceable under Civil Code 1671.
5. Mention attorney fees only if a clause or statute allows them, and cite it.
6. Show the math in a short table in the letter, with a per-day figure for interest after your date.
7. Leave out anything you cannot tie to a contract term or statute.

For the rest of the letter, see our guides to a demand letter for money owed and an unpaid invoice demand letter in California. The broader question of what losses to claim is covered in how to calculate damages for a demand letter.

When the add-ons are worth an attorney's pen

Interest, late fees and fee clauses are easy places for a demand letter to overreach, and an inflated number can hand the other side an argument. An attorney can match each add-on to its source, show the calculation, and leave out what the law does not support.

Through Talk to My Lawyer, a licensed California attorney writes your letter from your facts and documents (the contract, invoices and payment records), calculates the interest the law allows, and sends it on your behalf. Questions about loans, usury or fee recovery deserve a lawyer's look at your specific facts.