Late payment is the most common cash-flow problem freelancers face, and a late fee clause is the simplest protection against it. The short answer is yes, you can charge late fees, but the rules depend on where you and your client are based, and the fee only works if it was agreed before the work started.

In the UK, the Late Payment of Commercial Debts (Interest) Act 1998 gives every freelancer and small business an automatic statutory right to charge interest on overdue business-to-business invoices. You do not need a contract clause: the right exists by default. The rate is 8 percent above the Bank of England base rate, which works out to 12.5 percent a year at a 4.5 percent base rate. On top of the interest you can claim fixed compensation: 40 pounds for debts up to 999.99 pounds, 70 pounds for debts from 1,000 to 9,999.99 pounds, and 100 pounds for debts of 10,000 pounds or more.

In the US there is no federal equivalent, so late fee rights are primarily contractual. You need a clause in your contract for the fee to be enforceable, and the standard rate is 1 to 2 percent per month, or 12 to 24 percent a year. The fee must be disclosed before work begins: you cannot add it retroactively to an invoice that had no such terms.

To add a late fee, issue a revised or supplementary invoice rather than editing the original. Show the original amount as a line item, add a late payment interest line with the calculation shown, rate, number of days and amount, and reference the legal basis or contract clause. The daily interest formula is simple: invoice amount times annual rate, divided by 365, times the number of days overdue. A 1,000 dollar invoice at 18 percent a year accrues about 49 cents a day, which sounds small until it runs for months.

The fee works best as a deterrent. Most clients pay at the first or second reminder, and the mere existence of a late fee clause makes late payment less likely. Use a fixed escalation ladder: a friendly reminder on the due date, a statement with the fee accruing at 14 days, and a pause on work at 30 days. Beyond that, the tools are real: freelance protection laws in New York, Illinois, California and elsewhere provide double damages and attorney's fees, and a demand letter citing the applicable statute often resolves things by itself.