Definition
A freelance developer sends a €3,000 invoice in March. The due date passes, so she chases by email. The client replies "we are looking into it this week". Second reminder, shorter answer.
Third reminder, no answer at all. She has no lawyer, no wish to go to court over one invoice, and starts assuming the money is gone.
One step is missing, and it is the only one that changes anything.
A mise en demeure (formal notice to pay) is the letter where you stop asking and start demanding.
It names the unpaid Invoice, the amount due, the deadline you grant and what you will do once it passes.
It needs no lawyer, no commissaire de justice (judicial officer) and no form: a lettre recommandée avec accusé de réception (registered letter with acknowledgement of receipt) is enough, and the acknowledgement is the point, because it dates delivery.
The official page on amicable debt recovery requires a letter carrying "interpellation suffisante", meaning wording clear and formal enough to qualify as a formal notice.
A letter inviting the client to kindly settle the matter when convenient is not a formal notice, whatever heading sits at the top of the page.
It is almost never mandatory. Its value lies elsewhere: it fixes the date on which the client knows, in writing and provably, that he is in default.
It is the first document a judge will read if things go further, and it is very often the letter that triggers the transfer, because it shows that the next step is already written.
What the letter claims, and what the unpaid invoice really costs you
A useful formal notice fits on one page. It carries the words "mise en demeure" in the subject line, the date, your details and the client's, then the invoice number and date.
Next come the principal amount, the Late payment penalties and the Recovery fee owed as of right on every invoice paid late between businesses.
It closes with the deadline you grant and the announcement of what follows, usually a Payment order.
On penalties, one misunderstanding travels everywhere.
Between businesses they run from the day after the payment date stated on the invoice, with no reminder required. Your letter does not create them, it totals them.
Their rate and the fixed fee already appear on the invoice and in your Terms and conditions: the formal notice merely presents the bill.
The grace period you grant, on the other hand, is set by no text at all: you choose it, and you had better choose one you can hold to, since you will have to do what you announced once it expires.
That leaves the question nobody asks: how much has this unpaid invoice cost you so far? Under the micro scheme, the answer is nothing.
The €3,000 invoiced in March never reached the account, so it belongs in no declaration.
Had this developer declared it by mistake, she would have paid €3,000 × 25.6% = €768 in contributions at the 2026 rate.
She is taxed under BNC, the non-commercial income category, in the general scheme and not with Cipav, whose rate differs.
On top would have come the CFP, the vocational training contribution, charged at 0.2% in 2026 for a liberal activity, so €6 more.
€774 would have gone out on money never received, and that contribution rate does not cover income tax, which is worked out separately.
The trap fits in three words: invoiced is not collected.
An unpaid invoice is declared nowhere, because your turnover is what actually reached you during the period: the Cash receipt decides, never the issuing of the document.
Two symmetrical mistakes follow. Declaring an invoice the day you send it makes you pay contributions on money you do not hold.
And on the day the client finally pays, even two years later, that sum enters the turnover of the year of the transfer, not of the year of the invoice.
Two letters carry this name: yours, and the URSSAF one
The same term covers a letter of an entirely different nature, the one you receive when contributions have gone unpaid after a declaration, or after a taxation d'office (assessment imposed for want of a declaration).
It is not negotiated like a commercial reminder.
It opens a period at the end of which URSSAF can issue a contrainte, an instrument that, unless you file an opposition before the court in time, carries the effects of a court judgment and opens the way to seizure without any judge having ruled.
The difference in nature is absolute: your own letter has no enforceable force, this one prepares one.
| The letter | What it produces |
|---|---|
| Your formal notice to a client | No immediate constraint: it dates the default, totals the penalties and prepares the court procedure |
| The URSSAF formal notice | It opens the period after which a contrainte can be issued: absent an opposition, it carries the effects of a judgment |
What to do with it is therefore the opposite of waiting: challenge it within the stated deadline if the amount is wrong, or ask for an URSSAF payment plan if your cash flow cannot follow.
Late-payment surcharges are added to the contributions due, and their waiver is requested from the director of your URSSAF with an explanation of the situation, a major client leaving you unpaid being precisely an admissible reason, as the official page on waiving surcharges sets out.
One last point weighs more than the surcharges: your pension rights are calculated on contributions actually paid, never on those merely owed.
A debt left alone costs you quarters of pension credit, and those are not recovered by a later transfer. This page is up to date as of 5 September 2026.
Frequently asked questions
Does the formal notice have to be sent by registered post?
No text requires it, but everything else points that way.
The acknowledgement of receipt establishes the date on which the client received the letter, and that date is exactly what you will have to prove if the procedure continues.
An email is easy to dispute, a registered letter far less so.
My client ignores the formal notice, what now?
You move to court-based recovery, and the simplest route needs no lawyer: a file lodged with the court can produce a payment order, which the client may formally oppose.
A référé-provision (interim payment order) and a full assignation en paiement (payment claim) also exist, both slower and more expensive.
Should an unpaid invoice go into my declared turnover?
No. The micro scheme works on sums actually received during the period being declared.
As long as the transfer has not arrived, the invoice appears neither in your turnover declaration nor in the calculation of your annual ceilings.
How long can I wait before acting?
Less time than you would think, and the period differs depending on whether your client is a business or a private individual.
An old debt becomes legally unrecoverable, so check the limitation period that applies to your situation before letting another year go by.
To find out how much time you have left and leave with the letter already written, go through Formal demand for an unpaid invoice: it starts by asking who owes you money, and everything else follows from that.