Definition
Spring arrives, your income tax return opens online, and you go through the usual sections: salaries, rental income, donations.
Nowhere is there a line for the €24,000 your business collected. That is normal: it lives on an annex you have to ask for, form 2042-C-PRO.
Form 2042-C-PRO is the déclaration complémentaire des revenus des professions non salariées (the supplementary return for self-employed income).
It goes with the main form, the 2042, and it is where a micro-entrepreneur reports the year's Turnover.
In the online return it only appears once you tick the self-employed income section at the step where you choose which sections apply: nobody offers it to you, which is the first reason micro-entrepreneurs reach the end of their return having declared nothing at all.
The form calculates nothing. You enter your gross turnover, never your profit, and the administration then applies the Standard allowance attached to the line you filled in.
That is the whole mechanism of the Micro-tax regime: one figure declared, one percentage removed automatically, no receipts required.
What you enter, worked through on €24,000
Take a service provider taxed as BIC (bénéfices industriels et commerciaux, industrial and commercial profits) who collected €24,000 over the calendar year.
On form 2042-C-PRO she enters €24,000, and nothing else. The administration removes the 50% allowance that applies to BIC services in 2026, which is €12,000.
Those €12,000 of taxable profit, and only those, join the rest of the household income and go through the progressive scale.
The same amount collected as a liberal professional taxed as BNC (bénéfices non commerciaux, non-commercial profits) would attract a 34% allowance in 2026, leaving €15,840 taxable, and 71% for the sale of goods, leaving €6,960.
The figure written down is the same in all three cases: the line you choose is what changes the outcome.
A quiet floor completes the picture, since in 2026 the allowance never falls below €305, raised to €610 for mixed activities, that doubling being the administrative reading of the two allowances rather than the wording of the tax code itself.
Never take your costs off before writing the amount. A designer who collected €24,000 and spent €6,000 on equipment and subscriptions does not declare €18,000, he declares €24,000.
The allowance is deemed to cover all of his business costs, and subtracting them a second time means under-declaring his income.
One detail settles the year-end edge cases: a December invoice paid in January belongs to the following year, because what counts is the date of the Cash receipt, never the invoice date.
The flat-rate tax option exempts you from nothing
This is the most widespread mistake on this form.
Many micro-entrepreneurs who opted for the Flat-rate income tax believe their tax is settled, since they already paid with every declaration during the year, at the rates in force in 2026, 1% of their turnover on the sale of goods, 1.7% on BIC services and 2.2% on BNC liberal work.
Those rates cover income tax alone, with social contributions coming on top and weighing far more heavily. So they fill in nothing in the spring.
Form 2042-C-PRO is compulsory with or without the versement libératoire, and even with zero turnover. Under the option, turnover goes in the box reserved for micro-entrepreneurs who chose the prélèvement libératoire, as the DGFiP page on the scheme states.
This entry triggers no second taxation.
It feeds three things: the household's effective tax rate calculation, the revenu fiscal de référence (reference tax income) and the retirement savings deduction ceiling.
That reference tax income is no administrative detail, since it governs access to benefits, to means-tested rates, and the right to keep the flat-rate option in later years, which is judged on the reference tax income of the year before last, not on last year's.
A year left undeclared distorts it for a long time.
Two declarations that share nothing but the figure
The Turnover declaration sent to URSSAF and form 2042-C-PRO carry the same number, but feed different calculations.
The first, monthly or quarterly, triggers your social contributions; the second, annual, triggers your income tax. Two administrations, two calendars, and neither fills in the other for you.
Hence a simple check before you submit: the turnover declared to URSSAF for the periods of the calendar year should match the amount entered on form 2042-C-PRO, since both rest on the turnover actually collected.
A quarterly declaration filed in January still covers the year that has just ended: that is the year it counts for, not the month you filed it.
Without the flat-rate option, this entry feeds Withholding tax: the administration recalculates your instalments from the profit it has just reconstructed, then takes them from your bank account.
The filing timetable, which varies with your département of residence, is published every year on impots.gouv.fr.
Frequently asked questions
Do I have to file if I collected nothing this year?
Yes. The return stays compulsory even with zero turnover, and you simply enter a zero.
A business that was registered but never found a client is declared exactly like any other, otherwise your reference tax income is calculated on an incomplete picture.
Where do I find form 2042-C-PRO in the online return?
At the step that asks you to select the sections that concern you, by ticking the one for self-employed income. The annex is then added to your path.
Without that box, the return finishes normally and never asks you for your turnover.
I am employed and a micro-entrepreneur, do I file two returns?
No, one single income tax return, with this annex added to it.
Your salary is pre-filled by your employer, your turnover never is: adding it is down to you, on the line matching the category of your activity.
What happens if I use the wrong line?
The administration applies the allowance of the line you filled in, not the one for your actual activity.
Declaring €24,000 of liberal services on the sale-of-goods line would move the allowance from 34% to 71% in 2026, so taxable profit from €15,840 to €6,960: an amended return is needed.