The good news is that you can still file your tax return, but the bad news is that you'll have to pay a fine.
Filing your tax return late in France means your tax bill will jump by at least 10 percent but this will go up the longer you leave it, so you should act sooner rather than later.
Here's how much you can expect to pay, depending on how late you file:
10 percent if you make your declaration before receiving formal notice from the tax office
20 percent if you make your declaration within 30 days of the formal notice
40 percent if you do not make a declaration within 30 days of the formal notice
80 percent if the tax office discovers that you are doing undeclared work or illegal activity, with no formal notice.
On top of that you may also be expected to pay late interest on your tax bill. Since January 1st 2018, this has amounted to 0.2 percent for every month you are late with your declaration. These interests apply until the last day of the month in which the return is filed.
If you do not have to pay income tax, you are still expected to make a declaration that will allow you to obtain a notice of non-taxation, which can be useful for certain administrative procedures. However you will not be subjected to the fines for making a delayed declaration.
Finally, here's a reminder of the 2019 deadlines…
The paper form should have been completed and submitted by May 16th while the deadline for online forms varied in different areas. If you live in départements 1 to 19 it was May 21st, for départements 20 to 49 it was May 28th and for départements 50 plus the deadline was June 4th.