How to modify a tax return already filed
How to correct a mistake in your return:amending and complementary documents
In the event of an error in the tax return, there are several possibilities depending on the type of error:
If you have overpaid or under-declared Corporation Tax to be offset or refunded (favourable situation for the taxpayer), you can request rectification of the self-assessment submitted within four years of the deadline for submitting it.
If you have underpaid or over-declared Corporation Tax to be offset or refunded (unfavourable situation for taxpayer), you must present a complementary self-assessment, ticking the "complementary self-assessment" box on form 200, indicating the number of the receipt of the previous self-assessment and filling in the "To be deducted (exclusively in the case of a complementary declaration)" box with the amount of the result of the previous self-assessment.
If the error refers to an information box or the account number to which the refund is requested, or another error that does not affect the result of the settlement, you must submit a request for rectification of the self-assessment before four years have elapsed since the deadline for submitting it.