How to modify a tax return already filed

In the event of an error in the declaration, there are several possibilities depending on the type of error:

If you have paid too much or declared a lower amount to be refunded for Corporate Tax (favorable situation for the taxpayer) , you can request the rectification of the self-assessment submitted before four years have passed since the deadline for submitting the aforementioned self-assessment ended.

If you have underpaid or declared a higher amount to be refunded for Corporation Tax (unfavorable situation for the taxpayer) , you must submit a supplementary self-assessment , checking the "supplementary self-assessment" box on form 200, indicating the receipt number of the previous self-assessment and filling in the "To be deducted (exclusively in the case of a supplementary 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 application for rectification of the self-assessment before four years have passed since the deadline for submitting the aforementioned self-assessment ended.