Filled in Form 12 - What happens next

The Form 12 is not the same as the Form 11, and it won't give you a calculation of tax due.

The Form 12 is generally completed by PAYE Taxpayers, who have had their tax already calculated for them in their employments, and where they can declare additional Non PAYE income of less than a specified amount.

What will happen next is, you will get a P21 Balancing Statement for the year. It will state "treatment of result" on the bottom of the first page. 3 possible outcomes:

1) Balanced - nothing owed to anybody
2) Overpayment - happy days, you get a repayment
3) Underpayment - You owe additional tax*

In relation to the underpayment, the usual treatment is that your tax credits are reduced for future years (usually 2), by 50% of that amount per year. The caveat* being that, the underpayment is greater that €100. If it is less than this, then you will not have to pay it back, although if you look for an amended P21, and make a claim for repayment, the underpaid amount first gets collected at that time

You can of course - i'm just outlining the standard treatment that happens.

If there is a figure outstanding you can clear it via Cheque no bother.