If you think you’ve underpaid tax, you might have to complete a tax return.
If this is the case, normal self-assessment time limits apply.
To pay an amount up to £3,000 through an adjustment to your tax code for the following year, you should file a return by 31 December following the end of tax year.
Otherwise, tax still due for the last tax year must be paid by 31 January following the end of the tax year in which the income arose.
If you think you haven’t paid enough tax, contact HMRC.
You might be asked to complete a tax return. Be aware if you don’t do this, you’ll usually have to pay penalties and interest when the underpayment does come to light.