When you have not paid enough tax during the year. HMRC will usually collect this by adjusting your tax code the following year, spreading the repayment across 12 months.
Not sure your deductions are correct?
Related guides
Expense Reimbursements vs Taxable Benefits: What Belongs on Your Payslip
Mileage at 45p/25p, the £6 a week working-from-home allowance, BIK, P11D entries and what should never appear in your taxable pay. Updated for 2026/27.
Read guideHow HMRC Uses Payroll Data to Calculate Your Tax (RTI Explained)
Real Time Information (RTI), Full Payment Submissions, dynamic coding and why your tax code can suddenly change mid-year. Plain English UK guide for 2026/27.
Read guide