Why HMRC can request more tax after PAYE deductions

Katerina Galkina · EN · 07/10/2026

Identify the calculation you received

PAYE deductions do not guarantee that the final annual tax liability equals the amount withheld. HMRC reconciles information after the tax year. A wrong code, multiple jobs, pensions, benefits or information received later can create a difference.

HMRC distinguishes ↗ a P800 from Simple Assessment. Self Assessment is a separate system. Read the document’s title, tax year, amount and payment instructions. Check unexpected correspondence through a Personal Tax Account opened independently, rather than paying through a link in an unexpected text.

Reconcile the underlying figures

Gather P60s for each job, P45s, final payslips, pension information, company-benefit records and savings-interest statements. Create a table showing each source, income, tax already deducted and the figure used by HMRC. Look for duplicated employment, incorrect income or missing deductions.

If employer data is wrong, ask payroll to check what it reported. Also tell HMRC exactly which figure is incorrect and what it should be. Saying “my employer already deducted tax” is insufficient without reconciling the totals. Keep documents, call dates and case references.

Check how the underpayment will be collected

For a P800 underpayment ↗, HMRC can usually collect through a changed tax code when conditions are met, including an amount below £3,000 and sufficient income. Otherwise it explains another payment route. Check before making a separate payment if the amount may already be included in payroll deductions.

A Simple Assessment ↗ may be issued where collection through the code is unavailable. Check the deadline and payment details on the current bill. If you cannot pay on time, contact HMRC before the deadline to discuss available arrangements. Instalments are not automatic.

Challenge an incorrect calculation

For Simple Assessment, HMRC says to query incorrect figures ↗ within 60 days of the bill date. Identify the disputed entries, correct amounts and supporting records. If HMRC disagrees, its decision letter explains the appeal process; the normal appeal period is 30 days from that decision letter.

A query does not itself postpone payment. Observe the deadline unless HMRC confirms a deferral. After correction, obtain the revised calculation and check your current tax code so that the same information error does not continue into the next year.