How to Correct a Tax Error with HMRC UK 2026: Voluntary Disclosure

Discovering that you have underpaid tax is stressful, but the worst thing you can do is ignore it. HMRC's approach to errors depends heavily on how and when they come to light. A proactive, voluntary disclosure -- where you tell HMRC before they find out -- attracts significantly lower penalties than a disclosure made only after an investigation has begun. This guide explains the main routes to correct a tax error in 2026.

Why Voluntary Disclosure Matters

HMRC's penalty regime under Schedule 55 of the Finance Act 2009 (for filing failures) and Schedule 56 (for late payment) contains a crucial distinction between prompted and unprompted disclosures.

An unprompted disclosure -- where you come forward before HMRC has any reason to suspect a problem -- attracts the lowest possible penalties, sometimes reduced to nil for a genuine mistake. A prompted disclosure (made after HMRC has written to you or opened an enquiry) carries higher minimum penalties.

The quality of the disclosure also matters. HMRC judges disclosures on three factors: telling (how fully you disclosed), helping (how much you assisted the investigation) and giving (whether you paid what was owed promptly). Full marks on all three means the maximum possible reduction in penalties.

Step 1: Work Out What You Owe

Before contacting HMRC, calculate the underpayment as accurately as possible. You need to know:

• Which tax years are affected (HMRC can generally go back 4 years for innocent errors, 6 years for careless errors, and 20 years for deliberate behaviour)
• The amount of income or gains that were not declared
• The tax that should have been paid, including the effect on your Personal Allowance (GBP 12,570 for 2026/27) and which rates applied
• Interest that will be charged (HMRC charges interest from the original due date)

If the error involves rental income, self-employment, offshore accounts or any area where HMRC has a specific disclosure facility, use the relevant campaign route rather than a general contact.

The Let Property Campaign

If your underpayment relates to undeclared rental income, HMRC's Let Property Campaign is the correct route for individual landlords. You register online at gov.uk, notify HMRC that you intend to make a disclosure, and then have 90 days to prepare and submit the full disclosure.

The campaign covers all UK and overseas rental income that has not been declared. It does not cover properties owned through a limited company -- those require a different approach via the Business Tax Account or a formal disclosure under Code of Practice 9 (COP9) if the amounts are large.

Penalties under the Let Property Campaign for an unprompted, innocent error can be as low as 0%. For careless errors, the minimum penalty is typically around 15% of the tax owed before reductions. Providing full cooperation can bring this down to 10% or lower.

Correcting a Self Assessment Return

If the error is in a Self Assessment return that was filed less than 12 months ago, you can simply amend the return online via your Personal Tax Account. No penalty applies to a corrected return within the amendment window, as long as the amendment is made before HMRC opens an enquiry.

For older returns -- up to 4 years back -- you need to write to HMRC's Self Assessment office with a letter setting out the error and including the corrected figures.

Certificate of Tax Deposit

If you have identified a potential liability but are not yet certain of the exact amount -- perhaps because the calculation is complex or disputed -- a Certificate of Tax Deposit (CTD) can be used to stop further interest accruing while you finalise the figures. You deposit money with HMRC against the anticipated liability. The CTD earns a modest rate of return while it sits with HMRC.

CTDs are particularly useful where a tax dispute is ongoing and you want to demonstrate good faith without accepting liability for a specific amount.

Code of Practice 9: Serious Cases

For disclosures involving deliberate conduct (for example, deliberately concealing income over multiple years), HMRC may require the disclosure to be handled under COP9, also known as the Contractual Disclosure Facility. This process involves a formal offer of immunity from prosecution in exchange for full cooperation and a complete disclosure.

COP9 is a serious process and requires specialist tax investigation advice. Do not attempt to handle a COP9 case without a qualified accountant or tax lawyer. Failing to make full disclosure under COP9 removes the protection from prosecution.

Paying What Is Owed

Any tax owed as part of a voluntary disclosure should be paid as quickly as possible. HMRC charges late payment interest from the original due date. For 2026/27, the late payment interest rate is linked to the Bank of England base rate plus a margin -- check the current rate on gov.uk before you calculate.

If you genuinely cannot pay the full amount immediately, contact HMRC to agree a Time to Pay arrangement. HMRC is generally willing to agree instalment plans of up to 12 months for straightforward cases.

Get the Calculation Right Before You Disclose

Use the CalcHub income tax calculator to work out how much tax should have been paid in each affected year before you submit your disclosure:

Income Tax Calculator -- CalcHub