Every year, HMRC issues between three and four million P800 tax calculations to employees and pensioners whose tax affairs are handled through PAYE rather than self assessment. Most recipients treat the P800 as gospel: a definitive statement from the tax authority of what they owe, or what they are owed. Yet since 2021, tax specialist Tim Good has been warning that a significant number of these calculations are wrong, and that the error consistently works against the taxpayer rather than in their favour.

What exactly is a P800?

A P800 is HMRC’s reconciliation of an individual’s tax position for a given tax year, issued to those who are not required to submit a self assessment return. Drawing on information supplied by employers, pension providers and other third parties, HMRC works out whether the correct amount of tax has been paid through PAYE over the course of the year. Where it has not, a P800 is sent showing either a repayment due or additional tax owed. For the millions of people who receive one each year, often between June and November, it is generally accepted at face value, since few taxpayers have the means or inclination to check HMRC’s own workings.

Tim Good

That trust is precisely what makes the flaw identified by Tim Good so concerning.

The discovery: beneficial ordering rules ignored

In 2021, Good, a tax lecturer and director of Absolute Accounting Software, was asked by taxpayers to review P800 calculations that did not match the results produced by conventional self assessment software. Tracing the discrepancy back to its source, he found that HMRC’s P800 process was failing to apply the long-established “beneficial ordering” rules set out in section 25(2) of the Income Tax Act 2007.

In plain terms, for income tax calculations from the 2008/09 tax year onwards, taxpayers with a mixture of non-savings income, savings income and dividend income are legally entitled to have their personal allowances and reliefs set against whichever category produces the lowest overall tax bill. Self assessment calculations follow this rule correctly. The P800 process, Good discovered, did not.

When challenged by software developers and tax professionals, HMRC explained that PAYE and self assessment are managed on entirely separate systems, so calculations operate independently for each, with the department stating that it proactively tries to identify affected customers and calculate their correct position manually to protect them from the risk of an incorrect PAYE calculation. HMRC’s own PAYE team acknowledged that its allocation of allowances and reliefs was not always in the taxpayer’s best interests, but said it was not in a position to update the underlying logic, describing the problem as a known issue that work was under way to resolve.

The consequences for taxpayers

The practical effect of the error is that certain taxpayers with income spread across non-savings, savings and dividend categories can be charged more tax than the law requires. The mechanism, according to Good, lies in how the P800 process handles the savings rate band, worth up to £5,000, alongside the personal savings allowance of £1,000 or £500. Where allowances are allocated against interest before dividends, rather than in the order that minimises the overall bill, a taxpayer can be deprived of tax-free savings income and instead see additional dividends taxed at a rate of 10.75%.

The scale of the individual impact varies considerably. In many cases the difference between the P800 figure and the legally correct figure will be negligible. In a meaningful number of cases, however, the gap runs to somewhere between £50 and a few hundred pounds, and in some instances the overcharge can exceed £1,000. One illustrative example, based on 2026/27 rates, involves a taxpayer with £4,000 of non-savings income, £8,000 of gross interest and £40,000 of dividends. Correctly applying beneficial ordering produces a liability of £4,026.23, whereas the flawed P800 methodology generates a bill of £4,617.48, an overcharge of just under £600 on that individual case alone.

Crucially, the error is understood to be one-directional. It can leave a taxpayer overpaying, but does not appear to ever result in an underpayment, which raises awkward questions about why the underlying calculation engine has not been prioritised for repair in the years since the problem first surfaced.

Tim Good’s claim that the flaw persists

Five years on from HMRC’s initial acknowledgement of the issue, Good says the same fundamental error is still occurring. Having reviewed further examples supplied by accountants in 2026, he traced the discrepancies back to precisely the same failure to apply beneficial ordering that he first identified in 2021. The issue has also surfaced independently in professional discussion forums, where practitioners comparing the interaction between the starting rate for savings and the personal allowance have spotted calculations that plainly do not follow the statutory rules.

HMRC maintains that it is aware of the problem and is working to align the P800 methodology with the self assessment calculator. A manual process exists to catch miscalculated cases, with roughly 100,000 affected for the 2025/26 tax year, an estimated 20,000 for 2026/27, and a projected fall to around 3,000 once legislative changes take effect from April 2027. HMRC has stated that fewer than 0.05% of the PAYE population has typically been affected over the past decade, with an average discrepancy of less than £80, and says nearly all cases are corrected through the end-of-year reconciliation process.

Good remains sceptical that tinkering with beneficial ordering is the right approach at all. Reacting to changes introduced by the Finance Act 2026, which will alter how allowances are allocated from April 2027 without meaningfully simplifying the calculation, Good argued that the rules should be abolished altogether, paired with an increase in the savings rate band and personal savings allowance to compensate taxpayers. He was blunt about the prospects of that happening, adding only that readers should not hold their breath.

Sarah Weston of the Low Incomes Tax Reform Group has echoed the underlying concern, stressing that taxpayers should never simply assume an HMRC calculation is correct, particularly where multiple income sources or reliefs are involved, while acknowledging that the ordering rules are genuinely difficult for ordinary taxpayers to understand or check for themselves.

What taxpayers and agents should do

For anyone receiving a P800 with a combination of employment or pension income alongside savings interest or dividends, the practical advice is to recalculate the figure independently using software that correctly applies beneficial ordering, and to challenge HMRC directly if a discrepancy emerges. The greater risk sits with unrepresented taxpayers who have no accountant to flag the error and are unlikely to question a letter from HMRC. With Making Tax Digital for income tax reducing the extent to which HMRC routinely issues end-of-year calculations that agents can cross-check, the P800 saga stands as a cautionary tale about the value, and the increasing scarcity, of independent verification within the UK tax system.