The Cash-Flow Shock: Why Your January 2027 Tax Bill Can Feel Twice as Large

Every autumn, HMRC begins reminding self assessment taxpayers about the 31 January filing and payment deadline. For many sole traders and higher earners, the figure on the screen comes as a genuine surprise. The tax due for the year looks broadly right, yet the amount payable is much larger. Often, the explanation is not an error. It is payments on account.

What payments on account actually are

Payments on account are advance payments towards your next year's Income Tax and Class 4 National Insurance. They are not a separate tax. They are a mechanism for collecting tax earlier, rather than waiting until after the tax year has ended.

HMRC generally requires payments on account where your previous year's self assessment bill was more than £1,000, and less than 80% of that liability was already collected at source. Each payment is normally half of the previous year's relevant tax bill. The first instalment is due on 31 January in the current tax year, and the second on 31 July after it ends.

For the 2026/27 tax year, that means the first payment on account is due on 31 January 2027, alongside any balancing payment still owed for 2025/26. The second instalment is due on 31 July 2027. When both fall in the same January, the cash requirement can feel roughly double what you expected.

Why the January bill can look double

The 31 January 2027 payment is really two things added together. First, there is the balancing payment for 2025/26: the difference between the tax actually due for that year and any payments already made. Second, there is the first payment on account for 2026/27.

If your 2025/26 liability was £6,000 and you made no payments on account during that year, the January calculation may show the £6,000 balance plus a £3,000 first instalment for 2026/27. The total is £9,000, not because the tax rate has changed, but because HMRC is collecting next year's tax in advance. A further £3,000 would then be due by 31 July 2027.

This is why the bill can feel disproportionate. It is a cash-flow effect, not necessarily a sign that your tax affairs have gone wrong.

The one-off income trap

The bigger difficulty arises when the previous year was unusually profitable. Suppose a sole trader had a quiet 2024/25 but won a large one-off contract in 2025/26. The 2025/26 profit might be £80,000, well above the normal £40,000. HMRC will use that higher figure to set the 2026/27 payments on account, even if the trader expects to return to a more modest level.

The same can happen with a large bonus, a one-off consultancy project, or income from an activity that HMRC treats as trading rather than a capital gain. A large capital gain alone does not usually drive payments on account, because Capital Gains Tax is excluded from the calculation. But if the underlying activity is trading income, or if the year's income tax liability has risen sharply, the following year's instalments can be based on profit that may never be repeated.

The result is a familiar pattern: a strong year produces a large tax bill, and the next January demands advance payments based on that strong year just as income is falling back. The taxpayer is paying tax in advance for profit they may not earn again.

Reducing payments on account with SA303

There is a legitimate remedy. If you expect your 2026/27 income tax liability to be lower than the 2025/26 figure on which the instalments are based, you can ask HMRC to reduce your payments on account. This is done using form SA303, either through your HMRC online account or on paper.

The reduction must be based on a reasonable estimate, not wishful thinking. If you reduce payments too far and your actual liability turns out to be higher, HMRC may charge interest on the shortfall. Deliberately excessive reductions can also attract penalties. Keep your workings, and revisit the estimate if circumstances change.

A sensible approach is to calculate what you expect to earn in 2026/27, estimate the Income Tax and Class 4 National Insurance due, and reduce the instalments to reflect that figure. If your income recovers later in the year, you can adjust again. The key is to act before the relevant deadline: 31 January 2027 for the first instalment, and 31 July 2027 for the second.

Budgeting for 31 January and 31 July 2027

Payments on account reward planning. From the moment a strong year becomes clear, set aside a proportion of each payment received for tax. A common approach is to separate the tax element into a savings account as income arrives, rather than waiting for the January statement.

It also helps to map the two deadlines together. The 31 January 2027 bill may include the 2025/26 balance plus the first 2026/27 instalment. The 31 July 2027 bill will normally be the second instalment alone. If you reduce payments on account using SA303, update your budget immediately so the later balancing payment does not become a second shock.

Finally, do not assume a large January figure is automatically correct. Check the calculation, confirm whether payments on account have been included, and compare the figures with your records. If something looks wrong, query it before the deadline rather than after.

Payments on account are a collection mechanism, not a penalty. Once you understand why the January bill looks the way it does, it becomes easier to plan for it, reduce it where appropriate, and avoid being caught out by a bumper year that has already passed. This is general guidance and not a substitute for professional advice on your own circumstances.