The January Self Assessment peak is managed by moving work earlier rather than by working longer in January. The practical levers are a records deadline written into the engagement letter, sorting clients by complexity and setting a target month for each group, filing early for clients who gain from it, such as those who can have tax collected through their tax code if the return is online by 30 December, and deciding by early December which returns will not be finished and telling those clients.

Why does so much work arrive at once?

For the 2025 to 2026 tax year, GOV.UK sets an online filing deadline of 11:59pm on 31 January 2027, and any tax owed must be paid by the same date. Paper returns were due by 31 October 2026. The first payment on account for 2026 to 2027 is also due on 31 January, so many clients face a balancing payment and a payment on account together. Because nothing forces an earlier filing, clients who are not chased send records late, and the work compresses into December and January.

Two newer pressures sit on top. Someone who needed to register for a return had to tell HMRC by 5 October; if they register later, HMRC sets a filing deadline 3 months from its letter, but payment is still due by 31 January. And clients already within Making Tax Digital for Income Tax have a quarterly update due by 7 February for the period to 5 January. HMRC will not apply penalty points for late quarterly updates in 2026 to 2027, but all updates must be sent before the return for that year can be submitted. The full penalty rules are on Self Assessment deadlines and penalties.

How should a practice sort its Self Assessment clients?

Sort clients once a year, in the spring, into groups that set a target month and a fee:

Group Typical client Target month
Simple Employment or pension income plus one extra source June to September
Coding out candidates Employees or pensioners likely to owe under £3,000 By early December
Trading and property Sole traders and landlords with year ends near 5 April September to November
Complex Capital gains, foreign income, trusts, partnership shares Records by summer, finished by November
Known late payers of records Clients who were late last year Records deadline earlier than others

The point of the table is not the labels but the commitment: each client has a month in which the work is booked, so capacity can be checked months before January.

Which clients gain from filing early?

  • Coding out. GOV.UK says HMRC will automatically collect a Self Assessment bill through the tax code if the person owes less than £3,000, already pays tax through PAYE, and filed a paper return by 31 October or an online return by 30 December. A part payment cannot be made to get under the limit, and coding out is not available if there is not enough PAYE income or if the client would pay more than 50% of their PAYE income in tax. See coding out underpaid tax.
  • Payments on account. Each payment is usually half of last year’s tax, unless last year’s bill was under £1,000 or more than 80% was collected at source. If income has fallen, the client can ask HMRC to reduce them online or on form SA303, but interest is charged if the reduction proves too large. Early figures make that decision safer; see Self Assessment payments on account.
  • Large bills. A client who learns in October that they owe a large sum has three months to save, set up regular payments or, if needed, contact HMRC about a payment plan. GOV.UK points clients whose bill is not yet overdue to a Budget Payment Plan; the Time to Pay page covers overdue debts.

How do records deadlines and pricing help?

A records deadline only works if it is written into the engagement letter, explained to the client and enforced. A common pattern is a date such as 31 October for records, a written reminder a month before, and a clear statement that records received later will be dealt with in order of receipt and may cost more. Some practices charge a surcharge for records after the date; others give a discount for records by the summer. Either way, the client must know before the year starts. The routine for following up is on the chasing client records page.

New clients are the other trap. Set a cut off date, such as 30 November, for taking on new Self Assessment clients for that January, because HMRC agent authorisation, professional clearance and identity checks all take time. A prospect who has not registered for Self Assessment at all needs to be told about the registration rules on the registering for Self Assessment page.

How should capacity be planned for December and January?

  • From October, count the returns not yet started, in preparation, at review and awaiting approval every week.
  • Convert those counts into hours using each group’s budget and compare them with staff hours available after holidays.
  • Protect reviewer time, because review is usually the bottleneck rather than preparation.
  • Schedule the coding out group to finish before mid December so the 30 December date is not at risk over Christmas.
  • Tell clients early about payment timing. GOV.UK says Faster Payments and debit card payments arrive the same or next day, but a first Direct Debit takes 5 working days, and online payment services may be slow at busy times.

The year round system that sits behind this is described in workflow and deadline management.

What should happen to returns that will miss 31 January?

Decide early, ideally by the middle of December, and write to the client. Explain the cause, the expected filing date and the penalties. GOV.UK lists an initial £100 penalty for a late return; after 3 months, daily penalties of £10 a day up to £900; after 6 months, a further 5% of the tax due or £300, whichever is greater; and after 12 months, another 5% or £300. Separately, late payment penalties of 5% of the unpaid tax arise at 30 days, 6 months and 12 months, with interest on top.

Because the payment penalties run from the payment deadline whatever happens to the return, a client whose return will be late should pay a careful estimate by 31 January. Whether a penalty can be appealed is a separate question, covered on the reasonable excuse page. GOV.UK lists relying on someone else to send a return as something that may count, but paragraph 23 of Schedule 55 to the Finance Act 2009 says reliance on another person is not a reasonable excuse unless the taxpayer took reasonable care to avoid the failure, so a client who sent records late is unlikely to succeed.

How might the plan look in numbers?

Illustration only. A practice has 600 returns and four people who prepare them. It budgets 2 hours for simple returns, 4 for trading and property and 8 for complex ones, giving 300 simple, 250 trading and 50 complex returns, or 2,000 hours. If each preparer can give 100 hours a month to Self Assessment from June, the practice needs five months of steady work. Starting in June and finishing most work by the end of November leaves December and January for late records and corrections, rather than for 300 returns arriving at once.

What mistakes make the peak worse?

  • Setting a records deadline but accepting everything late without consequence.
  • Leaving coding out clients until after Christmas.
  • Taking on new Self Assessment clients in January.
  • Booking reviewer leave in late January.
  • Not warning clients about payments on account until the bill arrives.
  • Forgetting the February quarterly update for Making Tax Digital clients.

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Sources

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 8 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.