Short answer: online Self Assessment returns for 2025 to 2026 are due by 31 January 2027. Practices that get through January without hiring do four things from November: count and rank every return, set one records cut-off, move preparation to whoever has the hours while seniors keep review, and get payment figures to clients by mid-January.
Last season shows the size of the queue. For the 2024 to 2025 tax year HMRC expected 12,029,168 returns. It received 11.49 million by 31 January 2026, 475,722 of them on the final day, and estimated that about 1 million people missed the deadline (GOV.UK: 11.48 million beat the Self Assessment deadline). Much of that final-day volume is agents filing for clients whose records arrived late.
What a missed deadline costs the client
The late filing penalty starts the day after the deadline, whether or not any tax is owed (GOV.UK: Self Assessment penalties):
| Time after 31 January | Late filing penalty |
|---|---|
| 1 day | £100 |
| After 3 months | £10 a day, up to £900 |
| After 6 months | 5% of the tax due or £300, whichever is greater |
| After 12 months | Another 5% or £300, whichever is greater |
Late payment is charged separately: 5% of the unpaid tax at 30 days, 6 months and 12 months, plus interest. A client who receives their figure on 30 January has very little time to arrange payment, so the practical deadline for the practice is earlier than HMRC’s.
A November to January plan
The plan below works back from 31 January. Practices that start in November have about twelve working weeks, minus the Christmas break.
Early November: count and rank every return
List every client due a return with three facts: records in or not, complexity (employment only, self-employed, rental, capital gains), and last year’s filing date. Simple returns with records already in are finished first; they clear the list fastest.
Mid-November: one records request, one cut-off date
Send each outstanding client a single list of what is missing and a date, such as 30 November. Clients who miss it move down the queue and are told in writing what that means for their filing date.
November to mid-January: separate preparation from review
Preparation is production work: income and expense schedules, rental and capital gains workings, and a draft return in your software. Review, client questions and approval are judgement work. Keep seniors on judgement and move preparation to whoever has the hours.
Weekly: review in fixed blocks
Book review time in partners’ diaries every week and feed drafts in by readiness. Queues stall when drafts arrive one at a time in the last fortnight.
By 15 January: payment figures to every client
Send the balancing payment and any payment on account with the 31 January due date. The last two weeks are then for filing and stragglers, not for first drafts.
Clients who joined Making Tax Digital for Income Tax from April 2026 still file a 2025 to 2026 return by 31 January 2027 in the usual way, and their quarterly updates add to the same winter workload. See what MTD quarterly updates mean for practice workload.
Where the extra hours come from
Self Assessment is an hours problem in a fixed window. A hire made in November will not be recruited and trained before 31 January. The realistic options are overtime, turning work away, or temporary preparation help that works inside your own tax software while your team keeps review, client contact and filing approval.
That last option is what EarthOne does. Qualified Indian Chartered Accountants prepare self-assessment returns to draft stage for your review, at £16.50 an hour. If a return takes about 2.5 hours to prepare, 40 returns is 100 hours, or £1,650, paid once, with one month’s notice. The work is white-label and covered by a UK GDPR Data Processing Agreement. See self-assessment outsourcing or how this works in IRIS.
Short of Self Assessment hours before 31 January?
Tell us roughly how many returns you have open and which software you use. We reply within one business day with who would prepare them and what it would cost.
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