Self Assessment deadline 2027: a week-by-week plan for small practices

Written by the Mipp team

Published: 8 October 2026 | Last checked against GOV.UK: 8 October 2026 | Accurate as of October 2026

The online Self Assessment deadline for the 2025/26 tax year is 11:59pm on Sunday 31 January 2027, and any tax owed for 2025/26 is due by the same time (GOV.UK: Self Assessment deadlines). Because 31 January 2027 falls on a Sunday, treat Friday 29 January as your practical deadline for anything that needs a client, a bank or your own team. This plan works back from that date, week by week, for sole practitioners, small practices and bookkeepers who file returns for their clients.

Self Assessment deadlines for 2025/26 at a glance

These are the dates that shape the season. The tax year being reported is 6 April 2025 to 5 April 2026.

See the full HMRC deadlines calendar for 2026/27.

Clients who registered after 5 October 2026 get a letter or email from HMRC with a different filing deadline, 3 months from the date of that letter or email. They must still pay what they owe by 31 January 2027 (GOV.UK).

Why 31 January 2027 on a Sunday changes your plan

The online filing deadline does not move. GOV.UK gives the deadline as 11:59pm on 31 January 2027, so a return filed on the Sunday is on time. Everything around the return is different, though.

• Payments can need to arrive early. GOV.UK says that if a payment deadline falls on a weekend or bank holiday, the payment must reach HMRC on the last working day before, unless the client pays by Faster Payments or by debit or corporate credit card (personal credit cards are not accepted) (GOV.UK: pay your Self Assessment tax bill). For a cheque or Bacs payment, that means it must arrive by Friday 29 January 2027. Bacs payments usually take 3 working days.
• Clients are harder to reach. Approvals, missing figures and signatures need a client who is at their desk. Plan for those to be finished by midweek.
• Your own team needs the weekend back. A Friday cut-off protects the people doing the work.

The practical rule: aim to have every return approved by Wednesday 27 January and filed by Friday 29 January 2027. Use the weekend only for genuine last-minute cases.

The season in four phases

Each phase below ends with the question to ask at that point. When is the Self Assessment deadline for your practice? Earlier than HMRC's, if you want a calm January.

October: build the chase list and capacity plan

October is the planning month. Before anything else:

1. List every client who needs a 2025/26 return, including company directors, landlords and partners who file personally. Add anyone who registered late, and note their HMRC filing date if it differs.
2. Sort them into four groups: records already in, records expected, usually late, and new this year.
3. Count your capacity. Divide the returns you still need to prepare by the working weeks left before 29 January and by the number of people preparing them. That number is your weekly target. Write it down.
4. Check authorisation for every new client before you need to talk to HMRC about them.
5. Spot clients who will need Making Tax Digital next year. Sole traders and landlords with qualifying income over £30,000 for 2025/26 must use Making Tax Digital for Income Tax from 6 April 2027. Qualifying income is turnover from self-employment and property, before expenses (GOV.UK: work out your qualifying income). You are looking at exactly those figures now, so flag them as you go.
6. Send the first records request with a clear date for the records to arrive.

November: first chasers and early returns

• Send a friendly chaser to everyone still in "records expected" around the middle of the month.
• Prepare the simplest returns first. Each one finished in November is one fewer in January.
• Clients already in Making Tax Digital for Income Tax (qualifying income over £50,000 for 2024/25) have a quarterly update due by Saturday 7 November 2026, covering 6 April to 5 October 2026 on standard update periods (GOV.UK: send quarterly updates). HMRC will not apply penalty points for late quarterly updates in the 2026/27 tax year, but the updates must still be sent before the client can submit their 2026/27 return. Their 2025/26 return is filed in the normal way by 31 January 2027.

December: second chasers and approvals

• 30 December 2026 is the last day to file online if an eligible client wants their bill collected through their tax code. They must owe less than £3,000 and already pay tax through PAYE, for example as an employee or pensioner (GOV.UK).
• Send a firmer chaser to the "usually late" group with your own cut-off date for records.
• Plan holiday cover. In England and Wales the bank holidays are Friday 25 December, Monday 28 December (substitute day for Boxing Day) and Friday 1 January (GOV.UK: bank holidays).
• Start sending finished returns to clients for approval as soon as they are ready, not in a January batch.

January: final notices, approvals and filing

• Week one: final notice to anyone whose records are still outstanding.
• Send each return for approval with the tax figure and clear payment instructions, including any first payment on account for 2026/27.
• File by Friday 29 January 2027.
• Remind clients paying by cheque or Bacs that their payment must reach HMRC by Friday 29 January. Faster Payments and card payments can be made over the weekend.
• For clients in Making Tax Digital, the next quarterly update is due by Sunday 7 February 2027, one week after the Self Assessment deadline.

Week-by-week Self Assessment season plan

Fill in the last column with your own numbers from the October capacity count. We have deliberately left it blank: your target depends on your client list, not on an average.

Track every return by stage, not by memory

A list of names is not a plan. What makes a season manageable is knowing, at a glance, where each return is stuck. Six stages cover the whole journey:

Run your chasers from the "Waiting on client" list and your approval reminders from the "With client" list, and the season stops depending on memory.

These are the stages in Mipp's Compliance view. It lists every client's Self Assessment return with its stage, shows counters for Overdue, Due this week and Due this month, groups deadlines into Overdue, Due this week, Due this month and Later, and lets you move several returns to a new stage, mark them done or push their dates in one go. Mipp tracks where each return is; you still send your own emails to clients and file through your usual tax software.

A 10-point checklist for the Self Assessment season

1. List every client who needs a 2025/26 return, including directors, landlords and partners.
2. Note any client who registered after 5 October 2026 and their HMRC filing date.
3. Check you are authorised to act for every new client.
4. Count returns per person per week until Friday 29 January 2027.
5. Send records requests with a clear records date.
6. Flag sole traders and landlords with qualifying income over £30,000 for 2025/26: they need Making Tax Digital from 6 April 2027.
7. File eligible PAYE clients by 30 December 2026 if they want their bill collected through their tax code.
8. Book approval slots with clients before the holidays.
9. Tell clients what they owe on 31 January 2027, including any first payment on account, and that cheque or Bacs payments must arrive by Friday 29 January.
10. Keep every return at a stage so you always know who to chase.

Setting this up in Mipp

If you want to run this plan in Mipp, our accountancy practice management software for small firms, rather than a spreadsheet:

• Import your client list from a CSV file, up to 500 rows at a time. Include a "Self assessment (yes/no)" column and each client's Self Assessment return is scheduled as they come in.
• Tick "MTD for Income Tax" on a client's profile and Mipp adds their quarterly update dates and final declaration alongside the return.
• Use the January capacity plan reminder. Accountancy workspaces get a "January capacity plan" item in the Compliance view's My practice tab, a nudge to do the capacity count in this article.
• Subscribe to the private calendar feed so every deadline appears in Google, Outlook or Apple calendars.

Deadline dates in Mipp are a planning aid, calculated from the information you enter using standard UK rules. Always check them against HMRC and Companies House. Mipp does not provide tax advice.

Frequently asked questions

When is the Self Assessment deadline for 2025/26?

The online deadline is 11:59pm on 31 January 2027. Paper returns had to reach HMRC by 11:59pm on 31 October 2026. Tax owed for 2025/26 is also due by 31 January 2027 (GOV.UK).

Does the deadline move because 31 January 2027 is a Sunday?

No. The online filing deadline is still 11:59pm on 31 January 2027. Payments are different: if a payment deadline falls on a weekend, the payment must reach HMRC on the last working day before, Friday 29 January 2027, unless it is made by Faster Payments or by debit or corporate credit card (GOV.UK).

When do clients have to pay the tax they owe?

By 31 January 2027 for the balancing payment for 2025/26 and any first payment on account for 2026/27, and by 31 July 2027 for the second payment on account. Payments on account are not needed if the client's tax bill for the previous year was under £1,000, or if more than 80% of their tax was paid outside Self Assessment, for example through their tax code (GOV.UK: payments on account).

What is the penalty for filing a 2025/26 return late?

An initial £100 penalty. After 3 months, £10 a day up to a maximum of £900. After 6 months, a further 5% of the tax due or £300, whichever is greater, and the same again after 12 months. Late payment brings penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months, plus interest (GOV.UK: penalties). See GOV.UK for how penalties for late Self Assessment returns work, and for penalties under Making Tax Digital for Income Tax.

When should a practice start chasing clients for records?

In October. Send the first request with a records date, a friendly chaser in mid-November, a firm chaser with your cut-off in early December and a final notice in the first week of January. The week-by-week table above shows the full sequence.

Do clients in Making Tax Digital still file a 2025/26 return by 31 January 2027?

Yes. Clients who started Making Tax Digital for Income Tax from 6 April 2026 file their 2025/26 return in the normal way, and the current penalties apply to it (GOV.UK). Their quarterly updates for 2026/27 run alongside, due 7 November 2026, 7 February 2027 and 7 May 2027 after the first one in August.

How can a small practice track which clients still owe records?

Keep every return at a stage and run your chasers from the "Waiting on client" list. A shared spreadsheet works if it is kept up to date; a tool such as Mipp's Compliance view does it with counters and bulk updates.

Plan your season in Mipp

See every client's Self Assessment deadline in one view. Start a 14-day free trial. Plans start at £19 a month for one person; see pricing. A card is taken at signup and nothing is charged until day 15.

Using a spreadsheet today? See how Mipp works as a CRM for accountants or as practice management software for small firms. Running a bookkeeping practice? See Mipp for bookkeepers.

Sources

All checked on 8 October 2026. Accurate as of October 2026.

1. GOV.UK, Self Assessment tax returns: deadlines. https://www.gov.uk/self-assessment-tax-returns/deadlines
2. GOV.UK, Self Assessment tax returns: penalties. https://www.gov.uk/self-assessment-tax-returns/penalties
3. GOV.UK, Register for Self Assessment. https://www.gov.uk/register-for-self-assessment
4. GOV.UK, Pay your Self Assessment tax bill (including weekend rule and payment times). https://www.gov.uk/pay-self-assessment-tax-bill
5. GOV.UK, Pay your Self Assessment tax bill: through your tax code. https://www.gov.uk/pay-self-assessment-tax-bill/through-your-tax-code
6. GOV.UK, Understand your Self Assessment tax bill: payments on account. https://www.gov.uk/understand-self-assessment-bill/payments-on-account
7. GOV.UK, Find out if and when you need to use Making Tax Digital for Income Tax. https://www.gov.uk/guidance/find-out-if-and-when-you-need-to-use-making-tax-digital-for-income-tax
8. GOV.UK, Work out your qualifying income for Making Tax Digital for Income Tax. https://www.gov.uk/guidance/work-out-your-qualifying-income-for-making-tax-digital-for-income-tax
9. GOV.UK, Use Making Tax Digital for Income Tax: send quarterly updates. https://www.gov.uk/guidance/use-making-tax-digital-for-income-tax/send-quarterly-updates
10. GOV.UK, Penalties for Making Tax Digital for Income Tax. https://www.gov.uk/guidance/penalties-for-making-tax-digital-for-income-tax
11. GOV.UK, UK bank holidays. https://www.gov.uk/bank-holidays