Self Assessment 2026/27: Deadlines & Penalties

HMRC Self Assessment 2026/27 - who must file, the 5 October registration deadline, 31 January online filing, how to pay, and the penalties for missing a date.

Most UK employees never touch Self Assessment — PAYE handles their tax. But if you’re self-employed, a landlord, a high earner with untaxed income, or have specific situations like HICBC or capital gains, HMRC expects you to file an annual Self Assessment return. This guide covers who, when, and what happens if you miss a deadline for the 2026/27 tax year.

Who must file Self Assessment for 2026/27

HMRC requires a return if, in the tax year 6 April 2026 – 5 April 2027, you:

  • Were self-employed with gross trading income above £1,000 (you can claim the Trading Allowance to offset small amounts)
  • Were a partner in a business partnership
  • Received more than £10,000 from dividends (or have investment tax to pay even under that)
  • Received untaxed rental income above £1,000 (or above the Rent-a-Room scheme limit for lodgers)
  • Had capital gains above the annual exempt amount and need to pay CGT
  • Earned more than £150,000 from any source (even all-PAYE) and HMRC has asked you to file
  • Owe High Income Child Benefit Charge (HICBC) because your adjusted net income exceeded £60,000 and your household claims Child Benefit (see the HICBC guide)
  • Had foreign income that requires UK declaration
  • Are a director of a limited company and HMRC has requested a return
  • Need to claim tax relief (pension higher-rate top-up, Marriage Allowance in some cases, gift aid on higher-rate income)

If unsure, HMRC’s self-assessment tool walks you through a few questions and gives a yes/no.

The 2026/27 deadlines in order

5 October 2027 — register for Self Assessment if it’s your first time filing for the 2026/27 tax year. Missing this deadline alone isn’t fined, but it pushes the rest of your timeline.

31 October 2027 — deadline for paper returns for 2026/27. Almost no one files on paper anymore; if you do, this is your cut-off.

31 January 2028 — deadline for online returns for 2026/27 AND the deadline to pay any tax owed for 2026/27. Most people file and pay on or near this date.

31 July 2028 — second payment on account for 2026/27 (if you have payments on account — see below).

Payments on account — the quirk that catches people out

If your 2026/27 tax bill is over £1,000, HMRC typically asks for two payments on account toward the following year’s tax — half due 31 January 2028 and half due 31 July 2028. Each payment is 50% of the previous year’s liability.

So in January 2028 you may owe:

  1. Balancing payment for 2026/27 (the actual bill)
  2. First payment on account for 2027/28 (half the 2026/27 bill)

This effectively bunches 1.5 years of tax into one January. Plan cash flow accordingly. You can reduce payments on account if you know 2027/28 will be lower, but overestimate and you’ll pay interest on the shortfall.

Penalties if you miss deadlines

Late filing (after 31 January 2028 online):

  • 1 day late — £100 automatic penalty (even if no tax owed)
  • 3 months late — £10/day for up to 90 days (max £900)
  • 6 months late — 5% of tax due or £300, whichever is higher
  • 12 months late — another 5% of tax due or £300

Late payment (after 31 January 2028):

  • 30 days late — 5% of unpaid tax
  • 6 months late — another 5%
  • 12 months late — another 5%
  • Plus daily interest from the due date at HMRC’s current rate

Penalties stack. Filing late AND paying late creates two separate penalty tracks. HMRC accepts “reasonable excuse” appeals but the bar is high (serious illness, bereavement, HMRC system outage).

How to file

  1. Register first time: apply on gov.uk/register-for-self-assessment. HMRC posts a Unique Taxpayer Reference (UTR) and an activation code for the online service. Allow 2-3 weeks.
  2. Gather documents: P60, P45(s), P11D, CIS statements, bank interest certificates, dividend vouchers, rental income/expense records, pension contribution receipts, Gift Aid declarations.
  3. Log in at gov.uk/log-in-file-self-assessment-tax-return using your Government Gateway credentials.
  4. Complete the return — supplementary pages appear based on what you declare. Self-employment uses the SA103, UK property uses SA105, etc.
  5. Submit — you’ll see your tax liability immediately. HMRC issues a receipt with payment reference.
  6. Pay via bank transfer, Direct Debit, online banking, or card. Payment reference format is your UTR followed by “K”.

Common mistakes that trigger HMRC follow-up

  • Missing bank interest — banks report directly to HMRC via common reporting standards. If your return omits interest they see, they’ll notice.
  • Dividend under-declaration — same story; brokers report to HMRC.
  • Untaxed rental income — HMRC cross-references with Land Registry and letting-agent data.
  • Capital gains on property disposal — must be reported within 60 days of completion on a separate property CGT return as well as on Self Assessment.
  • High-earner PAYE without self-assessment — if HMRC has previously asked you to file, you must keep filing until they formally remove the obligation.

MTD for Income Tax — changes coming

From 6 April 2026, Making Tax Digital for Income Tax (MTD ITSA) becomes mandatory for sole traders and landlords with qualifying income above £50,000. Instead of a single annual return you’ll submit quarterly updates through compatible software, then finalise the year with an end-of-period statement. See the MTD guide for the full roll-out.

If you’re under the £50,000 threshold, 2026/27 Self-Assessment continues as described above.

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