Self Assessment deadlines 2026/27: every key date and what happens if you miss them

Self Assessment 2026/27 at a glance

If you earned income in the 2025/26 tax year that was not taxed at source (self-employment, rental, dividends, foreign income, capital gains), you need to file a Self Assessment return in the 2026/27 filing window. Four dates matter:

Get to 31 January 2027 with the return filed and the tax paid, and you are done. Miss it, and HMRC starts charging penalties from the following day.

5 October 2026: register if this is your first SA year

If you have never filed Self Assessment before, you must register with HMRC by 5 October 2026 if you became liable to file in the 2025/26 tax year. Common triggers:

• You started self-employment
• You started receiving rental income from a property
• You became a company director (in some cases)
• Your total income passed £100,000
• You started receiving foreign or investment income above the allowances
• You had capital gains above the annual exempt amount

Registration is done online at gov.uk/register-for-self-assessment. HMRC then posts your Unique Taxpayer Reference (UTR) within 10 working days (or up to 21 if you live abroad). You need the UTR to log in and file, so register early.

Missing the registration deadline can trigger a 'failure to notify' penalty separate from late filing. The amount depends on how late you register and how much tax is at stake.

31 October 2026: paper return deadline

If you choose to file on paper rather than online, the return must reach HMRC by midnight on 31 October 2026. Posting on the 30th is risky; allow extra days for delivery. The form is SA100, plus supplementary pages depending on your income sources (SA105 for property, SA103 for self-employment, and so on).

Almost no one files on paper now. Online filing gives you three extra months (to 31 January), automatic tax calculation, and an instant submission receipt. The only common reasons to file on paper are very specific (trustees, Lloyd's underwriters, MPs). For everyone else, file online.

31 January 2027: the big one

By midnight on 31 January 2027, three things must be done:

1. Submit your 2025/26 online tax return. Through the HMRC online service or commercial software. You get a submission receipt immediately.
2. Pay the balancing payment for 2025/26. That is the difference between your total 2025/26 tax bill and any payments on account you already made in January and July 2026.
3. Pay the first payment on account for 2026/27. If you are in the payments-on-account system, this instalment is due alongside the balancing payment.

The combined bill on 31 January is often larger than people expect, particularly for newly self-employed taxpayers who do not realise the payment on account is on top of the balancing payment. We strongly recommend running the numbers in October or November so there are no surprises in the January cash flow.

31 July 2027: second payment on account

The second instalment of your 2026/27 tax (calculated from the 2025/26 liability) is due by midnight on 31 July 2027. Same payment methods as January: bank transfer, direct debit, debit card, or through your bank.

If your income has dropped this year and you expect the final 2026/27 bill to be lower than 2025/26, you can ask HMRC to reduce the payments on account. This is done either through your online account or by filing form SA303. Be careful, if you reduce them too far and the final bill is higher, HMRC charges interest from the original due dates.

Penalties if you miss the 31 January deadline

Two penalty regimes run in parallel and you can attract one without the other: late filing and late payment.

Late filing penalties

• 1 day late: automatic £100 penalty, even if you owe no tax
• 3 months late: £10 per day, capped at 90 days (£900 maximum)
• 6 months late: 5% of the tax owed, with a minimum of £300
• 12 months late: a further 5% of the tax owed, minimum £300

So a return filed 12 months late with £5,000 of tax owed attracts £100 + £900 + £300 + £300 = £1,600 in fixed penalties before interest on the unpaid tax.

Late payment penalties

• From 1 February: daily interest on the unpaid tax
• 30 days late: 5% of the unpaid tax
• 6 months late: another 5% surcharge
• 12 months late: a further 5% surcharge

The full breakdown, and how to appeal a penalty if you have a genuine reasonable excuse, is in our late filing penalties guide and late filing vs late payment explainer.

What counts as a 'reasonable excuse'?

HMRC accepts a narrow set of reasons for filing or paying late without penalty. Most of the successful appeals fall into these categories:

• Serious or life-threatening illness around the deadline
• Death of a close relative or partner shortly before the deadline
• An unexpected stay in hospital that prevented filing
• An HMRC system failure (rare, but it happens)
• A fire, flood, or theft that destroyed your records
• Postal delays you could not have predicted (paper filers)

The following are not accepted as reasonable excuses: forgetting, finding the system too difficult, your accountant being late, being too busy at work, or relying on someone else to file for you.

How to pay your Self Assessment bill

HMRC accepts payment by:

• Online or telephone bank transfer (Faster Payments): usually credits the same day if sent before mid-afternoon. The fastest reliable method.
• Direct Debit: set up through your HMRC online account. First-time Direct Debits take up to 5 working days; later payments are quicker.
• Debit card online: usually credits same day. HMRC no longer accepts personal credit cards.
• Through your bank or building society: using a paying-in slip from HMRC. Can take 3 working days.
• By cheque through the post: not recommended for anything close to the deadline. Allow at least 3 working days.

Always include your UTR plus K as the payment reference (for example 1234567890K) so HMRC matches the payment to your account.

Our recommended filing timetable

For an unstressed January, work backwards from 31 January 2027:

Clients who leave it to the last week always pay more, in stress if not in pounds.