Self Assessment deadlines and penalties for 2025/26

The key dates

Your 2025/26 tax return covers the tax year that ran from 6 April 2025 to 5 April 2026. The deadlines you need to know are:

• 5 October 2026 — deadline to register for Self Assessment if you're new to it
• 31 October 2026 — deadline for paper tax returns
• 31 January 2027 — deadline for online returns and for paying any tax you owe for 2025/26
• 31 January 2027 — first payment on account for 2026/27 (if applicable)

What happens if you file late

HMRC's late-filing penalties for a standard Self Assessment return escalate the longer you leave it:

• The day after the deadline: an automatic £100 penalty, even if you owe nothing
• 3 months late: £10 a day, up to 90 days — a further £900 maximum
• 6 months late: another £300, or 5% of the tax due if that's higher
• 12 months late: a further £300, or 5% of the tax due if higher

These penalties apply even if you have no tax to pay, and they're on top of any late-payment interest and penalties on the tax itself.

Late payment costs too

If you pay your tax bill late, HMRC charges interest from the due date, plus a penalty of 5% of the unpaid tax at 30 days, again at 6 months, and again at 12 months. Interest rates change in line with the Bank of England base rate, so the longer a balance sits unpaid, the more it grows.

Payments on account

If your last bill was over £1,000, HMRC usually asks for two "payments on account" towards next year's tax — due 31 January and 31 July — each worth half your previous bill. It catches a lot of people out in their first year, so it's worth planning for.

How to stay ahead of it

• Register early if it's your first return — getting a UTR can take a couple of weeks
• Keep your income and expense records up to date through the year
• Get your figures to your accountant well before Christmas, not in late January
• Set money aside for the bill as you go, so 31 January isn't a shock

The simplest way to never see a penalty is to get your figures in early. We send clients a short checklist, prepare the return, show you exactly what you owe, and file it once you're happy — for a fixed £298.80 a year.