If you’re filing a UK Self-Assessment for the 2025/26 tax year, here’s every deadline you need to know — and a quick action list under each so you don’t miss anything important.
The dates
- 5 October 2026 — register for Self-Assessment if you’re a new taxpayer (sole trader, new landlord, etc.)
- 31 October 2026 — paper return deadline (if you still file on paper, which we strongly suggest you don’t)
- 30 December 2026 — file online if you want HMRC to collect tax owed (under £3,000) via your tax code next year
- 31 January 2027 — online return deadline AND payment deadline for 2025/26 tax owed AND first payment on account for 2026/27
- 5 April 2027 — last day to make tax-relievable contributions for 2026/27 (pension top-ups, ISA, EIS investments)
- 31 July 2027 — second payment on account for 2026/27
What you’ll need to gather
If you haven’t started yet, here’s a kickoff checklist:
- All P60s, P45s, P11Ds from any employments
- Self-employed bookkeeping (or bank statements + invoices/receipts if you don’t use software)
- Rental income statements and expenses (for landlords)
- Dividend vouchers (from your Ltd company, share investments)
- Interest statements from savings accounts
- Pension contribution statements (especially anything outside the workplace scheme)
- Any property sale completion statements (if disposing of a non-main-residence)
- Charity donation receipts (Gift Aid)
Avoiding penalties
HMRC’s penalty regime is unforgiving and automated:
- 1 day late: £100, even if no tax is owed
- 3 months late: £10/day for 90 days = £900 extra
- 6 months late: another £300 (or 5% of tax due if greater)
- 12 months late: another £300 (or 5% of tax due if greater)
If you’re going to be late, file the return anyway, even with estimates, and amend later. The £100 fixed penalty applies; the daily charges only kick in after 3 months. Don’t ignore it — that’s how penalties spiral into thousands.
When you should ask for help
We see two kinds of people in our office every January:
- People who’ve left it too late and need rescuing
- People who’ve been winging it for years and want to know what they’ve been missing
The reality: if your tax situation involves more than a single PAYE income, an accountant typically pays for themselves through the deductions and reliefs you didn’t know you could claim. And during the rest of the year, we’re a phone call away when something tax-relevant happens — so you don’t end up surprised in January.
If you’d like a free 20-minute review of your situation, book a call — we’ll tell you straight whether we can save you anything.