Self Assessment 2025/26 — deadlines, who files, penalties
UK Self Assessment 2025/26 — who needs to file, the £100 penalty, payments on account, and what changes from MTD in April 2026. Pillar + 6 spokes.
About this hub
This SmartBooks resource hub brings together plain-English guides on self assessment & personal tax for UK accountancy firms, landlords, sole traders and small businesses. Each guide answers one question directly, so you can read only what you need.
There are 7 guides in this hub, listed below with a short summary of each. The hub is kept current as HMRC rules change. To see how SmartBooks supports this work, start free or book a 15-minute demo.
Guides in this hub
Self Assessment 2025/26 — deadlines, who files, penalties
Pillar guide to UK Self Assessment for 2025/26 — due 31 January 2027.
Read the guideDo you need to file a Self Assessment tax return in 2025/26?
The full trigger list — self-employed, directors with untaxed income, landlords, high earners.
Read the guideSelf Assessment deadlines and key dates (2025/26)
Register by 5 Oct 2026, paper return 31 Oct 2026, online + payment 31 Jan 2027, second payment on account 31 Jul 2027.
Read the guideHow to register for and file a UK Self Assessment tax return
Step-by-step — CWF1 / SA1 / SA401, UTR, Government Gateway, the SA100, and how to pay.
Read the guideSelf Assessment penalties — how they escalate
£100 automatic, £10/day after 3 months, 5% surcharges at 6 and 12 months — plus separate late-payment penalties + interest.
Read the guideSelf Assessment payments on account — why your first bill feels double
Advance payments toward next year. Triggered when bill over £1,000 and under 80% taxed at source.
Read the guideSelf Assessment for company directors and dividends
Directors file because of untaxed income, not the role itself. Dividends above £500 trigger it.
Read the guide
Bring your next MTD submission to SmartBooks.
Create a SmartBooks workspace now, or book a demo if you want to walk through the workflow with the team first.