Self Assessment Calendar: 2025/26
Staying on top of Self Assessment deadlines is critical for accountants managing multiple clients. Missing a single date can trigger penalties that compound quickly. Here's your complete calendar for the 2025/26 tax year.
Key Dates at a Glance
| Date | Deadline |
|---|---|
| 5 April 2026 | End of the 2025/26 tax year |
| 6 April 2026 | Start of the 2026/27 tax year |
| 5 October 2026 | Register for Self Assessment (if new) |
| 31 October 2026 | Paper return deadline (2025/26) |
| 30 December 2026 | Online return deadline for PAYE coding adjustment |
| 31 January 2027 | Online return deadline + balancing payment due |
| 31 July 2027 | Second payment on account due |
Penalty Structure
HMRC's penalty regime for late Self Assessment filing is straightforward but punitive:
Late Filing Penalties
- 1 day late: £100 fixed penalty
- 3 months late: £10/day for up to 90 days (max £900)
- 6 months late: 5% of tax due or £300 (whichever is greater)
- 12 months late: Further 5% of tax due or £300 (whichever is greater)
Pro tip: The £100 penalty applies even if your client owes no tax or is due a refund. There is no excuse for a late nil return.
Late Payment Penalties
Late payment interest accrues from the due date. Additionally:
- 30 days late: 5% of tax unpaid
- 6 months late: Further 5% of tax still unpaid
- 12 months late: Further 5% of tax still unpaid
ITSA: Making Tax Digital for Income Tax
From April 2026, Making Tax Digital for Income Tax Self Assessment (ITSA) begins its phased rollout:
Phase 1 (April 2026)
Self-employed individuals and landlords with income over £50,000 must:
- Keep digital records
- Submit quarterly updates to HMRC
- File an end-of-period statement
Phase 2 (April 2027)
Extended to those with income over £30,000.
This represents a fundamental shift from annual returns to quarterly digital reporting. Accountants need to prepare clients now for this transition.
Practical Tips for Accountants
1. Start Early
Don't wait until January. Begin gathering client information in April and aim to file by November. This gives time to resolve queries and spreads your workload.
2. Automate Where Possible
Manual data entry across dozens of clients is error-prone and time-consuming. Use software that pulls data directly from digital records and pre-populates returns.
3. Communicate Proactively
Send clients a checklist of what you need well before the deadline. Many delays are caused by waiting for client information, not by the filing itself.
4. Plan for Payments on Account
Ensure clients understand that payments on account are due in January and July. Unexpected tax bills cause the most friction in client relationships.
How ac-co.ai Supports Self Assessment
ac-co.ai is building tools to help accountants manage the Self Assessment workflow efficiently:
- Client deadline tracking across your entire practice
- Digital record integration ready for ITSA quarterly reporting
- Automated data collection from connected sources
- HMRC API filing for direct submission
Planning your practice's transition to ITSA quarterly reporting? Contact us to discuss how ac-co.ai can help.