Self Assessment Deadlines & Penalties: What Happens If You Miss the 31 January Deadline?
Every year, millions of UK taxpayers must complete a Self Assessment tax return. Missing the deadline is surprisingly common — and often costly.
If you’re a sole trader, freelancer, landlord, or company director, understanding Self Assessment deadlines, penalties, and interest is essential to avoid unnecessary fines from HMRC.
This guide explains exactly what happens if you miss the deadline, how penalties escalate, and how to stay compliant.
Self Assessment Deadline in the UK
The most important date to remember is:
📅 31 January
By 31 January, you must:
- Submit your online Self Assessment tax return
- Pay any tax owed for the previous tax year
- Pay your first payment on account (if applicable)
Example: For the 2024/25 tax year, the deadline is 31 January 2026.
Missing this date — even by one day — triggers an automatic penalty.
£100 Late Filing Penalty Explained
If your Self Assessment return is submitted after 31 January, HMRC immediately issues a:
❌ £100 fixed late filing penalty
- Applies even if no tax is owed
- Applies even if you are one day late
- Forgetting or being busy is not a valid excuse
This £100 penalty is unavoidable unless HMRC accepts a valid reasonable excuse.
Daily Penalties After 3 Months
If your tax return remains unfiled for 3 months after the deadline, penalties increase significantly.
- £10 per day
- Charged for up to 90 days
- Maximum daily penalty: £900
Total penalties at this stage:
£100 (initial penalty) + £900 (daily penalties) = £1,000
Further Penalties After 6 and 12 Months
- After 6 months: £300 or 5% of tax due (whichever is higher)
- After 12 months: Another £300 or 5% of tax due
- In serious cases, penalties can reach 100% of the tax owed
HMRC is far stricter than in the past — late filing is actively enforced.
Interest on Late Tax Payments
Filing late is one issue — paying late is another.
- Interest charged from 1 February
- Calculated daily
- Rate = Bank of England base rate + 2.5%
Interest continues to build until the full balance is paid — even if you later submit your return.
What Happens If I Miss the Self Assessment Deadline?
- £100 automatic fine issued
- Interest starts on unpaid tax
- Daily penalties after 3 months
- Extra penalties at 6 and 12 months
- HMRC payment demands or enforcement action
Ignoring HMRC letters only makes the situation worse.
Can You Appeal a Self Assessment Penalty?
Yes — but only in limited circumstances.
- Serious illness or hospitalisation
- Bereavement close to the deadline
- HMRC system failures
- Unexpected events beyond your control
Missing the deadline due to poor planning or misunderstanding is not accepted.
How to Avoid Self Assessment Penalties
- Register early for Self Assessment
- Keep accurate income and expense records
- File well before 31 January
- Set aside money for tax payments
- Use a professional accountant
Let 11TaxAssist Handle Your Self Assessment
Managing deadlines, penalties, and HMRC compliance can be stressful — especially when you’re running a business.
11TaxAssist supports UK taxpayers with:
- Self Assessment tax returns
- Penalty prevention and appeals
- Late filing support
- HMRC correspondence
- Ongoing tax planning
👉 Get expert Self Assessment support today with 11TaxAssist