Filing your Self-Assessment tax return on time is essential to avoid fines from HM Revenue & Customs (HMRC).
Here’s a straightforward explanation of the key deadlines, how they work, and what happens if you file late.
When You Need to File Your Tax Return
Paper Returns
- If you file a paper return, the deadline is 31 October after the end of the tax year.
Example: The 2024/25 tax year ends on 5 April 2025.
Paper deadline = 31 October 2025.
Online Returns
- If you file online, the deadline is 31 January after the end of the tax year.
Example: Online deadline for 2024/25 = 31 January 2026.
If You Receive a Notice to File Late
If HMRC only sends you a notice to file after 31 October, you get 3 months from the date of the notice to file, whether on paper or online.
Example:
- Notice received on 25 November
- Deadline to file = 25 February
Paper Returns and Tax Calculation
- If you file your paper return by 31 October (or within 2 months of receiving the notice, if later), HMRC will calculate your tax for you if you want.
- If you file your paper return after 31 October but still within the 3-month notice period, you must calculate the tax due yourself.
Penalties for Filing Your Tax Return Late
Even if you owe no tax, late filing can still result in penalties.
Here’s what can happen:
| How Late | Penalty | Details |
| 📆 1 day late | £100 fixed penalty | Applies even if no tax is due |
| 🕒 3 months late | £10 per day | Up to 90 days (max £900) |
| ⏳ 6 months late | 5% of tax due or £300 | Whichever is greater |
| 🗓️ 12 months late | Another 5% or £300 | Whichever is greater |
If the Delay is Deliberate:
- Deliberate (not concealed) → penalty can go up to 70% of the tax due (or £300 if higher).
- Deliberate & concealed → penalty can reach 100% of the tax due (or £300 if higher).
What “Tax Liability” Means
Your tax liability is:
- The total tax due for the year
- Minus any tax already deducted at source
- It’s not reduced by payments made after the year end.
If multiple penalties apply (e.g., failure to notify and late filing), HMRC will make sure the same liability isn’t penalised twice for the same issue.
But — if you also pay the tax late, you’ll face both:
- Late filing penalty
- Late payment penalty
Final Tips to Avoid Penalties
- Mark your calendar for 31 October (paper) and 31 January (online).
- If you get a late notice, remember the 3-month rule.
- File early to avoid last-minute issues.
- Even if you can’t pay the tax right away, filing on time can help reduce penalties.
In short:
Filing your return late can be expensive. A simple £100 fine can grow quickly with daily penalties, percentage charges, and interest.
If you’re unsure when or how to file, it’s best to get advice early or use a professional tax agent to keep you on track.
