Self Assessment Penalties — How to Avoid Them

Missing a Self Assessment deadline isn’t just stressful — it’s expensive. HMRC’s penalty system means that even a single day late can trigger an automatic fine, and the charges mount up quickly the longer things are left. Here’s how the penalties work and what you can do to avoid them.

The Key Self Assessment Deadlines

Deadline	What It’s For
5 October	Register for Self Assessment if you’re new to it
31 October	Paper tax return submission deadline
31 January	Online tax return deadline and tax payment due
31 July	Second payment on account (if applicable)

The 31 January deadline is the one most people miss. It covers both the submission of your online Self Assessment return and payment of any tax owed — including your first payment on account for the following year if required.

What Are the Penalties for Missing the Deadline?

Late Filing Penalties

How Late	Penalty
1 day late	£100 automatic penalty
3 months late	£10 per day (up to 90 days = £900)
6 months late	Additional £300 or 5% of tax owed (whichever is higher)
12 months late	A further £300 or 5% of tax owed — or up to 100% if HMRC believes you’re deliberately withholding information

Late Payment Penalties

• 30 days late: 5% of the unpaid tax
• 6 months late: A further 5%
• 12 months late: A further 5%

On top of all of this, HMRC charges interest on any unpaid tax from the day it was due. You can find the current interest rates on HMRC’s rates and allowances page.

What If You Can’t Pay Your Tax Bill?

The important thing to know is that filing your return and paying your tax bill are two separate things. Even if you can’t pay what you owe, you should still file your return on time to avoid the late filing penalty on top of the late payment penalty.

If you’re struggling to pay, HMRC’s Time to Pay arrangement lets you spread the cost over a period of time. You need to contact HMRC to set this up — ideally before the deadline.

Can Penalties Be Appealed?

Yes — but only if you have a genuine “reasonable excuse.” HMRC accepts appeals in cases such as serious illness, a family bereavement, or technical failures with HMRC’s own systems. Forgetting, being busy, or not knowing about the deadline are not considered reasonable excuses.

How to Avoid Self Assessment Penalties

• File early — you can submit your return as soon as the tax year ends on 5 April, even though the deadline isn’t until 31 January. Filing early means you know exactly what you owe with plenty of time to plan
• Set money aside throughout the year — a common rule of thumb is to save around 20–30% of your profit to cover your tax bill
• Keep your records up to date — good bookkeeping means your return is straightforward to prepare and less likely to contain errors
• Claim all your allowable expenses — read our guide on what expenses sole traders can claim to make sure you’re not paying more tax than you need to
• Don’t ignore letters from HMRC — if they write to you, respond promptly. Ignoring correspondence can escalate problems quickly

What About MTD for Income Tax?

From April 2026, Making Tax Digital for Income Tax introduces quarterly reporting for self-employed individuals with income over £50,000. MTD carries its own points-based penalty system for missed quarterly submissions — separate from the existing Self Assessment penalties. Read our full guide on MTD for Income Tax to find out what you need to do.

Let PBAS Handle It For You

The simplest way to avoid Self Assessment penalties is to have someone manage it for you. At PBAS, we prepare and file Self Assessment returns for sole traders and self-employed professionals — accurately, on time, every year. No penalties, no stress, and no nasty surprises.