Self Assessment penalties: how much you’ll pay if your tax return is late in 2025/26

The truth is that Self Assessment penalties are very predictable, very automated, and often much harsher than people expect. HMRC does not look at your circumstances before issuing them. The penalties simply switch on once deadlines are missed.

In this guide, I will explain in plain English what the penalties are, how much they cost in real pounds, and what you can still do if you are late or about to be late. No scare tactics, no judgement, just clear information and practical next steps.

Quick overview of Self Assessment penalties

If you miss a Self Assessment deadline, you can be hit with more than one type of penalty at the same time. This is where people get caught out.

Here is the simple overview.

You may face penalties for:

• Filing your tax return late
• Paying your tax bill late
• Interest charged daily on unpaid tax
• Errors or inaccuracies on your return

Each of these stacks up separately. Filing late and paying late are treated as two different problems by HMRC.

Late filing penalties: how the £100 fine and extra charges work

What is the £100 Self Assessment penalty?

If you miss the online filing deadline of 31 January, HMRC automatically issues a £100 late filing penalty.

This applies even if:

• You owe no tax
• You are due a refund
• You only missed the deadline by one day

There is no grace period.

Timeline of late filing penalties

Here is how late tax return fines build up over time.

Day 1 late (from 1 February)

• £100 fixed penalty

3 months late

• £10 per day penalty
• Charged for up to 90 days
• Maximum of £900

6 months late

• Additional penalty of £300 or 5 percent of the tax due, whichever is higher

12 months late

• Another £300 or 5 percent of the tax due
• Higher penalties if HMRC believes the delay was deliberate

Example: filing 2 months late

You file your tax return at the end of March.

Penalties:

• £100 late filing penalty
• No daily penalties yet

Total penalty: £100

Example: filing 8 months late

You file your return in September.

Penalties:

• £100 initial penalty
• £900 daily penalties (maximum reached)
• £300 six-month penalty

Total penalty so far: £1,300

This is before any late payment penalties or interest.

Late payment penalties and interest: what if you pay your tax bill late?

Filing your return and paying your tax are two separate deadlines. You can file on time and still be penalised if you do not pay.

How Self Assessment late payment penalties work

HMRC charges late payment penalties based on how long the tax remains unpaid.

The key dates are:

• 30 days late
• 6 months late
• 12 months late

Penalty percentages

• 5 percent of unpaid tax after 30 days
• Another 5 percent after 6 months
• Another 5 percent after 12 months

That is up to 15 percent in penalties alone.

Interest on unpaid tax

On top of penalties, HMRC charges daily interest on unpaid tax. This interest starts from 1 February and continues until the tax is paid in full.

Interest rates can change, but it adds up quietly in the background.

Example: £2,000 tax bill paid 3 months late

Let’s say:

• Tax due: £2,000
• Paid in late April

Costs:

• Late payment penalty after 30 days: £100 (5 percent of £2,000)
• Interest for roughly 3 months: around £25 to £30

Total extra cost: around £130

Example: £5,000 tax bill paid after 8 months

Costs:

• 5 percent penalty after 30 days: £250
• 5 percent penalty after 6 months: £250
• Interest: roughly £150 to £200

Total extra cost: around £650 to £700

These figures rise further if payment goes past 12 months.

Penalties for mistakes on your tax return

Self Assessment penalties are not only about lateness. HMRC can also penalise you for errors.

Careless vs deliberate mistakes

HMRC looks at why the mistake happened.

• Careless mistakes include missing income, misunderstanding rules, or using wrong figures
• Deliberate mistakes include knowingly leaving income out or inflating expenses

The penalties depend on:

• Whether the mistake was careless or deliberate
• Whether you told HMRC yourself
• How much tax was underpaid

What this means in practice

For careless errors:

• HMRC can charge up to 30 percent of the extra tax owed

For deliberate errors:

• Penalties can reach 70 percent or more

The good news is that honest mistakes can usually be corrected by amending the return. Doing this quickly and openly reduces penalties significantly.

Using a service that checks your return before submission greatly reduces this risk.

Can HMRC reduce or cancel penalties?

Sometimes, yes. But only in specific circumstances.

What counts as a reasonable excuse?

HMRC may cancel penalties if you had a genuine reason for missing the deadline and acted promptly afterwards.

Common examples that may count:

• Serious illness or hospitalisation
• Bereavement close to the deadline
• Technical issues with HMRC systems
• Unexpected events that made filing impossible

Examples that usually do not count:

• Forgetting the deadline
• Being too busy at work
• Not understanding the rules
• Waiting for information that you could reasonably estimate

HMRC is strict. Evidence helps.

What to do if you are late or think you will be

If you are already late or fear you will be, the worst thing to do is nothing.

Here is what actually helps.

1. File the return as soon as possible

Late filing penalties stop increasing once the return is submitted.

Even if you cannot pay yet, filing reduces the damage.

2. Pay what you can

Paying something reduces:

• Late payment penalties
• Daily interest

HMRC prefers partial payment to no payment.

3. Set up a Time to Pay arrangement

If you cannot pay in full, you may be able to agree a Time to Pay plan with HMRC.

This spreads payments over several months and can prevent further penalties if arranged early.

4. Keep records and evidence

If you believe you have a reasonable excuse, keep evidence and contact HMRC promptly.

Feeling overwhelmed? You are not alone

Most people who incur Self Assessment penalties are not trying to avoid tax. They are busy, unsure, or overwhelmed.

This is why many choose to use an online Self Assessment filing service, especially when deadlines are close.

A good service can:

• File your return quickly and correctly
• Reduce the risk of mistakes and penalties
• Make sure all income is declared properly
• Charge a clear fixed fee with no surprises

If time or confidence is an issue, getting help can cost far less than the penalties you risk.

Final thoughts on Self Assessment penalties

Self Assessment penalties are not random and they are not personal. They follow strict rules, fixed timelines, and automatic systems.

The longer you wait, the more expensive it gets. But the moment you act, the damage starts to slow down.

If you are worried about late tax return fines, HMRC penalties for late filing, or Self Assessment late payment penalties, the best move is to deal with it now rather than hoping it will go away.

And if you want it handled properly without stress, getting started online can be the simplest and safest option.