# Self Assessment late filing and late payment penalty calculator (2026 rates)

> Filing date, tax due and payment date in; the £100 penalty, daily penalties, 6 and 12 month charges, late payment penalties and interest out.

- Page to cite: https://aeolookup.com/accountants/self-assessment-late-filing-penalty-calculator/
- Publisher: AEO Lookup (https://aeolookup.com/)
- Section: Accountancy practices, Deadlines and penalties
- Published 1 October 2026
- Key fact: On the GOV.UK scale, a return filed on 20 April with £4,800 of tax paid the same day costs £340 in penalties (£100 for the return, £240 for paying more than 30 days late) and about £80 in interest; filed and paid on 20 August it costs £1,780 in penalties plus about £205 in interest, because the daily penalties and the 6-month charges have arrived.
- Sources: GOV.UK "Self Assessment tax returns: penalties" and "Rates and allowances: HMRC interest rates" (7.75% from 9 January 2026; base rate plus 4% since 6 April 2025), both read 2026-10-01.

A client who files in April thinks the damage is £100. It is, for the return. The tax is a separate clock, and the interest runs on top. The calculator works both clocks and adds them, so you can show a late client what waiting another month costs.

**Free tool on the page:** Self Assessment late filing and late payment penalty estimator. It runs in the browser at https://aeolookup.com/accountants/self-assessment-late-filing-penalty-calculator/#tool and asks for no sign-up.

## Late filing: four steps

The GOV.UK scale for a late Self Assessment return:

1. **£100** the day after the deadline, even if no tax is due.
2. **From 3 months late: £10 a day**, for up to 90 days, so up to £900.
3. **At 6 months late:** 5% of the tax due or £300, whichever is greater.
4. **At 12 months late:** another 5% or £300, whichever is greater, and more if HMRC decides the delay was deliberate.

This is the scale for returns due before April 2027. GOV.UK's page on penalties for Making Tax Digital for Income Tax, updated 30 March 2026, says that from April 2027 the new points-based penalties will apply to everyone who submits a personal Self Assessment return, so check which regime a late return falls under before quoting these figures for a later year.

A return filed on 20 April is only at step 1 for the return, though the tax paid that day is already past the 30-day payment penalty below. One filed on 20 August has collected step 2 in full and step 3: on £4,800 of tax, that is £100 plus £900 plus £300, which is £1,300 for the return alone.

## Late payment: three steps and interest

Late payment is charged separately:

- **5%** of the tax unpaid 30 days after the deadline;
- another **5%** of what is still unpaid at 6 months;
- another **5%** at 12 months;
- **interest** every day from the deadline, at Bank Rate plus 4% since 6 April 2025, which is 7.75% from 9 January 2026.

On £4,800 paid on 20 August, that is two 5% charges (£480) plus about £200 of interest. Add the filing penalties and the client has paid about £1,980 for a six-and-a-half-month delay.

## What the calculator does not model

- **Payments on account.** The tax figure here is the balancing payment. Payments on account due on 31 January and 31 July carry interest but not the late payment penalties.
- **Reasonable excuse.** HMRC can cancel a penalty where there was a reasonable excuse, and the appeal should be made as soon as the return is in.
- **The points-based regime.** Clients in MTD for Income Tax move to penalty points for late updates. Our [MTD deadline checker](https://aeolookup.com/accountants/mtd-income-tax-quarterly-deadline-checker/) covers it.
- **Partnerships.** Partnership returns carry penalties for each partner.
- **Paper returns.** The paper deadline is 31 October, three months earlier, and the same scale runs from that date.

## The two clocks, side by side

| Months late | Filing penalties (on £4,800 tax) | Payment penalties | Interest at 7.75% |
|---|---|---|---|
| 1 | £100 | £240 | about £29 |
| 3 | £100 | £240 | about £91 |
| 6 | £1,000 (daily penalties running) | £240 | about £185 |
| 7 | £1,300 | £480 | about £216 |
| 12 | £1,300 | £480 | about £372 |
| 13 | £1,600 | £720 | about £398 |

The jumps are at 3 months, when the daily penalties start, and at 6 and 12 months, when both clocks charge at once. A client who is going to be late is best told that the cheapest late month is the first and the most expensive are the seventh and thirteenth.

## Using it with a client

The useful thing is the comparison. Run it with today's date, then with the date the client says they will get the records to you, and send both. A difference of a few hundred pounds does what a reminder letter cannot. The [records request letter maker](https://aeolookup.com/accountants/records-request-letter-template-accountants/) has a final-letter stage that says the penalty is the client's, not the practice's, which is the line to hold.

This is general information, not tax advice.

## Short answers from this page

**What is the penalty for filing a Self Assessment return late?** £100 as soon as the return is late. After three months, £10 a day for up to 90 days. At six months and again at twelve months, 5% of the tax due or £300 if greater. Late payment adds 5% of the unpaid tax at 30 days, six months and twelve months, plus interest. A return filed on 20 April with £4,800 of tax paid that day costs £340 in penalties and about £80 in interest.

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General information for UK small firms, not legal, tax or financial advice.