Different penalty regime under MTD. If you're MTD-mandated, HMRC's new points-based late-submission penalties + tiered late-payment penalties replace the old flat-fee system. See MTD Penalties Explained for the new rules.
The three deadlines that matter
UK Self Assessment has three deadlines that trigger penalties if missed: 5 October (register for the tax year you started), 31 January (file online + pay balancing tax), and 31 July (second payment on account). The penalty ladder escalates from £100 to potentially thousands. This guide covers the exact schedule and what actually gets charged.
The three deadlines that matter
5 October following the tax year you started self-employment — deadline to register.
31 January following the tax year end — deadline to file online + pay the balancing tax + first payment on account (if applicable).
31 July — deadline for the second payment on account.
Paper filing is 31 October — three months earlier than online. Not recommended for UK freelancers today.
The penalty ladder (2025/26 rules)
Late filing: - Day 1 after deadline: £100 flat penalty. - 3 months late: additional £10/day (up to £900). - 6 months late: additional 5% of tax due (or £300, whichever higher). - 12 months late: another 5% or £300.
Late payment: - 30 days late: 5% of tax owed. - 6 months late: additional 5%. - 12 months late: additional 5%. - Interest also accrues from day 1 at HMRC's official rate.
Failure to Notify (missed registration): - Percentage of tax owed, ranging 0-100% depending on culpability and disclosure timing.
How penalties actually get calculated
The £100 flat penalty applies even if you owed no tax that year (i.e. penalty for late-filing a nil return). It's automated — no HMRC judgment. You can appeal a late-filing penalty if you have a 'reasonable excuse' (serious illness, bereavement, HMRC error), which HMRC assesses case-by-case.
Interest on unpaid tax is at HMRC's late-payment rate (currently 7.75% — Bank of England base + 2.5%). It compounds. A £5,000 unpaid bill accrues around £387 of interest over 12 months.
Worked example
You owed £4,200 for 2024/25 but filed 8 days late and paid 53 days late. Late-filing penalty: £100 flat (immediate). Late-payment: 5% of £4,200 = £210 (triggered at day 30, i.e. 2 March). Interest at 7.75% for 53 days: ~£47. Total penalty + interest: £357 on top of the £4,200 tax. Filing on time and setting up a Time to Pay arrangement would have avoided the 5% surcharge — the flat £100 late-filing penalty is the only unavoidable cost if you'd filed on time but couldn't pay.
Reference table
| Delay | Late filing penalty | Late payment penalty | Interest |
|---|---|---|---|
| 1 day | £100 flat | None yet | None yet |
| 30 days | £100 flat | 5% of tax owed | Accruing |
| 3 months | £100 + £900 (£10/day) | 5% of tax owed | Accruing |
| 6 months | £1,000 + 5% (min £300) | 10% total (2 × 5%) | Accruing |
| 12 months | £1,000 + 10% (min £600) | 15% total (3 × 5%) | Accruing |
Deadline-management checklist
- 5 October: registration deadline (year of starting)
- December: gather records and start return early
- 31 January: filing + balancing payment + payment on account 1
- 31 July: payment on account 2
- Set up HMRC direct debit for automatic payment
- Enable HMRC email notifications for deadline reminders
- If cash-tight: request Time to Pay before the deadline, not after
Yes. It's a flat penalty for late filing regardless of tax owed. Only avoided by filing on time or having HMRC accept a reasonable-excuse appeal.
HMRC's payment plan for tax debt — spread the bill over up to 12 months. Interest applies but no additional penalties provided you set it up on or before 31 January and stick to the plan.
Serious illness (yours or close family), bereavement, HMRC IT failure, natural disaster affecting records. Not: forgot, was busy, didn't know.
Yes — you avoid the late-payment penalty ladder. You still get the £100 late-filing penalty but save 5-15% on the tax amount.
No. HMRC penalties are not allowable business expenses.
Sources & official references
Related on FreelanceToolkit UK
Deadline countdown
Interactive tool. Estimates only — always verify with the full calculator or HMRC guidance for final decisions.
This guide is general information based on UK rules for the 2025/26 tax year. It is not personal tax or legal advice. For decisions affecting your tax position or legal exposure, consult a qualified accountant or solicitor.