Paying your Self-Assessment tax late is one of the most common and most avoidable financial mistakes made by self-employed individuals and company directors in the UK. HMRC imposes automatic penalties and interest charges from the first day after the payment deadline passes, and those charges accumulate quickly. This guide explains exactly what penalties apply in 2025/26, the payment deadlines you need to meet, and the steps you can take to reduce or appeal a penalty.
Self-Assessment Payment Deadlines 2025/26
| What Is Due | Deadline | What Happens If You Miss It |
| First payment on account (50% of previous year’s tax) | 31 January 2025 | Interest from 1 February 2025 |
| Second payment on account (50% of previous year’s tax) | 31 July 2025 | Interest from 1 August 2025 |
| Balancing payment (any tax still owed for 2024/25) | 31 January 2026 | Penalties + interest from 1 February 2026 |
| First payment on account for 2025/26 | 31 January 2026 | Interest from 1 February 2026 |
HMRC Late Payment Penalties The Penalty Timeline
| How Late | Penalty | Basis | Notes |
| 1 day – 30 days late | Interest only | 2.5% above BoE base rate | No fixed penalty yet |
| 30 days late | 5% | % of unpaid tax | Fixed penalty added |
| 6 months late | 5% further | % of unpaid tax | 2nd fixed penalty |
| 12 months late | 5% further | % of unpaid tax | 3rd fixed penalty — deliberate non-payment can be 70–100% |
Example: If you owe £10,000 in Self-Assessment tax and pay 6 months late, you will face: 5% penalty at 30 days (£500) + 5% penalty at 6 months (£500) + daily interest for 6 months (approximately £200–£300 depending on Bank of England rate) = total additional cost of approximately £1,200–£1,300.
What Is the HMRC Late Payment Interest Rate?
HMRC charges interest on late Self-Assessment payments at 2.5% above the Bank of England base rate. With the Bank of England base rate at 4.5% in early 2025, the HMRC late payment interest rate is 7.0% per annum. Interest is charged daily from the day after the payment deadline until the date payment is received.
This rate changes when the Bank of England changes its base rate AccFirm advises clients on the current rate when managing outstanding tax liabilities.
Payments on Account Why So Many People Get Caught Out
One of the most common causes of unexpected tax bills and late payment penalties is payments on account the advance payments HMRC requires from Self-Assessment taxpayers whose tax bill exceeds £1,000. If you received a large unexpected tax bill in January, you may have also been required to make a first payment on account for the following year at the same time effectively paying 150% of your expected annual tax in a single payment.
AccFirm helps clients plan for payments on account well in advance, ensuring they have the cash available to meet both their balancing payment and their first payment on account in January without financial strain. We also advise on applying to reduce payments on account if your income has fallen.
How to Appeal a Self-Assessment Late Payment Penalty
HMRC will consider cancelling a late payment penalty if you have a ‘reasonable excuse’ for the late payment. HMRC’s definition of a reasonable excuse is quite specific it does not include running out of money, misunderstanding the deadline, or relying on a third party (such as an accountant) who failed to file on time.
HMRC does accept the following as reasonable excuses in some circumstances:
- A serious or life-threatening illness that prevented you from managing your tax affairs
- The death of a close family member shortly before the payment deadline
- HMRC’s own service failure (e.g., their online service was unavailable)
- A genuine postal delay beyond your control (though this is rarely accepted for online payments)
To appeal a penalty, log in to your HMRC Personal Tax Account and submit an appeal through the Self Assessment section, or write to HMRC directly. AccFirm assists clients with penalty appeals and HMRC correspondence on their behalf.
How to Avoid Self-Assessment Late Payment Penalties
- Set a calendar reminder for 31 January and 31 July — every year without exception
- Set money aside monthly treat HMRC as a monthly expense, not a once-a-year bill
- File your return as early as possible HMRC calculates your tax bill as soon as you file, giving you time to plan
- Plan for payments on account if your previous year’s bill was over £1,000, payments on account are due
- Apply to reduce payments on account if your income has fallen you can apply online through HMRC
- Use an accountant AccFirm reminds every client of upcoming deadlines and manages all HMRC submissions proactively.
AccFirm files Self-Assessment tax returns for sole traders, company directors, landlords, and high earners across London. We manage all payment planning and HMRC correspondence on your behalf. Contact us for a free consultation.