Self Assessment late filing and late payment penalties

Two clocks start when a Self Assessment deadline is missed: one for the return, and one for the tax. They have different official schedules, and interest can run on unpaid tax at the same time. This guide restates the published lists in our own words and explains what our penalty calculator does — and what it deliberately does not do.

Late filing: the four official steps

The Self Assessment penalties page says you will get the following late-filing penalties if you send the return late: an initial £100 penalty; after 3 months, additional daily penalties of £10 per day, up to a maximum of £900; after 6 months, a further penalty of 5% of the tax due or £300, whichever is greater; after 12 months, another 5% or £300, whichever is greater. All partners are charged if a partnership return is late. The same page tells you to send the return as soon as possible.

The first £100 is a flat amount on that list. It is not described as a percentage of the bill. The later steps do use the tax due, which is why a large unpaid liability makes the six-month and twelve-month charges much bigger than £300, while a small liability still meets the £300 floor at those points. Daily penalties are capped at £900, which is 90 days at £10. Once that cap is reached, the next official increase is the six-month percentage (or £300) step, not a further daily amount.

To turn “after 3 months”, “after 6 months” and “after 12 months” into a number of days, our calculator uses conventions that we state on the tool page: daily penalties start after 90 days (the first £10 day is day 91); six months is treated as 181 days, which is the span from 31 January to 31 July in a year that is not a leap year; twelve months is treated as 365 days. If your deadline is unusual, or you need HMRC’s own day count, use the official “estimate your penalty” service as well. The online filing date for the year most people are completing now is 23:59 on 31 January 2027; a paper deadline of 31 October 2026 has already passed, so a paper return that is still outstanding is already late on that track.

Late payment: three 5% charges, plus interest

If you pay the tax late, the official page says you will get penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months. You will also be charged interest on the amount owed. Those 5% steps are separate from the late-filing list. A return that is filed on time but paid 40 days late can attract a late-payment charge without a late-filing charge; a return that is both late and unpaid can attract both lists at once.

Interest is real, and it is not included as a pound figure in our calculator. The official interest-rates page, updated on 6 November 2026, explains that late-payment interest is set at the Bank of England base rate plus 4% from 6 April 2025 (it was plus 2.5% on or before 5 April 2025). The current table on that page shows 7.75% from 9 January 2026 and 8.00% from 24 November 2026. Because the rate moves, inventing a pound interest total here would go stale quickly. If you need the interest, read that official table and HMRC’s own calculation, not a made-up amount on this site.

Failure to notify is a different penalty

If you register after 5 October and do not pay all of your tax bill by 31 January, you may get a “failure to notify” penalty. The penalties page says this is based on the amount still left to pay and that you will receive it within 12 months after HMRC receives the return. It then points to more information on failure-to-notify penalties. That further information is factsheet CC/FS11, updated on 19 March 2026.

The factsheet explains that HMRC may charge a penalty if you fail to notify, or do not notify on time, and that the amount depends on the quality of any disclosure and on whether the behaviour was delayed, deliberate, or deliberate and concealed. Unprompted disclosure — telling HMRC before you had reason to believe they were about to find the failure — has a lower minimum than prompted disclosure. The published ranges are percentages of potential lost revenue, not a single flat fee. We do not fold those ranges into the late-filing calculator, because collapsing them into one box would pretend a precision the factsheet does not give a casual reader. If this situation applies, read CC/FS11 and still file and pay as soon as you can.

HMRC will not charge a failure-to-notify penalty if you have a reasonable excuse, the failure was not deliberate, and you told them without unreasonable delay after the excuse ended. What counts as a reasonable excuse depends on the circumstances and on the person. This guide cannot decide that for you. If you disagree with a penalty notice, the official penalties page points to the appeal route and to paying the penalty in the meantime as directed on the notice.

What to do in practice

The cheapest response to a missed date is almost always to file and pay now. Daily filing penalties, once they have started, add £10 a day until the cap. Interest continues on unpaid tax until it is paid. Setting up the official payment method, and submitting the return online for free on GOV.UK, stops the problem getting larger while you read about it.

Use the deadlines guide to confirm which date you missed, and the payments on account calculator if part of what you owe is a January instalment towards next year rather than last year’s balancing amount. A planning estimate from our tax calculator is not a penalty notice and not a substitute for HMRC’s figures.

Partnerships, reasonable excuses, and official estimates

If a partnership return is late, official wording is that all partners will be charged a penalty. That can surprise people who assumed only the nominated partner would be involved. The same published list still applies; the difference is who receives a charge, not the steps on the list. Each partner then has the ordinary official routes for payment and, if they believe they have a reasonable excuse, for appeal.

A reasonable excuse is something HMRC may accept when you appeal. This guide cannot list every situation that will or will not qualify, because official appeal guidance treats it as fact-specific. Health, a serious system outage, or a delay that was genuinely outside your control sometimes features in people’s appeals; “I forgot” or “I was too busy in January” is a weaker starting point. If you appeal, follow the instructions on the notice and the tax-appeals pages on GOV.UK, and do not assume that filing late by a few days will be waived automatically.

HMRC also offers its own service to estimate a penalty for late Self Assessment returns and payments. Use that official estimate alongside ours if you want a second reading of the same published schedule. Ours is a planning arithmetic with the day-count conventions stated on the tool page. Neither estimate is a penalty notice, and neither replaces the amount on a notice HMRC has actually issued.