HMRC Late Payment Penalties and Interest: What You Owe When the Tax Itself Is Late

There are two different ways to be late with Self Assessment, and HMRC penalises them separately. Filing the return late triggers one set of charges. Paying the tax late triggers another, entirely distinct set. You can be on time with one and late with the other, and you can collect penalties from both at once. Most people who focus on getting the return in are caught out by the second system, because it carries on running quietly in the background on any tax that has not actually been paid. Late filing carries its own £100 charge and escalating penalties, a separate regime from the late-payment penalties below.

The Three Late Payment Penalties: 30 Days, 6 Months, 12 Months

HMRC charges a penalty of 5% of the tax that is still unpaid at three separate points: 30 days after the due date, 6 months after the due date, and 12 months after the due date. Each one is a fresh 5% charge calculated on whatever remains outstanding at that moment, so the three can stack into 15% of the original bill if nothing is paid across the whole period. HMRC sets out this structure in its Self Assessment penalties guidance, which lists penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months.

Because each surcharge is calculated on the balance still outstanding when it falls due, paying down the bill reduces every later charge. Clearing half the debt before the six-month point halves the six-month and twelve-month surcharges. There is no daily element to these penalties, unlike the filing daily charges, so the three trigger dates are what matter.

On a larger balance the same percentages produce much larger numbers. A £20,000 bill left unpaid for over a year collects £1,000 at each of the three points, a £3,000 penalty total, before any interest is counted and entirely separate from anything charged for filing the return late.

Each of the three charges is triggered purely by the calendar, not by any action HMRC takes. The 30-day, six-month and twelve-month points are measured from the payment due date of 31 January, so the first surcharge lands at the start of March, the second at the end of July, and the third the following January, on whatever tax is still outstanding when each date passes. There is no warning letter you have to wait for and no assessment that has to be raised first. If the balance is clear before a given trigger date, that surcharge simply never arises, which is why the dates rather than HMRC contact are what you plan around.

Interest Runs Daily on Top of the Penalties

Penalties are not the only cost of paying late. HMRC also charges interest on the unpaid tax, and it runs from the day after the payment was due until the day it is paid in full. Interest is not a one-off; it accrues every day the balance is outstanding, so it grows steadily rather than landing in fixed lumps like the surcharges.

The rate is set by formula. Late payment interest is the Bank of England base rate plus 4 percentage points, a margin that increased from the previous 2.5 points from 6 April 2025. Because it is tied to the base rate, the figure moves when the base rate moves, so the precise rate on any given day should be checked against the current published HMRC rate, but the base-rate-plus-4-points formula is the constant. The Chartered Institute of Payroll Professionals tracks these HMRC interest rate changes as they are announced, which is a useful cross-check when a rate has just moved.

The interest is not compounded in the way a credit card balance is; HMRC applies the daily equivalent of the annual rate to the tax outstanding on each day, so it is closest to simple interest accruing day by day on the live balance. What makes it grow faster than people expect is the combination of the higher margin and the time involved. At a headline rate of around eight per cent, a £6,000 balance left unpaid for a full year carries roughly £480 of interest on top of the £900 in surcharges, and a larger balance scales that in direct proportion. None of this interacts with the surcharges, which are calculated separately on their own trigger dates, so the two costs run alongside each other rather than one replacing the other.

It helps to understand how the interest is built up day by day. HMRC works out the interest on the balance outstanding on each individual day, so it is the daily equivalent of the annual rate applied to whatever is still owed that day. The practical effect is that the running interest charge falls every time you make a payment, because the next day is calculated on a smaller balance. A part payment never stops the clock entirely while anything remains owed, but it does shrink the daily amount from that point forward, which is why paying something is always better than paying nothing while you wait to clear the full bill.

Why This Is Separate From the Filing Penalties

It is worth being precise about the two systems, because conflating them leads people to think they are safe when they are not. The filing penalties begin with the automatic £100 charge the day a return is late, covered in detail in our guide to the £100 late filing penalty and what comes after it. Those charges are about the return not being submitted. The late payment penalties and interest in this article are about the tax not being paid. A person who files on 31 January but cannot pay the bill avoids every filing penalty and still faces the full late payment regime. The reverse is also true: someone who pays an estimated amount on time but files the return months late faces filing penalties while having reduced or removed the payment penalties.

This is why filing early, even on estimated figures, is useful beyond stopping the filing clock. Once you know the figure, you can pay something against it, and every pound paid before each surcharge date shrinks the penalty calculated at that date and slows the daily interest.

How to Stop the Payment Penalties Escalating

If you cannot pay the bill in full, the route that stops enforcement and caps the damage is a formal payment arrangement with HMRC. A Time to Pay arrangement spreads the debt over monthly instalments. For Self Assessment debts of £30,000 or less it can usually be set up online within 60 days of the payment deadline, as GOV.UK explains in its guidance on paying an HMRC bill when you are in difficulty. The online route normally also requires that you have filed all your returns and have no other outstanding tax debts or existing payment plans. Larger or more complicated debts are still eligible but have to be agreed by contacting HMRC directly, and that is where professional Time to Pay negotiation tends to make the difference, because HMRC sets the monthly figure from a detailed look at your income, essential outgoings and other debts.

The amount HMRC agrees to is driven by what it judges you can realistically afford after essential living costs, not simply by dividing the debt into equal slices. The stronger and more honest the financial picture you put forward, the more likely the plan is to be both approved and sustainable. Overstating what you can pay leads to a defaulted plan, which is worse than no plan, because a default lets the suspended penalties and enforcement resume from where they left off. The aim is a figure you can actually keep up every month for the full term.

When you set the plan up online for a Self Assessment debt within the threshold, HMRC asks for the reference number of the tax you cannot pay, your bank details, and a breakdown of your income and spending, and it uses those figures to propose a length and a monthly amount. Plans set up this way are typically expected to clear the debt over a manageable number of months rather than stretching indefinitely, and longer or larger arrangements that fall outside the online route are negotiated case by case once HMRC has examined the same affordability information in more detail. Setting up a direct debit for the instalments matters, because a missed manual payment can be enough to count as a default.

A Time to Pay arrangement does not switch off interest, which keeps running on the outstanding balance, but it does stop further late payment penalties from being charged on the tax covered by the plan and it halts enforcement action while the plan is honoured. Where a debt is too large to clear even over instalments, the question moves into the broader territory of Time to Pay, an IVA or bankruptcy, which sets out the options when the bill is beyond what monthly payments can realistically cover.

1. 1File the return first, even on estimated figures, so the actual liability is known and can be quantified.
2. 2Pay as much as you can against the bill before each surcharge date, because every payment reduces the next 5% charge and the daily interest.
3. 3If you cannot pay in full, set up a Time to Pay arrangement online (debts of £30,000 or less) or by contacting HMRC for larger amounts.
4. 4Keep to the arrangement, because a defaulted plan lets the penalties and enforcement resume.

Common Questions About Late Payment Penalties

Do the late payment penalties apply if I owe no tax?

No. The late payment penalties are a percentage of the tax actually unpaid, so a return showing a nil or refund position attracts no late payment penalty. The filing penalties are different: the £100 fixed filing penalty applies even where no tax is owed.

Can I appeal a late payment penalty?

The 5% surcharges can be challenged on reasonable excuse grounds in the same way as filing penalties, where a genuine reason beyond your control prevented payment. The interest is different and cannot be appealed at all, because it is compensation rather than a penalty.

Does a Time to Pay arrangement stop the interest?

No. Interest continues to accrue on the outstanding balance throughout a Time to Pay arrangement. What the arrangement stops is further late payment penalties on the covered debt and any enforcement action, provided you keep up the instalments.