Every year, hundreds of thousands of people discover what happens if you file your tax return late — usually by receiving a penalty notice from HMRC that feels disproportionate to how late they actually were. The frustrating part is that the penalty structure is designed to escalate, so a small delay becomes an expensive one if it’s left unaddressed.

We deal with this regularly with clients who come to us having missed a deadline, sometimes without even realising they were registered for Self Assessment in the first place. The good news is that penalties can often be reduced or appealed — but only if you act quickly and understand the rules. The bad news is that ignoring the notice never makes it better.

This post covers the full penalty timeline for both Self Assessment and Corporation Tax, what HMRC is likely to do at each stage, and how to handle it if you’ve already missed a deadline.

The moment a Self Assessment tax return passes its deadline — 31 January for online filers — HMRC issues a fixed £100 penalty. What catches a lot of people off guard is that this penalty applies regardless of whether any tax is actually owed. You could have no liability whatsoever, file one day late, and still receive a £100 charge.

This is one of the most commonly misunderstood aspects of the system. People assume that if they don’t owe any tax, there’s no real urgency to file on time. That assumption is wrong, and it’s a costly one.

The £100 penalty is automatic and generated by HMRC’s systems — it doesn’t require any human review or decision. By the time most people receive the notice, the deadline has already passed and the charge is confirmed. The only route to removing it is a successful appeal on reasonable excuse grounds, which we’ll come to shortly.

The practical takeaway here is straightforward: file on time even if you think you have nothing to pay. The cost of the penalty isn’t just financial — it can also trigger HMRC to take a closer look at your affairs.

If the return remains unfiled after the initial penalty, HMRC’s penalty structure moves through several stages:

• 3 months late: Daily penalties of £10 per day begin to accumulate, up to a maximum of £900. That’s 90 days of charges running alongside the original £100.
• 6 months late: A further penalty is added — either 5% of the tax due or £300, whichever is higher.
• 12 months late: Another 5% or £300 charge is applied on top of everything already accrued.

By the time a return is a full year overdue, a taxpayer with a modest liability could easily be facing over £1,600 in penalties before interest is even factored in. And interest is charged on unpaid tax separately from the penalty amounts.

There are also late payment penalties to consider, which run in parallel to the late filing charges. If you file on time but don’t pay what’s owed, HMRC adds 5% of the outstanding amount at 30 days, again at 6 months, and again at 12 months.

The system is designed so that the longer you leave it, the harder it becomes to dig out. We’ve seen clients who delayed dealing with a straightforward return and ended up with a penalty bill larger than the tax they owed. It’s entirely avoidable with the right support in place.

Limited companies face a different penalty structure for late Corporation Tax returns, and from 1 April 2026 those penalties have been increased to account for inflation — so if your company has a filing date on or after that date, the numbers below are the current ones.

• 1 day late: A £200 fixed penalty.
• 3 months late: A further £200 fixed penalty.
• 6 months late: HMRC will estimate the tax owed and apply a penalty of 10% of that unpaid figure.
• 12 months late: Another 10% penalty on the outstanding tax.

There’s an additional sting for repeat lateness: if a company files its Corporation Tax return late three times in a row, each of the fixed £200 penalties rises to £1,000. HMRC tracks filing history, and serial late filers are treated more harshly.

It’s worth noting that the Corporation Tax payment deadline and the filing deadline are separate. Payment is generally due nine months and one day after the end of the accounting period, while the return itself must be filed within twelve months. Getting one right doesn’t mean the other is handled — both need to be on your radar.

If you work with an accountant, these deadlines should be managed for you. If you’re doing it yourself, a simple calendar reminder set well in advance is the minimum protection you should have.

Yes — and this is genuinely worth pursuing if you have a credible reason for the delay. HMRC will consider cancelling or reducing a penalty if you can demonstrate a reasonable excuse. That term has a specific meaning in practice, and it tends to cover situations outside your control: a serious illness, a bereavement, a technical failure on HMRC’s own systems, or postal delays beyond your control.

What it doesn’t cover is forgetting, being too busy, or assuming someone else had filed on your behalf. HMRC takes a fairly strict view of what qualifies, and appeals based on vague inconvenience are generally rejected.

Appeals for Self Assessment penalties can be made online through your tax account or by post using form SA370. If you paid the penalty but believe your appeal is valid, HMRC will refund the amount with interest if they agree with you.

Our advice is always to pay the penalty even while the appeal is in progress. If the appeal is rejected, unpaid penalties continue to accrue interest — so withholding payment while you wait can make the bill larger.

If you’re unsure whether your circumstances constitute a reasonable excuse, it’s worth getting a professional view before submitting. A poorly framed appeal can actually work against you by confirming HMRC’s position. We help clients with this fairly regularly, and the outcome often depends on how the grounds are presented.

The most effective solution is also the most obvious one: don’t leave the filing until the last few days before the deadline. Self Assessment returns for the 2025–26 tax year can be filed from April 2026 onwards — there’s no requirement to wait until January. Filing early means you know your liability well in advance, you have time to plan for the payment, and you eliminate the risk of last-minute technical problems locking you out of HMRC’s portal.

For limited companies, the same logic applies. Once your year-end accounts are prepared, the Corporation Tax return should follow promptly. Delays in getting information to your accountant are one of the most common reasons filings run close to the wire.

If you’re managing your own tax affairs and finding the deadlines stressful, it may be worth considering whether an accountant would help. The cost of a Self Assessment return prepared professionally is often less than a single late filing penalty — and the value of having someone who tracks deadlines on your behalf goes beyond the filing itself.

We handle Self Assessment returns for a fixed fee of £150, which includes preparation, submission, and advice on payments on account. It’s one of those things that’s much easier to hand over than most people expect.