Your Deadline Calendar Matters More Than You Think

Ask most sole traders and company directors what worries them about tax, and they will say the bill. In practice, it is the calendar that catches people out. A modest tax liability paid a few weeks late can attract a penalty far larger than the interest on the amount owed, and a pattern of late filing makes every future conversation with HMRC harder than it needs to be.

The reassuring news is that every Self Assessment and corporation tax deadline is fixed, predictable and published well in advance. Once you know the dates, they stop being a source of dread and become just another part of your business routine — like a standing order or a stock check.

The Main Self Assessment Dates

The tax year runs from 6 April to 5 April. Everything else hangs off those two dates, so it helps to think in terms of the tax year that has just ended rather than the current one.

• 5 October — deadline to register for Self Assessment if you have become newly self-employed or need to file a return for the first time.
• 31 October — deadline for paper returns. Very few people file on paper now, but the date still exists.
• 30 December — if you want any tax owed collected through your PAYE code rather than paid directly, your online return must be in by this date.
• 31 January — the big one. Online filing deadline, plus the date any balance owed for the previous tax year must be paid, plus your first payment on account.
• 31 July — your second payment on account.

Payments on account are advance instalments, each normally half of your previous year's liability, and they catch out plenty of first-time filers who assume a January payment clears everything. If your profits have dropped, you can ask HMRC to reduce them — but be careful, because reducing them too far attracts interest and penalties of its own.

What Happens When You Miss a Self Assessment Deadline

Late filing penalties are automatic and they stack. The £100 fixed penalty applies the moment the 31 January deadline passes, and it applies even if you owe no tax at all.

• £100 immediately after the deadline, and again if the return is still outstanding a year later.
• £10 per day once the return is three months late, capped at £900.
• 5% of the tax due, or £300 — whichever is greater — at six months late, and the same again at twelve months.

Late payment is penalised separately from late filing. You will pay interest from the day the tax was due until it is paid, plus a 5% surcharge at 30 days, a further 5% at six months and another 5% at twelve months. In other words, one missed January can compound into a genuinely painful sum by the following summer.

Limited Companies Face Different, But Equally Firm, Dates

If you trade through a limited company, Self Assessment is only part of the picture — and for directors, it is often the smaller part.

• Corporation tax payment is due nine months and one day after the end of your accounting period.
• The CT600 return is due twelve months after the end of the accounting period, so you pay before you file.
• Company accounts go to Companies House nine months after your year end (longer for a first set of accounts).
• The confirmation statement is due every twelve months, within fourteen days of the review date.
• PAYE and VAT run on their own monthly or quarterly cycles — usually the 22nd for electronic PAYE payments.

Late accounts at Companies House attract escalating fines, starting at £150 and rising to £1,500 for accounts more than six months overdue. Those penalties double if you file late two years running. Corporation tax late filing penalties are £100, then another £100, then £300, with further charges for consecutive failures.

How to Appeal a Penalty You Believe Is Unfair

HMRC does cancel penalties, but only when you can show a reasonable excuse — something genuinely outside your control that stopped you meeting the deadline. You normally have 30 days from the date on the penalty notice to appeal, and you can do it online or in writing.

Excuses that tend to succeed include serious illness or bereavement, a death or hospital stay close to the deadline, a failure of HMRC's own systems, or documents lost in the post. Excuses that rarely succeed include being too busy, not understanding the rules, relying on someone else who let you down, or simply not having the money — although HMRC will usually agree a payment plan if you ask before the deadline rather than after it.

When you appeal, be specific. State the dates, explain the sequence of events, and attach evidence such as medical letters or correspondence. Vague appeals get refused; well-documented ones often do not.

Practical Habits That Keep You Ahead

The simplest protection is to treat tax dates as business deadlines, not personal admin. Put all five Self Assessment dates in your calendar now, with a reminder two weeks before each one. Set aside a percentage of every payment you receive into a separate tax pot, so the money exists when January arrives.

Keep your records tidy through the year — bank statements, receipts, invoices — and reconcile them monthly rather than in a panic in December. If you use accounting software, check that it is set up with the correct year end and that your payments on account are recorded properly.

Finally, if a deadline is approaching and you know you will miss it, file what you can and speak to HMRC early. A return filed slightly late with an honest explanation is a far better position than silence, and a payment arrangement agreed in advance costs you nothing extra.