What Happens if You Miss a Tax Deadline?

© Ascannio / Shutterstock.com

Missing a self-assessment tax deadline isn’t something to be taken lightly, as it can have serious consequences including penalties, interest on unpaid taxes and the risk of an HM Revenue and Customs investigation.

The tax office may notify you of the deadline for filing your tax return, either through your personal account online, or by post. When you receive a notification, you must file a return on time by law, even if you don’t have any income for the year.

Tax return deadline UK

The tax year runs from 6th April to 5th April the following year. HMRC must receive your return and any money you owe by the deadlines it sets to avoid your business incurring financial penalties.

If you need to complete a tax return, but have never sent one before, you must tell HMRC that you are liable to file a return by 5th October. You can do this by registering for self-assessment online.

A paper tax return must be submitted by midnight on 31st October and an online return by midnight on 31st January. Should you require HMRC to collect the tax you owe from your pension and wages, you must submit your return online by 30th December. You need to check beforehand if you’re eligible to pay in this way.

Deadlines for paying owed tax

When it comes to paying the taxes you owe, the Self-Assessment payment deadline is 31st January the following year. For example, if you’re paying your taxes for the 2024/25 financial year, the deadline will be midnight on 31st January 2026.

If you make advance payments towards your bill (known as payments on account), there is usually a second payment deadline of 31st July. If you miss either the tax returns deadline or the payment deadline, you will normally incur a fine, unless you have what HMRC considers a “reasonable excuse”.

Many business people estimate their Self-Assessment tax bill before sending their tax return, giving them an idea of what the bill is likely to be, as this can help avoid a late payment penalty by assisting your budgeting. Different deadlines apply to different business types and taxes in the UK. For example, if you have a limited company, you may be able to file your tax return at the same time as your accounts with Companies House.

The deadline for filing your tax return is 12 months after the end of the accounting period it covers. There’s also a separate deadline for paying your Corporation Tax bill, which is usually nine months and one day after the end of the accounting period.

For newcomers to the business world, or for company directors who are continually busy, dealing with tax returns and deadlines can be a potential minefield, not to mention time consuming. For this reason, many businesses choose to commission a professional accountant who offers expert tax services in Cornwall.

Penalty for late tax return

A missed deadline for Self-Assessment returns usually results in an automatic fine of £100, with the potential for additional fines, depending on how late you are. In the case of VAT and Corporation Tax returns, there are also penalties for late filing. You will be charged an automatic fine of £100, even if you don’t owe any tax, for not filing your paperwork on time.

If you still haven’t filed your return three months after the deadline, you’ll be charged an additional fine of £10 a day, up to a maximum penalty of £900. Failing to file your return within six months of the deadline will then add a further penalty of either 5% of the total tax owed, or £300.

If the return is still outstanding 12 months after the deadline, another penalty of either £300, or 5% of the tax owed, will be added to your fine. This can leave SMEs with a massive debt outstanding to HMRC that can seriously dent their financial wellbeing.

HMRC calculates interest on unpaid taxes daily, based on the Bank of England base rate, plus an additional margin. The interest starts from the day after the deadline and continues to accumulate.

According to research, 35% of UK businesses leave filing their tax return until the last minute, waiting until late January, with 21% taking it right to the wire. Unfortunately, around 12% of businesses miss the deadline altogether, resulting in financial penalties they could have avoided by managing their books better, or enlisting the services of an accountant.

More than 640,000 businesses in the UK were estimated to have filed late in January 2025, according to a sample survey of 500 SMEs by Free Agent. Almost half (46%) of respondents admitted they dragged their feet when it came to completing and submitting the paperwork.

What might count as a reasonable excuse?

Only having a genuine “reasonable excuse” may prevent you from being fined for failing to meet a tax obligation. This can include the death of your partner or a close relative shortly before the deadline; an unexpected hospital stay that prevented you from dealing with tax affairs; a life-threatening or serious illness; the failure of your computer or software while preparing an online return; or a technical issue with HMRC’s online services.

Suffering a flood, fire or theft at your premises is also considered a reasonable excuse, as are postal delays that couldn’t have been predicted, or a delay relating to a mental illness or disability, including if you were relying on another person to send your return and they failed to do so.

If HMRC accepts you have a genuine reason for missing the deadline, you must then send your tax return or payment as soon as possible. It isn’t classed as a viable excuse if you simply didn’t have enough money to pay on time; if you found the HMRC online system too complex to use; you made a mistake on your return; or didn’t receive a reminder from HMRC.

How to avoid missing future deadlines

When you receive a late filing penalty and don’t have a reasonable excuse, pay the penalty as soon as possible to prevent further interest from accumulating. If you believe the penalty is unjustified and are considering appealing or negotiating payment terms, contact HMRC to discuss your options.

Avoid missing tax deadlines in the future by contacting DL Accounts, the leading professional accountant in Cornwall. We’ll help stay on top of your tax returns!