The key October self-assessment tax return deadlines to remember so you can avoid a shock bill

Forget Halloween or the clocks going back, October is an important month when it comes to taxes.

There are two key tax dates during October, affecting new landlords or self-employed people and also for those who want to file a paper tax return.

While most people see their tax taken automatically through the pay-as-you-earn system, untaxed income has to be declared to HMRC through self-assessment.

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This applies to the self-employed, those earning income from a side-hustle, investors, high-earning parents paying back Child Benefit and also landlords receiving rental income.

Frozen tax thresholds mean more people could be caught by the self-assessment net and it is important to get registered and to file in time to avoid the scare of an unexpected bill and penalties.

Myrtle Lloyd, HMRC’s chief customer officer, said: “Thousands of people join self assessment each year and anyone new to self assessment for the 2024/2025 tax year should register as soon as possible.”

Here are the key October self assessment dates to be aware of.

Deadline to register for self-assessment: 5 October

Anyone newly self-employed or earning untaxed income such as rent needs to inform HMRC and register for self assessment by 5 October.

This applies to earnings that cover the 2024/2025 tax year.

The taxman will generate a unique taxpayer reference for you so you can complete a tax return.

You can register online at Gov.uk.

You typically need to register for self-assessment if you are newly self-employed or a sole trader and have earned gross income over £1,000, received any untaxed income including pension income above £2,500 or if you received income of more than £1,000 from trading or providing services online.

Landlords also have to register if they received income from property, while investors may need to declare dividends or capital gains above the tax allowances.

You don’t need to register if you have already informed HMRC of your status in previous years.

There may be failure to notify penalties if you don’t register by 5 October and owe tax, according to the Low Incomes Tax Reform Group.

This could be worth up to 100% of the tax owed if HMRC thinks you are deliberately concealing income.

If you don’t send your tax bill on time, there will be an initial £100 penalty, with daily penalties of £10 per day after that, up to a maximum of £900.

After six months, there is a further penalty of 5% of the tax due or £300, whichever is greater and the same after 12 months.

There are also penalties of 5% of the tax unpaid at 30 days, six and 12 months.

So you can save a lot of hassle and money by registering before the deadline.

Even if you register after 5 October, HMRC will send you a letter or email with a different deadline to send your tax return by – this will usually be three months from the date on the letter or email.

HMRC has a free online tool that helps you to check whether you need to submit a self-assessment tax return.

31 October - paper tax return deadline

Once registered for self-assessment, there are two deadlines to be aware of, with one coming up soon.

You can file your return online by 31 January.

But anyone who has opted to complete a paper tax return must file it by the end of October.

HMRC must receive your forms by 11:59pm on 31 October 2025 or you could get a late filing penalty.

When to pay your tax bill

Whether you file by paper or online, any tax owed must be paid by 31 January 2026 for the 2024/2025 tax year.

This means that if you file a paper form in October, you could pay the tax owed at the same time or wait a few months.

You may prefer to keep your own cash in the bank until your tax bill needs to be paid, but there are benefits to filing and paying any taxed owed ahead of the self-assessment deadline, whether you use paper forms or go online.

Samuel Mather-Holgate, independent financial adviser at Mather and Murray Financial, said filing early could mean you avoid mega-high interest rates if you've underpaid, adding: “Getting your self-assessment tax return done early will ensure your payments on account are accurate. That way there’s no risk of paying too little and getting caught by the mega high interest rates from HMRC.”

But that assumes that landlords and the self-employed have readily accessible cashflow.

David Stirling, director at Mint Mortgages & Protection, said: "With the last Budget starting to really bite, increased National Insurance contributions hitting businesses and financial squeezes from all directions, I'm not sure many business owners are in a suitable position."