If you need to file a Self Assessment tax return for the 2025/26 tax year, the first deadline is fast approaching. Paper returns must reach HMRC by midnight on 31 October 2026. Online returns are due by 31 January 2027, which is also the date any tax owed must be paid. This article explains who needs to file, which deadline applies to you, what happens if you miss it, and the steps you can take now to make January less stressful.

Disclaimer: This article is for general information only and reflects HMRC rules and deadlines for the 2025/26 tax year at the time of writing in October 2026. Whether you need to file, and how much you owe, depends on your own circumstances. Always seek specific advice if you are unsure.

The Key Dates for the 2025/26 Tax Year

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026.

The deadline to tell HMRC you needed to file a return for the first time was 5 October 2026.

The deadline for paper returns is midnight on 31 October 2026.

If you file online by 30 December 2026, you can ask for tax owed of less than £3,000 to be collected through your PAYE tax code, if you are eligible.

The deadline for online returns, and for paying any tax owed for 2025/26 together with your first payment on account for 2026/27, if one is due, is 31 January 2027.

The second payment on account for 2026/27, if one is due, must be paid by 31 July 2027.

Paper or Online: Which Deadline Applies to You?

Most people file online. It is quicker, HMRC's system calculates the tax automatically, and the online deadline gives you three more months. HMRC recommends filing online for most taxpayers.

Paper returns are now used mainly by people who cannot use the online service, and by certain types of return that cannot be filed online through HMRC's own system. If you plan to file on paper, the return must reach HMRC by 31 October 2026. A paper return that arrives after that date is late, even though the online deadline has not yet passed.

Who Needs to File a Return?

You generally need to file a Self Assessment tax return for 2025/26 if any of the following applied to you.

You were self employed as a sole trader with gross income of more than £1,000, or you were a partner in a business partnership.

You received income from property above certain levels. HMRC generally requires a return if your rental income was £10,000 or more before expenses or £2,500 or more after expenses. Lower amounts may be dealt with through your tax code instead.

You had income that is not taxed at source, such as untaxed savings interest, dividends or foreign income, above the levels that can be collected through your tax code. Company directors are not automatically required to file just because they are directors, but most directors who take dividends above the dividend allowance will need to.

You made a capital gain that results in tax to pay, or you sold assets for more than £50,000 even if no tax is due, unless you have already reported and paid the tax through a Capital Gains Tax on UK property account.

You received income from abroad, or you were not resident in the UK but had UK income to report.

You need to pay the High Income Child Benefit Charge and are not paying it through your tax code.

You are claiming certain reliefs, such as higher rate tax relief on personal pension contributions, which is often easier to claim through a return.

HMRC also sends a notice to file to some people. If you receive one, you must file a return even if you think no tax is due, or ask HMRC to withdraw the notice.

Making Tax Digital for Income Tax began in April 2026 for sole traders and landlords with qualifying income above £50,000. If you are in that group, you still file your 2025/26 return in the normal way by 31 January 2027, because Making Tax Digital applies from 2026/27 onwards. Sole traders and landlords with qualifying income above £30,000 will join from April 2027, as explained in our Making Tax Digital guide.

What If You Missed the 5 October Registration Deadline?

If you needed to file for the first time and have not yet registered, do so as soon as possible. Registration can take some time, and you will need your Unique Taxpayer Reference before you can file online. Penalties for failing to notify HMRC are based on the tax that remains unpaid, so if you register, file and pay everything by 31 January 2027, a penalty for late notification is unlikely in most cases.

Penalties for Late Filing and Late Payment

Late filing penalties apply automatically, even if you have no tax to pay or you pay on time.

A return filed up to three months late attracts a fixed penalty of £100. After three months, a daily penalty of £10 applies for up to 90 days, to a maximum of £900. After six months, a further penalty applies of 5% of the tax due or £300, whichever is greater. After twelve months, another penalty of 5% of the tax due or £300 applies, and it can be higher in some cases.

Late payment penalties are charged on tax that remains unpaid. Under the rules that apply to 2025/26 Self Assessment returns, a penalty of 5% of the unpaid tax applies at 30 days, with further 5% penalties at six months and twelve months. Interest is also charged on late payments, currently at the Bank of England base rate plus 4%.

Worked Example 1: The Cost of Missing the Paper Deadline

Graham is a retired engineer who receives rental income from a flat and prefers to file on paper. He posts his 2025/26 return on 6 November 2026 and it arrives on 10 November. Because the paper deadline was 31 October, the return is late and he is charged a £100 penalty, even though he owes only £850 and pays it in full by 31 January 2027.

Had Graham filed online instead, his deadline would have been 31 January 2027, and there would have been no penalty. If he had filed online before 30 December 2026, he could also have asked for the £850 to be collected through his pension tax code over the following year.

Payments on Account: The January Surprise

Many people filing for the first time are caught out by payments on account. If your Self Assessment tax bill for the year is £1,000 or more, and less than 80% of your total tax was collected at source, for example through PAYE, HMRC usually asks you to make advance payments towards next year's bill. Each payment on account is half of the previous year's Self Assessment bill, due on 31 January and 31 July.

Worked Example 2: A First Year Sole Trader's January Bill

Priya started freelancing as a graphic designer in May 2025, alongside a part time employed role. Her Self Assessment tax bill for 2025/26, after the tax already paid through PAYE, is £4,000.

By 31 January 2027, Priya must pay the £4,000 for 2025/26 plus her first payment on account for 2026/27 of £2,000, making a total of £6,000. A further £2,000 is due on 31 July 2027. If Priya's income in 2026/27 is lower, she can apply to reduce her payments on account, but if she reduces them too far, interest is charged on the shortfall. Knowing this now, three months ahead, gives her time to set money aside.

Worked Example 3: Paying Through Your Tax Code

James is employed and also earns a modest income from tutoring. His 2025/26 Self Assessment bill is £1,200. Because he still has PAYE income, owes less than £3,000 and files online before 30 December 2026, he can ask HMRC to collect the £1,200 through his tax code during 2027/28. Instead of paying £1,200 in January, he pays around £100 a month through his salary. Payments on account would not apply, because his bill is just over £1,000 but more than 80% of his total tax is collected through PAYE.

Other Ways to Spread the Cost

If you cannot pay your tax bill in full, you may be able to set up a Time to Pay arrangement with HMRC. For Self Assessment debts within certain limits, this can often be done online, provided your returns are up to date. Setting up an arrangement does not stop interest, but it can prevent further late payment penalties and enforcement action.

Some people choose to make regular payments towards their tax bill during the year, using HMRC's budget payment plan, so there is less to pay in January.

Separately, HMRC is sending around 1.8 million Simple Assessment letters for 2025/26 to people whose tax can be calculated from information HMRC already holds, such as pension income or savings interest. If you receive one before 31 October 2026, payment is due by 31 January 2027. If you receive it on or after 31 October, payment is due within three months of the date of the letter.

Practical Steps to Take This Month

Decide now whether you will file on paper or online. If you are planning a paper return and are not confident it will arrive by 31 October, switch to online filing.

Gather your records, including P60s, P11Ds, bank interest statements, dividend vouchers, rental income and expenses, self employed income and expenses, pension contributions and charitable donations.

Check whether you will need to make payments on account in January, and set money aside if so.

If you have received any letter from HMRC, including a nudge letter about undeclared income, deal with it now rather than waiting for January. Our article on HMRC's letters to medical consultants explains how to respond to these letters.

If you are a company director, check that your salary and dividends are reported correctly. Our guide to paying yourself as a director explains how dividends are taxed.

Frequently Asked Questions

When is the deadline for a paper Self Assessment return?

For the 2025/26 tax year, paper returns must reach HMRC by midnight on 31 October 2026.

When is the online Self Assessment deadline?

The deadline for online returns for the 2025/26 tax year is 31 January 2027. This is also the deadline for paying any tax owed.

What happens if I miss the 31 October paper deadline?

If your paper return arrives after 31 October, you will normally be charged a £100 late filing penalty. If you have not yet filed, you can avoid this by filing online by 31 January 2027 instead.

Can I pay my Self Assessment tax through my tax code?

Yes, if you owe less than £3,000, you have income taxed through PAYE, and you file online by 30 December 2026, or on paper by 31 October 2026. HMRC will then collect the tax through your tax code over the following tax year.

What are payments on account?

They are advance payments towards next year's tax bill, usually required if your Self Assessment bill is £1,000 or more and less than 80% of your total tax is collected at source. Each is half of the previous year's bill, due on 31 January and 31 July.

Do I need to file a return if I have a side hustle?

If your gross income from self employment was more than £1,000 in the tax year, you generally need to register and file a return. Below that, the trading allowance usually means you do not need to report it.

I am in Making Tax Digital for Income Tax. Do I still file a 2025/26 return?

Yes. Making Tax Digital for Income Tax applies from the 2026/27 tax year for those with qualifying income above £50,000. Your 2025/26 return is filed in the normal way by 31 January 2027.

What if I cannot afford to pay my tax bill?

File your return on time anyway, to avoid late filing penalties, and contact HMRC about a Time to Pay arrangement. Interest will still be charged on the unpaid tax, but an arrangement can help prevent further penalties.

What CoreAcc Accountants Can Help You With

CoreAcc Accountants prepares and files Self Assessment tax returns for sole traders, landlords, company directors, healthcare professionals and individuals with more complex affairs. We make sure everything that should be reported is included, claim the reliefs and expenses you are entitled to, explain your January and July payments in advance, and help you prepare for Making Tax Digital if it applies to you. Filing early with us means you know your bill well before the deadline.

Get in Touch

If you need to file a 2025/26 tax return, are not sure whether you need to, or have missed the registration deadline, contact CoreAcc Accountants now. We will make sure your return is filed on time and that there are no surprises in January.

CoreAcc Accountants is an ACCA accredited firm of Chartered Certified Accountants based in Borehamwood, Hertfordshire. This article was published in October 2026 and reflects HMRC rules and deadlines for the 2025/26 tax year at the time of writing. It does not constitute professional or legal advice. Always seek specific advice tailored to your individual circumstances.

‍