10 tips to help filers hit the Self-Assessment tax return deadline 2026

Putting personal finance admin on the backburner is something many of us are guilty of, but leaving your Self-Assessment tax return until the final days of January puts you at risk of missing the deadline and receiving a fine. 

A tax return can require detailed information and time to ensure everything is correct. This might explain why almost 5.85 million people still need to file their online return for the 2024-25 tax year*. Leave it too late, however, and you risk missing the online submission deadline at midnight on January 31, and incurring an instant fine, with additional penalties the longer the return or any tax owed remains outstanding. 

Taking advantage of all the reliefs and allowances available to you has the potential to minimise your tax liabilities and increase your disposable income. This is particularly important now that almost two million more people are liable for the higher rate of tax in the 2025-26 financial year, compared to 2022-23*, as frozen income tax thresholds drag more earners into the 40% tax band as their pay rises.

With only a few weeks to go until the deadline, here are 10 things last-minute filers might consider:

1. Never assume tax returns don’t apply to you 

Many British taxpayers won’t need to file a return for the 2024-25 financial year because tax is automatically deducted from wages (known as Pay-As-You-Earn, or PAYE), pensions or savings. However, there are situations where completing a return is mandatory – particularly if tax is not deducted at source or you earn extra untaxed income. 

Reasons to file include include:

• Earning more than £1,000 from self-employment. Those earning less can take advantage of the trading allowance, which lets sole traders earn up to £1,000 without declaring it.
• Earning more than £1,000 from letting out a property or land, such as rental income from a buy-to-let portfolio, is another reason to file, along with earning other untaxed income from tips and commission, or from savings, investments and dividends, as well as foreign income.
• Receiving an income of over £10,000 from your savings and investments.

Other reasons why you might have to register for Self-Assessment include being a partner in a business partnership or claiming income tax reliefs on charitable donations or business expenses if you are self-employed. Those who need to claim higher or additional tax relief on pension contributions need to be proactive - they might be able to do it without a tax return but if they are Self-Assessment then it should be included. 

In addition, claiming 30% income tax relief on subscriptions to Venture Capital Trusts (VCT) and Enterprise Investment Scheme (EIS) new share issues also need to be done on a tax return. Those in a VCT Dividend Reinvestment Plan, where investors can claim a further 30% tax relief on reinvested dividends each year, should also claim this via a return. 

If you’re unsure if a Self-Assessment applies to you, HMRC has a handy tool to check.

2. Some rules have changed, so don’t file a return unnecessarily 

Pension tax relief: If claiming higher or additional rate tax relief is your only reason for filing and you’re PAYE, then you can now claim via your personal tax account, the HMRC app or by writing directly to HMRC with contribution details and the scheme name. With the Self-Assessment deadline looming, however, it may be wise to contact HMRC via webchat or by phone to ensure you complete the process correctly.

High income: Previously, earning above £150,000 required a return to be filed, but this rule no longer applies for 2024-25. If a high income was the only reason for filing last year, HMRC should have been in contact to outline your next steps. If not, get in touch directly.

Child benefit: Under the High Income Child Benefit Charge (HICBC), parents start repaying the benefit once one partner earns gross taxable income of more than £60,000 a year - and they lose it entirely above £80,000. While this used to require Self-Assessment, since last summer, parents can report and pay the charge directly through their PAYE tax code. This is optional, however, so those that still want to pay the charge via Self-Assessment can continue to do so. 

Future changes for sole traders and landlords: From April 2026, Making Tax Digital (MTD) will replace self-assessment for sole traders and landlords earning more than £50,000. They must keep digital records using approved software and submit quarterly income and expense summaries to HMRC. The threshold for this change drops to £30,000 from April 2027 and to £20,000 from April 2028. Those with income below these thresholds can opt into the new system early or continue filing a tax return as usual. While HMRC should contact those who need to make the switch, starting early can give you breathing space to find the right software or accountant to avoid any stress in the future.

3. Make sure you can actually file the return 

Downloading HMRC’s app or logging into the online portal are the simplest ways to complete and submit your tax return. However, sign-in glitches do happen, so leaving this process to the final hours of January 31 is unwise. 

First-time filers must ensure they have their Unique Taxpayer Reference (UTR) number to hand. This 10-digit code is essential for logging into the Self-Assessment platform and to submit your tax return. To obtain it, you must have informed HMRC of your intention to complete a tax return by October 5 last year, so if you haven’t done that, then get in touch with HMRC straightaway as you may face a fine. Once registered, it can take up to 10 days for a UTR to arrive by post, so act quickly if you don’t have it.

4. Allow enough time to collate relevant paperwork  

Tracking down P60s, rental statements, expense receipts, pension statements, charitable donation records and all the other key documents you need to complete your tax return takes time, particularly if you don’t keep organised records throughout the year. Accuracy is critical as input errors or misleading information, even if accidental, can lead to penalties.   

This is key for those with multiple income sources, such as from rental property, freelance work, savings and investments. 

Information to have ready includes:

• All sources of income - not only from your main job but also side hustles, property income, interest on savings and dividends and coupons from investments
• Any charitable donations eligible for Gift Aid
• Capital gains on assets sold outside pensions and ISAs
• Pension contributions and investment paperwork, including the sale of crypto assets

Documents may include:

• Pay slips
• Annual P60 forms
• Any investment and pension statements
• VCT or EIS paperwork and records of charitable gifts
• For those with buy-to-let properties, gather full details of income and expenses over the full tax year

Also, pay particular attention to any capital gains you made in the 2024-25 tax year. Assets sold after October 30, 2024, are subject to the increase in the lower and higher CGT rates to 18% and 24%, respectively, introduced by Chancellor Rachel Reeves from Budget Day. Your filing system may not automatically adjust for this, so use the adjustment calculator on Gov.uk to ensure accuracy.

5. Don’t miss out on allowances and reliefs that apply to you - including pension tax relief

This can be a great way to claw back some cash, as there is no point paying tax on things you don’t need to. Tax reliefs can apply to working from home, work uniforms you need to buy, repair and maintain and charity donations. There is a whole host of other allowances, such as a trading allowance of up to £1,000 for casual income. That might apply to those that buy and sell on trading sites such as Vinted, Depop and eBay, though remember this only applies if you have bought and sold items to make a profit. It doesn’t apply if you are simply trying to make a bit of cash from a wardrobe clear out.  

High-earners paying into a workplace pension or a Self-Invested Personal Pension (SIPP) should pay particular attention. If contributions are made via a relief-at-source scheme, which can include your workplace plan or a personal pension, then basic tax relief is added automatically, but higher and additional rate taxpayers must claim the extra 20% or 25% relief themselves – either via a tax return or directly with HMRC. Failing to do so means missing out on a healthy chunk of cash.

6. Prepare funds to pay your tax bill as this must happen by January 31  

Filing the return is only half the battle – you must also pay any tax owed for the 2024-25 tax year by January 31 which will require sufficient cleared funds available in your bank account ahead of the deadline. There are several payment options, such as via the HMRC app or an online banking transfer. You can also send a cheque by post or make a one-off Direct Debit payment.

If setting up a direct debt for the first time, allow five working days for processing. For bank transfers, it’s three working days. While debit cards or corporate credit cards are accepted, you cannot use a personal credit card. To avoid last-minute stress, make the payment as early as possible and double check the transaction has gone through - a payment glitch could result in a fine.

7. Don’t worry, there is help if you can’t pay the full bill upfront 

Ignoring the problem is not the solution. Rather than paying a large tax bill in one go, HMRC may allow you to set up a payment plan to spread the cost over a set period, though interest will apply. 

First, state how much you can afford to pay upfront, then HMRC will ask for details of your income and expenditure to assess affordability and may want to see evidence. Note, you must file your tax return to qualify for the scheme, owe less than £30,000 and be within 60 days of the payment deadline. That’s another reason to get that tax return filed on time.

You must also not have any other payment plans or debts with HMRC. While there is no time limit for repayment - as it depends on your circumstances - interest will be charged on any overdue amounts.

8. Expect penalties if you miss the submission deadline - but you can appeal 

People with a genuine excuse for filing a late return or payment can appeal a penalty if they can provide evidence. This could be the death of a partner or close relative, a serious or unexpected illness or IT challenges.  

Without a valid reason, fines escalate quickly, starting with an instant fine of £100, even if no tax is owed. After three months, there is an additional daily charge of £10 (capped at £900). The fines get even heftier if the delay continues beyond six months (5% of the tax due or £300, whichever is greater) or even 12 months (another 5% of tax due or £300).   

This means penalties can reach £1,600 for a late return, excluding fines on unpaid tax, as payment delays also incur fines: 5% of the amount owed after 30 days, six months and 12 months. Remember, interest is also charged on any tax paid late. Since January 9, this has been set at 7.75% (the Bank of England base rate plus 4%). Burying your head in the sand is not the answer as HMRC may use a debt collection agency to recover unpaid bills – with repercussions for your credit rating. No matter how bad your situation is, engage early rather than ignore the problem.

9. Missed something key? You can amend previous tax returns and claim a rebate  

If you file a return and then need to make an addition or amendment, you can tweak previous tax returns and potentially earn a rebate in the process – though depending on what you report you may also have more tax to pay as well. Once you’ve filed your 2024-25 return, you can amend it anytime from 72 hours after submission until January 31, 2027.   

That means you have until January 31 this year to tweak your 2023-24 tax return, which can be done via the app or on the platform if you filed your return online. For earlier tax years, write to HMRC explaining which tax year they are correcting and why. Remember, you can claim a refund up to four years after the end of the tax year it relates to – so, you have time but don’t leave it too long.

10. Learn from the process by starting your tax-year-end strategy now 

If your 2024-25 tax bill was higher than expected, use this moment to plan ahead and reduce next year’s liability. The tax return deadline can be a good time to kickstart your tax-year-end strategy and mop up any unused allowances such as your pension or ISA allowance or make additional charitable donations to reduce next year’s tax hit.

Tax efficiency has never been more important now that income tax thresholds will remain frozen until April 2031. It means more and more people will find themselves dragged into higher rates of tax as their income increases.

Ways to soften next year’s tax bill:

• Think about topping up a pension to secure more income tax relief or making donations to charity.
• Consider taking advantage of a workplace salary sacrifice scheme following the Autumn Budget decision to cap National Insurance (NI) relief on salary sacrifice pension contributions at £2,000 per year from April 2029 . There is time before the changes come into play, so taking advantage now could be a good idea as salary sacrifice allows employees to reduce their salary or bonus payments in exchange for increased pension contributions. Such schemes grant all income tax relief automatically at the contribution stage and offer relief to both employee and employer from National Insurance contributions (NICs), making pension saving even more tax efficient. Not every employer offers these schemes, but if yours does, now is a good time to increase your contributions.  

This is particularly effective for those nearing crucial tax cliff edges where an individual’s marginal rate can jump dramatically such as the £50,270 earnings threshold where the higher 40% tax rate kicks in or the £100,000 threshold, where people face an effective marginal tax rate of over 60% for earnings that fall between £100,000 and £125,140. 

There are, however, some aspects of salary sacrifice to bear in mind, for example:

• it impacts how much you can borrow: Credit providers typically calculate how much you can borrow based on your salary. A lower figure might influence what size mortgage you can get
• it impacts other earnings-related benefits such as life cover or statutory maternity pay
• changing this arrangement might only be allowed at certain times e.g. annually

As such, salary sacrifice might not be right for everyone.

• Look at carry forward rules if you’re fortunate enough to be able to make a lump sum contribution to your pension before the end of this financial year, perhaps the result of a generous bonus, business exit or inheritance. Once your current year pension annual allowance (of up to £60,000 or 100% of earnings, whichever is lower for those with total adjusted incomes below £260,000) has been used up, it is then possible to mop up any unused allowances from the three previous years providing you were a member of registered pension scheme at the time. That’s a potential contribution of up to £220,000 before tax-year-end.
• Other allowances to consider. You can also reduce your overall tax burden by using your £20,000 ISA allowance to make savings and investments work tax efficiently, crystallising any capital gains to maximise this year’s £3,000 Capital Gains Tax exemption, or using up your £500 Dividend Allowance.

Once your return is filed, check your tax code is correct

HMRC may adjust your tax code after you file, but mistakes happen so keep a close eye on your tax code. Incorrect codes can lead to overpayments or underpayments, and it’s your responsibility to ensure accuracy. 

Errors can occur if you earn money from more than one source, such as a part-time job or property rental income. They can also be wrong if you recently changed jobs, retired or filed a tax return and a technical glitch caused an inaccuracy. If something looks askew, check your personal tax account and either rectify it directly through the HMRC app or online, if possible, or contact HMRC directly.

*(as of January 5), according to HM Revenue and Customs