The Self Assessment deadline for filing your 2025/26 tax return online is 31 January 2027. It might feel a long way off, but every January we speak to Southampton sole traders, landlords and company directors who are scrambling to find paperwork at the last minute. Getting organised now means a calmer January, fewer mistakes and, very often, a lower tax bill.
This guide from Power Accountax, ICAEW Chartered Accountants in Southampton, explains every key date, who needs to file, what to gather, and what happens if you miss the Self Assessment deadline.
Self Assessment Deadline Dates for 2025/26
The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. These are the dates that matter for that year’s return:
- 5 October 2026 – register for Self Assessment if you need to file for the first time, for example because you started self-employment or began letting a property in 2025/26. You can register for Self Assessment on GOV.UK.
- 31 October 2026 – deadline for paper tax returns.
- 30 December 2026 – file online by this date if you want tax of under £3,000 collected through your PAYE tax code instead of paying it in one go.
- 31 January 2027 – the main Self Assessment deadline. This is the deadline for online returns, for paying any tax owed for 2025/26, and for your first payment on account for 2026/27.
- 31 July 2027 – second payment on account for 2026/27.
HMRC publishes the full list of Self Assessment tax return deadlines on GOV.UK if you would like to check them for yourself.
Who Needs to File a Self Assessment Tax Return?
You will usually need to complete a return by the Self Assessment deadline if, in the 2025/26 tax year, you were:
- Self-employed with income over £1,000
- A partner in a business partnership
- A landlord receiving rental income
- A company director taking dividends or other untaxed income
- Earning over £150,000, or receiving other untaxed income such as foreign income or large amounts of savings interest
- Selling assets such as shares, a second property or cryptocurrency where Capital Gains Tax may be due
If you are not sure whether you need to file, get in touch with our Southampton team and we will check for you.
What to Gather Before the Self Assessment Deadline
Having everything in one place is the single biggest time saver. Start collecting the following now, well before the deadline:
Personal and employment details
- Your UTR and Government Gateway details – your Unique Taxpayer Reference is on previous HMRC letters and in your HMRC online account.
- Employment details – your P60 and any P11D showing benefits in kind.
- Student loan details – if you are still repaying.
Business and rental records
- Self-employment records – sales invoices, expense receipts, bank statements and mileage logs for business journeys. Our bookkeeping service in Southampton can keep these up to date all year round.
- Rental income records – rent received, letting agent statements, repairs, insurance and mortgage interest statements.
- Online selling income – marketplace reports from Amazon, eBay, Etsy or Shopify. See our e-commerce accounting services.
Investments, gains and reliefs
- Dividends and interest – dividend vouchers and bank interest statements.
- Pension contributions and Gift Aid – these can reduce your tax bill if you are a higher rate taxpayer, so do not leave them out.
- Capital gains – purchase and sale details for shares, property or cryptocurrency, including exchange reports.
What Happens If You Miss the Self Assessment Deadline?
HMRC charges an automatic £100 penalty if your return is even one day late, even if you have no tax to pay.
After three months, daily penalties of £10 a day can apply for up to 90 days, with further penalties after six and twelve months. Interest is also charged on any tax paid late, and late payment penalties apply on top. GOV.UK explains the Self Assessment penalties in full.
If you think you may struggle to pay, speak to us early. HMRC Time to Pay arrangements are often available, and if you have already received a penalty, our team can help with HMRC enquiries, penalties and appeals.
Making Tax Digital Is Changing Self Assessment
If your self-employment and property income was over £50,000, you may already be sending quarterly updates under Making Tax Digital for Income Tax. You still need to meet the Self Assessment deadline for your final return.
From April 2027 the threshold falls to £30,000, so many more sole traders and landlords will be affected. Read our guides to MTD quarterly update deadlines and Making Tax Digital for the self-employed and landlords, or see our MTD service for the self-employed.
Tips to Make the Self Assessment Deadline Stress Free
- Book your accountant in the autumn, not in January.
- Keep a folder, paper or digital, for tax documents as they arrive.
- Check your HMRC online account for any payments on account already due.
- Set money aside for your tax bill each month, so 31 January does not come as a shock.
- File early. You do not have to pay until the Self Assessment deadline, but you will know your bill sooner.
Self Assessment Deadline FAQs
Can I file my tax return early?
Yes. You can file your 2025/26 return any time from 6 April 2026. Filing early does not mean you pay early, as the tax is still due by 31 January 2027.
What if I have never filed a return before?
You must register with HMRC by 5 October 2026. HMRC will then send you a Unique Taxpayer Reference so you can file before the Self Assessment deadline.
Do I need an accountant?
You can file your Self Assessment tax return online yourself, but many people find an accountant saves them time and tax, by claiming every allowable expense and relief and avoiding costly errors.
Let Southampton Accountants Handle Your Self Assessment
Power Accountax are ICAEW Chartered Accountants based in Southampton, and we have been preparing tax returns for sole traders, landlords, contractors and directors for over 20 years.
We will gather what we need from you, check for every allowable expense and relief, and file your return well before the Self Assessment deadline, with a fixed fee agreed up front.
Get your free quote today or contact us to book a free consultation.