How to File a Self Assessment Tax Return in the UK (2026)

How to File a Self Assessment Tax Return in the UK (2026)

August 10, 2026

You file your 2025/26 self assessment tax return by logging into HMRC’s Government Gateway, selecting the correct tax year, entering your income and expenses across each section, and submitting online before 31 January 2027. The whole process takes most people 30 to 60 minutes if their records are ready.That’s the short version. Below is the full walkthrough, from registration to payment.If you’d rather skip the DIY route and hand it to a qualified accountant, Tax Return Assist offers self assessment filing for self-employed individuals and personal tax return services starting from £99. But if you want to do it yourself, just keep reading.

Who actually needs to file a self assessment return?

Not everyone does. HMRC only requires self assessment from people with income that isn’t taxed at source through PAYE.The most common groups:Sole traders and freelancers earning more than £1,000 in a tax year. Landlords collecting rental income (even if it’s just one property). Company directors with income outside PAYE. People with savings, investments, or foreign income above certain thresholds. This includes anyone earning over £100,000 a year, regardless of how they receive that income.If you’re a sole trader working out whether this rule applies to you, our sole trader accountants page covers filing obligations specific to self-employment in more detail. If you’re a landlord unsure about your obligations, the page for property tax accountants has more detail on what landlords specifically need to report.HMRC has a free online tool on GOV.UK that tells you in about 2 minutes whether you need to file. Worth checking if you’re on the fence.

What are the key deadlines for 2025/26?

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. Here’s when everything is due:5 October 2026: Register with HMRC if you’re filing for the first time.31 October 2026: deadline for filing paper tax returns (if you’re still submitting by post).31 January 2027: online filing deadline AND payment deadline. Miss this and HMRC charges a £100 penalty automatically, even if you owe nothing.The penalties stack up fast after that. Three months late and HMRC adds £10 per day on top (up to £900). Six months late brings another 5% of the tax you owe or £300, whichever is higher. Same again at 12 months.Filing early doesn’t mean paying early, by the way. You can submit your return in April and still wait until 31 January to pay. But knowing your bill months in advance makes budgeting much easier.

What do you need before you start?

Gather everything before you open the HMRC website. Stopping halfway through to find a missing P60 is how returns take 3 hours instead of 1.Your Unique Taxpayer Reference (UTR). This is the 10-digit number HMRC sent you when you registered. It’s on previous tax returns and HMRC letters. If you’re unable to locate it, please review your HMRC online account.Your National Insurance number. Your National Insurance number can be found on your payslip, P60, or any HMRC correspondence.Income records. P60 from your employer (if you’re employed too), invoices or bank statements showing self-employment income, rental income figures, dividend statements, interest certificates from banks, and pension statements.Expense records. Receipts and records for any allowable business expenses you’re claiming. Office costs, travel, professional subscriptions, and tools. If your bookkeeping is up to date, this part is simple. But if it isn’t, that’s when the pain starts.Gift Aid and pension contributions are important. These reduce your tax bill, so please remember them.Your Government Gateway login. The login is a 12-digit user ID and password. Not the same as your UTR. If you don’t have one yet, the next section covers setting it up.

How do you register if it’s your first time?

If you’ve never filed before, you need to register with the HMRC and set up a Government Gateway account. Do the registration well before the October deadline because HMRC posts your UTR by mail, and that can take 10 working days (longer if you’re abroad).Here’s the process:Step 1. Go to GOV.UK and search for “register for self assessment.” HMRC routes you to the right form based on your situation. Sole traders use the online registration tool. Non-self-employed people (landlords and investors) fill in form SA1.Step 2. Create a Government Gateway account if you don’t already have one. You’ll need your name, email address, and National Insurance number. HMRC sends a user ID to your email and an activation code by post.Step 3. Wait for your UTR to arrive in the post. 10 working days for UK addresses. Use the activation code to complete your Government Gateway setup.Step 4.Log in and add Self Assessment to your HMRC services. You’re now ready to file.The most significant mistake here is leaving registration too late. If you register on 1 October and your UTR arrives on 15 October, you’ve barely got time to file a paper return by the 31 October deadline. Online registration is more forgiving, with a January deadline, but you still need your UTR before you can do anything. If you’d rather have the process handled for you, our registration services cover HMRC and Companies House registration on your behalf.

How do you actually file the return online, step by step?

Step 1: Log in. Go to GOV.UK, search “file your self assessment tax return,” and click through to the login page. Enter your Government Gateway user ID and password.Step 2: Select the tax year. Choose 2025/26. The system will show your personal details. Check they’re correct.Step 3: Please answer the tailoring questions. HMRC asks what types of income you had during the year. Self-employment? Rental income? Foreign income? Tick what applies. This determines which sections of the return you’ll need to complete.Step 4: Fill in the employment section. If you’re employed alongside being self-employed, enter your P60 figures here. Your employer’s PAYE reference, your gross pay, and the tax deducted. Please copy the numbers directly from the P60.Step 5: Fill in the self-employment section. This is where most people spend the bulk of their time. Enter your total business income (turnover) for the year. Then list your allowable expenses. HMRC breaks expenses into categories: office costs, travel, clothing, stock, legal fees, and so on. If your total expenses are under £1,000, you can claim the £1,000 trading allowance instead of itemizing.Step 6: Complete any other sections. Rental income goes in the property section. Savings interest, dividends, capital gains, foreign income, and pension contributions each have their own section. Only fill in what applies to you.Step 7: Review the tax calculation. Before you submit, HMRC shows you a summary: total income, total tax due, and any payments on account for next year. Check the numbers. If something looks wrong, go back and check your entries again.Step 8: Submit. Hit the submit button. HMRC sends a confirmation email with a reference number. Save it. Print it. Screenshot it. This is your proof of filing.The whole thing is a guided form. HMRC doesn’t expect you to know which box is which. You answer questions and fill in numbers. The system does the math.

How much tax will you actually owe?

That depends on your total taxable income after deducting your personal allowance and expenses. For 2025/26, the rates for England, Wales, and Northern Ireland are:Personal Allowance: £12,570 (you pay nothing on this portion).Basic rate (20%): on income from £12,571 to £50,270.Higher rate (40%): on income from £50,271 to £125,140.Additional rate (45%): on income above £125,140.Scotland has its own rates with 6 bands instead of 3, so Scottish taxpayers should check the Scottish Government website for exact figures.One thing that catches people out: if you earn between £100,000 and £125,140, your personal allowance gets clawed back at a rate of £1 for every £2 over £100,000. That creates an effective 60% tax rate in that band. Brutal, but legal.Self-employed people also pay Class 2 and Class 4 National Insurance on top of income tax. Class 2 is a flat £3.45 per week if profits exceed £12,570. Class 4 is 6% on profits between £12,570 and £50,270, and then 2% on profits above that.

What mistakes do people make most often?

After years of handling self assessment returns, the same errors come up again and again.One common mistake is forgetting to include all income. HMRC already knows about your PAYE income, bank interest, and dividends. If you leave something out, their systems will flag the mismatch. It’s not a question of if. It’s when.Not claiming legitimate expenses. People either claim nothing (overpaying tax) or claim everything, including personal expenses (inviting an inquiry). The rule is simple: if the expense was wholly and exclusively for business purposes, claim it. If your home broadband is 50% personal and 50% business, claim 50%. Small business accountants can help you get this right without going too far.Using the wrong tax year’s figures. The 2025/26 return covers income from 6 April 2025 to 5 April 2026. Not the calendar year. Not your financial year. HMRC’s tax year.Ignoring payments on account. If your tax bill is over £1,000 and less than 80% of it was collected at source, HMRC will ask for two advance payments towards your next year’s bill. These are due 31 January and 31 July. They surprise many first-time filers.Filing late because “I don’t owe anything.” The £100 penalty applies whether you owe tax or not. HMRC isn’t concerned about your reason. The penalty is automatic.

How do you pay your tax bill?

Once you’ve submitted your return, HMRC tells you exactly how much you owe and when it’s due. The deadline for payment in 2025/26 is 31 January 2027.You’ve got several payment options:Online banking or faster payment. Use the HMRC payment reference (your UTR followed by the letter K) and pay directly from your bank account. This is the fastest method. Money usually clears the same day or next day.Direct Debit. Set the payment up through your HMRC online account. It takes 5 working days to process the first time, so don’t leave it until 30 January.Debit card. Pay through the HMRC website. Personal credit cards are not accepted. Corporate credit cards are accepted, but a fee applies.Budget Payment Plan. If you know roughly what you’ll owe, you can set up weekly or monthly payments throughout the year. This spreads the hit instead of one lump sum in January.Can’t afford to pay? Please reach out to HMRC before the deadline to inquire about a time-to-pay arrangement. They’ll set up an installment plan. It’s much easier to arrange the payment plan before penalties kick in than after.

What about Making Tax Digital?

Big change here. From 6 April 2026, Making Tax Digital for Income Tax (MTD for ITSA) became mandatory for sole traders and landlords with qualifying income over £50,000.If that’s you, you will need to replace the old annual self-assessment return with quarterly digital submissions plus a final declaration. You’ll need MTD-compatible software (Xero, FreeAgent, QuickBooks, or similar), and your records must be kept digitally.The threshold drops to £30,000 from April 2027, then £20,000 from April 2028. So even if you’re below £50,000 now, the change is coming for most self-employed people eventually.For the 2025/26 tax year specifically, you’re still filing a traditional self-assessment return. MTD applies from 2026/27 onwards for those above the threshold.

Should you file it yourself or use an accountant?

If your tax affairs are simple (one source of self-employment income, a few expenses, no property or investments), you can probably file them yourself in under an hour. HMRC’s online system is genuinely usable.But if you’ve got multiple income sources, rental properties, capital gains, or foreign income, or you’re a company director with dividends, an accountant pays for itself in time saved and taxes reduced. A skilled accountant will spot reliefs and deductions you didn’t know existed. If you run a limited company alongside other income, our limited company accountant service covers both your company and personal filing obligations together.Tax Return Assist handles self assessment returns for sole traders, landlords, and small businesses across Essex and London. They file on your behalf as an HMRC-registered agent, so you don’t even need to touch Government Gateway yourself.

Ready to get your 2025/26 return filed?

Gather your records. Log into Government Gateway. Work through the sections. Submit. Pay by 31 January 2027. That’s the whole process.If you’d rather not deal with any of it, get an instant quote from Tax Return Assist. Call 02039377911, email contact@taxreturnassist.co.uk or use the online quote form. Their accountants will file your return, chase up any reliefs you’re owed, and make sure everything reaches HMRC on time.