Dev spent his first year as a self-employed electrician in Nottingham worrying about one envelope. His mate at the trade counter kept saying the taxman would catch up with him, and in August the letter finally landed: time to file Self Assessment online. He’d never done a tax return in his life. If you’re in the same position for the 2025/26 tax year, the whole job is easier than it looks, and this guide walks you through every step, deadline and fine before the January panic sets in.
Self Assessment at a Glance
| Who must file | Sole traders earning over £1,000 in 2025/26, plus others with untaxed income |
| Register by | 5 October 2026 |
| Online filing deadline | 11:59pm, 31 January 2027 |
| Paper deadline | 31 October 2026 |
| Payment due | 31 January 2027 |
| Late filing penalty | £100 straight away, rising after 3 months |
| Cost | Free on GOV.UK |
Do You Actually Need to File One?
Dev’s first question was blunt: does HMRC even want a return from him? For the 2025/26 tax year, you must file if you were a sole trader who earned more than £1,000 before expenses. You’re also in if you were a business partner, owed Capital Gains Tax, or had more than £2,500 in other untaxed income such as rent or tips. Parents hit by the High Income Child Benefit Charge, which starts at £60,000 and removes the benefit entirely at £80,000, often need a return too, although since 2025 HMRC can collect that charge through your tax code instead. Not sure? Run the free checker on GOV.UK, and while you’re signed in, check your tax code online so nothing else is quietly wrong.
Register by 5 October, Not in January
Here’s the deadline most first-timers miss. You must tell HMRC by 5 October 2026 that you need to send a return for 2025/26, and the simplest way is to register as self-employed with HMRC through GOV.UK. Once registered, your Unique Taxpayer Reference arrives by post in around 15 days, a 10-digit number you’ll need every time you file. It often shows up sooner in the HMRC app or your personal tax account, so keep an eye on both. Dev registered on a rainy Sunday evening in September and had his UTR before the month ended. Allow about three weeks for the full set-up, because you cannot send a return without that number, and registering late can bring a fine of its own.
Signing In: One Login or Government Gateway?
Signing in changed this year, so ignore older guides. Since 9 February 2026, new HMRC customers without an existing account create a GOV.UK One Login, using just an email address, a password and an identity check, and that single login now opens more than 200 government services. Already hold a Government Gateway user ID from an old job or a tax refund? Keep using it, because existing users will be moved across in phases before Gateway is switched off around 2029. If you’ve never set up either, our guide on how to set up a Government Gateway account explains which identity documents the checks accept. Dev used his passport and was through in about ten minutes at the kitchen table.
Gather Your Paperwork Before You Open the Form
Preparation saved Dev an entire evening. Before opening the return, collect your UTR, your National Insurance number, any P60 or P45 from employment, your invoices and sales records, receipts for expenses, and statements showing bank interest or dividends. Landlords should add rental income and costs. The online service lets you save your progress and come back later, so a missing figure never wrecks a session, but hunting receipts at 11pm on 31 January certainly might. While you’re collecting numbers, take five minutes to check your National Insurance record online, because filing Self Assessment is how self-employed people build qualifying years towards the State Pension. Keep every record for at least five years after the 31 January deadline, since HMRC can ask to see them.
Filling In the Return, Screen by Screen
The return itself surprised Dev with how much it helps you. Sign in at the official GOV.UK filing service, tell it about your year, and it shows only the sections that apply, self-employment for Dev, property pages for a landlord, with prompts along the way and an automatic calculation at the end. Nothing submits until you press the final button, and you can print the calculation for your records. A few groups cannot use the online form, including business partnerships, trusts and non-residents, who need commercial software instead. Returns for 2025/26 can go in any time from 6 April 2026. Fill yours in on a laptop rather than a phone; our comparison of the HMRC app and the HMRC website explains which jobs suit each.
Deadlines and the £100 Fine
Three dates decide whether this costs you anything. Paper returns must reach HMRC by 31 October 2026, online returns by 11:59pm on 31 January 2027, and the tax itself is due that same January night. File online by 30 December 2026 while owing under £3,000 and paying tax through PAYE, and HMRC can collect the bill through next year’s tax code instead. Miss the filing deadline and a £100 penalty lands immediately, even if you owe nothing. Three months late adds £10 a day up to £900, six months adds £300 or 5% of the tax due if that’s more, and late payment brings its own 5% charges plus interest. Locked out on deadline night? Here’s how to recover a lost Government Gateway user ID fast.
Payments on Account: The Sting in Year One
Dev’s biggest shock wasn’t the bill, it was the second bill. If you owe £1,000 or more, HMRC usually asks for payments on account: two advance instalments towards next year, each half of this year’s bill, due 31 January and 31 July. Owe £2,400 for 2025/26 and January’s total becomes £3,600, with another £1,200 the following July. Year one therefore hits hardest, so start saving now. Paying is quick by bank transfer, debit card or Direct Debit, though the Post Office option is gone. To spread the load, set up a weekly or monthly budget payment plan by Direct Debit in your online account; you can pause it for up to six months if work dries up. Can’t pay in full? Ask HMRC about a Time to Pay arrangement before the deadline.
Making Tax Digital: The Change That Started in April
One more change matters for anyone self-employed. Since 6 April 2026, sole traders and landlords whose qualifying income topped £50,000 in 2024/25 must follow Making Tax Digital for Income Tax, which means records kept in Making Tax Digital software and quarterly updates sent to HMRC. The threshold falls to £30,000 in April 2027 and £20,000 in April 2028, so most freelancers join eventually. Here’s the part first-timers get wrong: you still file a normal Self Assessment return for the tax year before you start, so Dev’s 2025/26 return goes in the usual way even though his quarterly updates began this spring. Exemptions exist for people who genuinely cannot use digital tools, and HMRC’s guidance pages set out how to apply for one.
One Evening Beats a January Panic
Dev filed in early December, paid by instalments, and the envelope that worried him all year turned out to be an hour’s work at the kitchen table. To file Self Assessment online without drama, the order matters: register by 5 October 2026, wait for your UTR, gather the records, then beat the 31 January 2027 deadline with weeks to spare. The service is free, it saves as you go, and it does the maths for you. Pick an evening this week and get registered.
Frequently Asked Questions
When is the deadline to file self assessment online for 2025/26?
Online returns for the 2025/26 tax year are due by 11:59pm on 31 January 2027, and any tax you owe must be paid by the same deadline.
How long does it take to get a UTR number?
Your 10-digit Unique Taxpayer Reference usually arrives by post around 15 days after you register, and often appears sooner in the HMRC app or personal tax account.
What happens if I file my tax return one day late?
HMRC charges an automatic £100 penalty the moment the 31 January deadline passes, even if you owe no tax, with daily fines starting after three months.
Do I need to file if I earned less than £1,000 self-employed?
Usually not. The £1,000 trading allowance covers small amounts of self-employment income, though you can still file voluntarily to pay National Insurance or prove your earnings.
Can I pay my self assessment bill in instalments?
Yes. A budget payment plan spreads the cost weekly or monthly by Direct Debit, and bills under £3,000 can often be collected through a PAYE tax code.
Disclaimer: This article is general information, not financial or tax advice. Rules and figures were correct as of August 2026; always check the latest guidance on GOV.UK or speak to a qualified adviser before making decisions about your tax.