Learning how to file your first Self Assessment can feel overwhelming, especially if you have recently joined the growing number of women running their own businesses. But with the right information, you will do it properly, meet every legal deadline and avoid penalties. Here is your no-nonsense guide to filing your 2025–26 return, covering every deadline that falls in 2026 and early 2027.
Who needs to file a Self Assessment tax return
You will need to register for Self Assessment if any of these applied during the 2025–26 tax year (6 April 2025 to 5 April 2026):
- You were self-employed as a sole trader and your total income before expenses was more than £1,000 (the trading allowance threshold). (Source:
- You were a partner in a business partnership. (Source:
- You had income that was not taxed at source, such as rental income, investments or overseas income. (Source:
- You need to pay Capital Gains Tax. (Source:
- Your income taxed through PAYE was more than £150,000. The old £100,000 filing threshold no longer applies on its own, although £100,000 is still the point where your Personal Allowance starts to reduce. (Source:
If none of these apply, you probably do not need to file. Even so, use HMRC’s online tool to be certain: if you are even slightly over a threshold, the cost of getting it wrong is far higher than the cost of checking. (Source:
Registration deadline and getting your Unique Taxpayer Reference
If you need to file, your first step is registration. You must register by 5 October 2026 if you need to send a return for 2025–26 and have never sent one before. The same deadline applies if you were registered previously but did not file a return last year. (Source:
After registration, HMRC will issue your Unique Taxpayer Reference (UTR), a 10-digit number you will use every year on your returns. You can find it in your HMRC online account or the HMRC app, or wait for it to arrive by post, which typically takes around 10 working days. Do not leave registration until late September: without a UTR, you cannot file. (Source:
Key deadlines: when to file and pay
For your first Self Assessment, covering 2025–26, here are the deadlines to know:
- 5 October 2026: last day to register for Self Assessment. (Source:
- 31 October 2026: deadline if you file a paper return. (Source:
- 31 January 2027: deadline for online submission and payment of tax owed. (Source:
If you are employed and owe less than £3,000, HMRC can collect what you owe through your PAYE tax code, spreading the cost across the following tax year rather than taking it in one lump sum. To arrange this, you must file your online return by 30 December 2026. (Source:
Which rates, allowances and digital rules apply
Income tax bands and allowances in 2026–27
For the tax year from 6 April 2026 to 5 April 2027, in England, Wales and Northern Ireland, the main figures are frozen at the same levels as 2025–26, so they apply both to the return you are about to file and to the year ahead:
- The Personal Allowance, your tax-free income, is £12,570. (Source:
- The basic rate of 20% applies to income from £12,571 to £50,270. (Source:
- The higher rate of 40% applies from £50,271 to £125,140. Above that, you pay the additional rate of 45%. (Source:
- If your income is more than £100,000, you lose £1 of Personal Allowance for every £2 you earn above that level. Your allowance reaches zero at £125,140. (Source:
Scotland sets its own income tax rates and bands, so check the Scottish figures if you live there. (Source:
Making Tax Digital for Income Tax (MTD IT)
Under Making Tax Digital for Income Tax, you must keep digital records and send quarterly updates. This applies if your combined self-employment and property income for 2024–25 was over £50,000, and the obligation starts from 6 April 2026. The threshold then falls to £30,000 from April 2027 and to £20,000 from April 2028. (Source:
If your income is below the threshold, or you are digitally excluded, you may be exempt. Even so, it is wise to check now whether the rules apply to you, because the falling thresholds mean most sole traders will be brought into the system by 2028. (Source:
How to prepare, submit and avoid penalties
Organise your documents early: collect invoices, bank statements, pension contributions, rental or dividend income and details of any foreign income. Have your National Insurance number and UTR ready. (Source:
Use HMRC’s online services or HMRC-recognised software to submit. Filing online gives you until 31 January, three months longer than the paper deadline, and you can file any time after 6 April 2026. Filing early does not mean paying early: it simply tells you the exact size of your bill while you still have months to plan for it. (Source:
Penalties for late returns and late payment
If you file your return late or pay your tax late, penalties apply. Missing the 5 October registration deadline can also lead to a penalty if it means tax goes unpaid as a result. (Source:
- A fixed penalty of £100 if your return is up to 3 months late, whether filed online or on paper. (Source:
- After 3 months late: £10 per day, up to a maximum of £900. (Source:
- After 6 months: a further 5% of the tax due or £300, whichever is greater. (Source:
- After 12 months: another 5% of the tax due or £300, whichever is greater. (Source:
- Late payment penalties: 5% of the tax unpaid at 30 days, 6 months and 12 months, with interest charged on top. (Source:
You can appeal if you have a reasonable excuse, such as a serious illness or HMRC service failure. Keep evidence to support your case, and note that “I forgot” and “I found it confusing” do not count. (Source:
Example timeline: first Self Assessment for a side hustle
Here is a typical example so you can see how deadlines, payments and allowances come together. Priya works full-time in marketing and started selling freelance design work in June 2025.
- June 2025 to 5 April 2026: she invoices £9,000 and keeps every receipt from day one. Her allowable expenses (software, stock and a share of her phone bill) come to £1,500, leaving a profit of £7,500.
- By 5 October 2026: she registers for Self Assessment. Her UTR arrives by post within about 10 working days.
- November 2026: she files online, well ahead of the deadline. She now knows her exact bill with more than two months to spare.
- Her bill: her salary already uses up her Personal Allowance, and she remains a basic-rate taxpayer overall, so her £7,500 profit is taxed at 20%. That gives a bill of £1,500. Because her self-employed profits are below £12,570, she owes no Class 4 National Insurance.
- 31 January 2027: she pays the £1,500. Done.
Two habits made this painless. First, Priya moved roughly 20% of every invoice into a separate savings account as she was paid, so the money for the bill was already sitting there. Second, she filed in November rather than joining the January rush, when HMRC’s phone lines are at their busiest and mistakes are most likely.
The payments on account trap most first-timers miss
If Priya had been freelancing full-time rather than running a side hustle alongside a salary, her first January could have looked very different. When your Self Assessment bill is over £1,000, and less than 80% of your total tax for the year was collected at source (for example through PAYE), HMRC will ask for payments on account: advance payments towards next year’s bill. (Source:
Each payment is 50% of the current year’s bill. The first is due on the same 31 January, and the second by 31 July. In your first year of full-time self-employment, that can mean paying roughly one and a half times your expected bill in a single January. It is the single biggest shock for new sole traders, so if you have left employment to work for yourself, budget for it from your very first invoice. (Source:
The bottom line
Your first Self Assessment is mostly admin, not advanced maths. Register by 5 October, keep records as you go, set aside 20 to 25% of your self-employed income, and file online well before 31 January. Do those four things and the process is genuinely manageable, whatever the horror stories suggest.
Most first-time filers are sole traders, and staying one is perfectly sensible while your income is modest. If you are wondering whether a different structure would serve you better as you grow, our guide to sole trader vs limited company explains the tax differences in plain English. And if you are still at the planning stage, start with our step-by-step guide to setting up a business today.
